Fachgebiet Materialmodellierung

Willkommen auf der Homepage des Fachgebiets Materialmodellierung (MM).

Das Fachgebiet beschäftigt sich mit multi-skaliger Modellierung von Materialstruktur und Materialeigenschaften unter Zuhilfenahme und Weiterentwicklung verschiedener computergestützter Berechnungsmethoden.

Forschungsschwerpunkte beinhalten funktionale Nanoteilchen, Ferroelektrika, transparente and organische Halbleiter sowie nanostrukturierte Metalle und Hochdruckphasen.

Im Bereich der Lehre bieten wir Vorlesungen zur Materialwissenschaft an, wobei der Schwerpunkt im Bereich theoretischer Modelle und computergestützter Beschreibung von Materialien liegt.

Unser Fachgebiet bietet darüber hinaus die Gelegenheit zu Bachelor-, Master- und Promotionsarbeiten an.

Aktuelle Informationen

PhD-graduation in the MM Group

We are happy to congratulate Ms Constanze Kalcher to her defense
„Creep of Cu-Zr metallic glasses and metallic glass composites: A molecular dynamics study“. The defense was followed by a buffet and an after-defense party.

We thank Ms Kalcher not only for this memorable day and the nice party but also for all the work she has done within the Materials Modelling group and we look forward to a continued good cooperation.

Role of oxygen and chlorine impurities in β−In2S3: A first-principles study

Authors: Elaheh Ghorbani and Karsten Albe

For environmental reasons there is a need for alternative Cd-free buffer materials in Cu(In,Ga)(S,Se)2 (CIGSSe) based thin film solar cells. In this context, β−In2S3 is one candidate material, whose optoelectronic properties can be affected by the presence of impurities. In this study, we investigate the impact of O and Cl impurities on the electronic and optical behavior of β−In2S3 by means of electronic structure calculations within density functional theory using hybrid functionals. We find that β−In2S3 is thermodynamically stable being in contact with both O and Cl reservoirs. Furthermore, we present evidence that O on interstitial sites (Oi) and Cl on 8c In sites (ClIn) cause low-temperature persistent electron photoconductivity. At room temperature, defect levels associated with Cl on S sites (ClS, ClS′, and ClS'') get thermally ionized and release free electrons into the system. Thus, the n-type conductivity of the In2S3 buffer layer increases. O impurities on S sites, in contrast, are electrically inert. Hence, we conclude that intentional doping by Cl is a means to improve the properties of β−In2S3 serving as buffer material.

DOI: 10.1103/PhysRevB.98.205201

Structure and Properties of Nanoglasses

Authors: Yulia Ivanisenko, Christian Kübel, Sree Harsha Nandam, Chaomin Wang, Xiaoke Mu, Omar Adjaoud, Karsten Albe, Horst Hahn

Nanoglasses represent a novel structural modification of amorphous materials, exhibiting properties and structural details that are markedly different from those observed in metallic glasses prepared by rapid quenching. In this review, the synthesis method and the techniques used for charactering the structure of nanoglasses are described together with our current understanding of their salient microstructural features. It is believed that the structure of nanoglasses consists of two distinct amorphous regions give rise to mechanical, thermal, and magnetic properties that are significantly different from those observed in rapidly quenched (RQ) metallic glasses. Nanoglasses, therefore, constitute a distinct new class of amorphous materials and thus opening up new opportunities for their potential use in a number of structural and functional applications.

DOI: 10.1002/adem.201800404

Pressure induced phase transitions and elastic properties of CaCO3 polymorphs: a density functional theory study

Authors: R. Belkofsi, O. Adjaoud and I. Belabbas

First-principles calculations, based on density functional theory, were carried out to investigate phase transitions, structural and elastic properties of three polymorphs of calcium carbonate, in the pressure range up to 40 GPa. Our calculations led to the following stability sequence: calcite-IIIb → calcite-III → calcite-IIIb → calcite-VI, where phase transitions of the first order occur at 4.3 GPa, 14.9 GPa and 18.2 GPa, respectively. From 4.3 to 40 GPa, the elastic properties, the acoustic wave velocities and the Debye temperature of each polymorph exhibit a linear dependence over pressure. A nonlinear behavior is observed from 2.0 to 4.3 GPa for the properties of calcite-IIIb. As there are no available experimental data on the elastic properties of calcite-III, calcite-IIIb, calcite-VI and their pressure dependence, our present findings can serve to a better understanding of the behavior of calcium carbonate in the Earth's mantle.

DOI: 10.1088/1361-651X/aacbed

Structure sensitivity of electronic transport across graphene grain boundaries

Authors: Delwin Perera and Jochen Rohrer

Graphene grown by large-scale synthesis methods usually contains grain boundaries. They can strongly affect the electronic and mechanical properties of graphene and it is promising to exploit them for the design of electronic components and sensors. Here, we consider semiconducting graphene bicrystals and study how grain boundary structure variations influence electron transport using density functional theory in conjunction with the nonequilibrium Green function method. We find that the size of the transport gap in these bicrystals is not changed by structure variations. Interestingly however, electron transport outside the transport gap is very sensitive to modifications of the grain boundary. We show that these results can be understood within the ballistic transport approximation and by inspecting the electronic density of states resolved in energy-momentum space. Our findings suggest that the electronic response of graphene bicrystals can be controlled not only by grain misorientation but also by manipulation of the grain boundary structure.

DOI: 10.1103/PhysRevB.98.155432

Correlated polarization-switching kinetics in bulk polycrystalline ferroelectrics. II. Impact of crystalline phase symmetries

Authors: R. Khachaturyan and Y. A. Genenko

Electric depolarization fields have a great impact on the polarization-switching kinetics in ferroelectrics although they are often neglected in statistical considerations. Analysis of statistical distributions and correlations of polarization and electric field during the field-driven polarization reversal in a bulk polycrystalline ferroelectric by means of the two-dimensional self-consistent mesoscopic switching (SMS) model has revealed that correlations, mediated by electrostatic fields, are mostly isotropic and short range at a typical scale of the mean grain size [Phys. Rev. B 96, 054113 (2017)]. However, the magnitude of emerging depolarization fields remains substantial and strongly influences the switching kinetics. It is known, on the other hand, that the effect of inhomogeneities, such as a granular structure, on the electric field pattern and local field magnitudes is considerably overestimated in two-dimensional simulations. Three-dimensional extension of the SMS model in the current study allows a realistic evaluation of the impact of spatial correlations on the polarization switching in ferroelectric ceramics and opens a possibility to consider materials of different phase symmetries. It is shown that bound charges at grain boundaries due to mismatching grain polarizations as well as the subsequent depolarization fields are essentially dependent on the crystalline symmetry. This explains great differences in statistical field distributions and polarization kinetics observed in ceramics of different phase symmetries. Field correlations are anisotropic, depend on the material symmetry, but remain in all cases short range at the scale of a grain size. This sheds light on the success of models assuming statistically independent switching of different regions. Evolution of the statistical field distributions in the course of polarization reversal is also symmetry dependent but temporal changes in distributions are not substantial which clarifies a good performance of models neglecting the feedback via depolarization fields.

DOI: 10.1103/PhysRevB.98.134106

MSE 2018 – Best Poster Award

Congratulations to our former colleague, Dr. Tobias Brink, for winning the poster competition on the MSE 2018.

Dislocation Analysis Tool for Atomistic Simulations

Book chapter (pp 1-14)

Author: Alexander Stukowski

Precise analysis and meaningful visualization of dislocation structures in molecular dynamics simulations are important steps toward physical insights. This chapter provides an introduction to the dislocation extraction algorithm (DXA), which is a computational method for identifying and quantifying dislocations in atomistic crystal models. It builds a bridge between the atomistic world of crystal defects and the discrete line picture of classical dislocation theory.

DOI: 10.1007/978-3-319-42913-7_20-1

PhD-graduation in the MM Group

We congratulate our dear colleague Mr. Daniel A. Barragan-Yani to his doctoral work on

„Frist-principles study of dislocations in Cu(In,Ga)Se2 Solar cell absorbes“

We thank Mr. Daniel Barragan-Yani not only for this memorable day with a nice after-defense party but also for all the work he has done within the Materials Modelling group.

Materials Modelling Team Building Days

27.08.2018 – 30.08.2018, Höchst/Odenwald

Control of polarization reversal temperature behavior by surface screening in thin ferroelectric films

Authors: Anna N.Morozovska, Eugene A.Eliseev, Ivan S.Vorotiahin, Maxim V.Silibin, Sergei V.Kalinin, Nicholas V.Morozovsky

Ferroelectric surfaces and interfaces are unique physical objects for fundamental studies of various screening mechanisms of spontaneous polarization by free carriers and possible ion exchange between the polar surface and ambient media. The theory of the polarization charge compensation at ferroelectric surface by ambient screening charges requires a detailed comparison of different screening models. In the article, we study the free energy of a thin ferroelectric film covered by a screening charge layer of different nature and calculate hysteresis loops of polarization and screening charge in the system at different temperatures. The dependence of the screening charge density on electric potential was considered for three basic models, namely for the linear Bardeen-type surface states (BS) and nonlinear the Fermi-Dirac (FD) density of states describing two-dimensional electron gas at the film surface, and the strongly nonlinear electrochemical Stephenson-Highland (SH) model describing the surface charge density of absorbed ions. Among considered surface screening models, the most various behavior of polarization and screening charge hysteresis loops is inherent to SH model. Obtained results give new insight to the understanding of bound charge compensation at ferroelectric surfaces at different temperatures, as well as they open the way to control the polarization reversal in thin ferroelectric films by appropriate choice of the surface charge nature and screening mechanism.

DOI: 10.1016/j.actamat.2018.08.041

Metallic glass nanolaminates with shape memory alloys

Authors: D. Şopu, K. Albe, J. Eckert

We model the deformation behavior of metallic amorphous Cu64Zr36/crystalline B2 CuZr nanolaminate systems using molecular-dynamics computer simulations. Amorphous-crystalline nanolaminates with shape memory alloys may be a material class which is combining the advantageous properties of metallic glasses with large-strain homogeneous flow at low temperatures and high stresses. We find that the deformation of the glassy and crystalline phases is a coupled process: martensitic transformation leads to shear band formation while the stress at the shear band tip induces martensitic transformation in the shape memory crystal. Moreover, the martensitic transformation changes the shear band morphology, stabilizes the shear flow and avoids a runaway instability. Finally, the critical volume fraction of the B2 layer for which the composite laminate shows a brittle-to-ductile transition is identified. The value of the critical volume fraction can be further decreased when the structure of the metallic glass is rejuvenated. Therefore, tailoring the architecture of metallic glass laminates with shape memory phases may allow the development of materials that exhibit large tensile ductility.

DOI: 10.1016/j.actamat.2018.08.034

Nanoferroics: State-of-art, gradient-driven couplings and advanced applications (Author’s review – invited article)

Authors: A.N. Morozovska, I.S. Vorotiahin, Ye.M. Fomichov, and C.M. Scherbakov

Ferroics and multiferroics are unique objects for fundamental physical research of complex nonlinear processes and phenomena that occur in them within micro- or nanoscale. Due to the possibility of their physical properties control by size effects, nanostructured and nanosized ferroics are among the most promising for advanced applications in nanoelectronics, nanoelectromechanics, optoelectronics, nonlinear optics and information technologies. The review shows that the thickness of the strained films, the size and shape of the ferroic and multiferroic nanoparticles are unique tools for controlling their phase diagrams, long-range order parameters, magnitude of susceptibility, magnetoelectric coupling and domain structure characteristics at a fixed temperature. Significant influence of the flexochemical effect on the phase transition temperature, polar and dielectric properties of nanoparticles is revealed for thin films and nanoparticles. The obtained results are important for understanding the nonlinear physical processes in nanoferroics as well as for the advanced applications in nanoelectronics.

DOI: 10.15407/spqeo21.02.139

The influence of anisotropic surface stresses and bulk stresses on defect thermodynamics in LiCoO2 nanoparticles

Authors: Peter Stein, Ashkan Moradabadi, Manuel Diehm, Bai-Xiang Xu, Karsten Albe

The demand for higher specific capacity and rate capability has led to the adoption of nanostructured electrodes for lithium-ion batteries. At these length scales, surface effects gain an appreciable impact not only on the electrochemical and mechanical behavior of the electrode material, but also on defect thermodynamics. The focus of this study is the distribution of surface-induced bulk stresses in a LiCoO2 nanoparticle and their impact on the migration of Livacancies. LiCoO2 is a prototypical cathode material, where the diffusion of Li is mediated by the vacancy mechanism.

For this investigation, elastic parameters and anisotropic surface stress components are computed using Density Functional Theory calculations. They are incorporated into a surface-enhanced continuum model, implemented by means of the Finite Element method. The particle geometry is derived from a Wulff construction, and changes in the formation energy and migration barriers of a Li vacancy are determined using the defect dipole tensor concept.

Within the considered nanoparticle, the surface stresses result in a highly heterogeneous bulk stress distribution with a vortex-like transition region between the tensile particle core and its non-uniformly stressed boundaries. Both the center and the exterior of the particle show enhanced formation energy and migration barriers for of a Li vacancy. These experience a reduction in the transition region in the particle, culminating in a peak increase in vacancy diffusivity and ionic conductivity by circa 10% each. For a particle at a length-scale of 10nm, this yields an overall increase in ionic conductivity by a mere 0.8%. This surface stress-enhanced conductivity decays rapidly with increasing particle size.

DOI: 10.1016/j.actamat.2018.07.046

Influence of Cu and Na incorporation on the thermodynamic stability and electronic properties of β-In2S3

Authors: Elaheh Ghorbani and Karsten Albe

The aim of this study is to understand the effect of Na and Cu incorporation in In2S3, which is representing a Cd-free buffer system for chalcopyrite-type thin film solar cells based on Cu(In,Ga)Se2 (CIGS). The formation energies and charge states of sodium and copper dopants in In2S3 are investigated by means of calculations based on electronic hybrid density functional theory using supercells of 320-atoms. Our results reveal a negative formation enthalpy of sodium in both In-rich and S-rich samples, which indicates the occurrence of side reactions and explains the existence of a chemically modified buffer layer in the presence of a Na-reservoir. Copper, in contrast, can be incorporated in large concentrations in In-rich In2S3 under n-type conditions, acting as an acceptor and thus limiting the n-type conductivity. For lower Fermi energies, however, reactions between Cu and the buffer material lead to the formation of Cu-containing secondary phases in the buffer side which is in qualitative agreement with experimental observations of Bär et al. [Bär et al., Appl. Mater. Interfaces, 2016, 8, 2120]. Sulfur rich samples are found to be more heavily doped under n-type conditions and we expect to have Na- and Cu-containing secondary phases formed under metal-poor growth conditio

DOI: 10.1039/c8tc01341a

From metallic glasses to nanocrystals: Molecular dynamics simulations on the crossover from glass-like to grain-boundary-mediated deformation behaviour

Authors: Tobias Brink, Karsten Albe

Nanocrystalline metals contain a large fraction of high-energy grain boundaries, which may be considered as glassy phases. Consequently, with decreasing grain size, a crossover in the deformation behaviour of nanocrystals to that of metallic glasses has been proposed. Here, we study this crossover using molecular dynamics simulations on bulk glasses, glass–crystal nanocomposites, and nanocrystals of Cu64Zr36 with varying crystalline volume fractions induced by long-time thermal annealing. We find that the grain boundary phase behaves like a metallic glass under constraint from the abutting crystallites. The transition from glass-like to grain-boundary-mediated plasticity can be classified into three regimes: (1) For low crystalline volume fractions, the system resembles a glass–crystal composite and plastic flow is localised in the amorphous phase; (2) with increasing crystalline volume fraction, clusters of crystallites become jammed and the mechanical response depends critically on the relaxation state of the glassy grain boundaries; (3) at grain sizes , the system is jammed completely, prohibiting pure grain-boundary plasticity and instead leading to co-deformation. We observe an inverse Hall–Petch effect only in the second regime when the grain boundary is not deeply relaxed. Experimental results with different grain boundary states are therefore not directly comparable in this regime.

DOI: 10.1016/j.actamat.2018.06.036

Designing nanoindentation simulation studies by appropriate indenter choices: Case study on single crystal tungsten

Authors: Saurav Goel, Graham Cross, Alexander Stukowski, Ernst Gamsjäger, Ben Beake, Anupam Agrawal

Atomic simulations are widely used to study the mechanics of small contacts for many contact loading processes such as nanometric cutting, nanoindentation, polishing, grinding and nanoimpact. A common assumption in most such studies is the idealisation of the impacting material (indenter or tool) as a perfectly rigid body. In this study, we explore this idealisation and show that active chemical interactions between two contacting asperities lead to significant deviations of atomic scale contact mechanics from predictions by classical continuum mechanics. We performed a testbed study by simulating velocity-controlled, fixed displacement nanoindentation on single crystal tungsten using five types of indenter (i) a rigid diamond indenter (DI) with full interactions, (ii) a rigid indenter comprising of the atoms of the same material as that of the substrate i.e. tungsten atoms (TI), (iii) a rigid diamond indenter with pairwise attraction turned off, (iv) a deformable diamond indenter and (v) an imaginary, ideally smooth, spherical, rigid and purely repulsive indenter (RI). Corroborating the published experimental data, the simulation results provide a useful guideline for selecting the right kind of indenter for atomic scale simulations.

DOI: 10.1016/j.commatsci.2018.04.044

Impact of polarization dynamics and charged defects on electrocaloric response of ferroelectric Pb(Zr,Ti)O3 ceramics

Authors: Florian Weyland, Andraž Bradeško, Yang-Bin Ma, Jurij Koruza, Bai-Xiang Xu, Karsten Albe, Tadej Rojac, Nikola Novak

The impact of charged point defects on the electrocaloric response of morphotropic Pb(Zr,Ti)O3 (PZT) ceramics is investigated by direct electrocaloric and polarization hysteresis measurements. The electrocaloric response determined for undoped, hard‐, and soft‐doped PZT is rationalized by considering the dipolar entropy change over polarization change and hysteresis losses. Surprisingly, the highest electrocaloric effect is observed in poled hard PZT with the field applied along the poling direction, which is related to the reduced hysteresis losses caused by internal electric fields associated with defect complexes. The weak hysteretic dependency of polarization as function of the electric field in hard PZT also allows inducing an inverse electrocaloric effect. The experimental results are compared with model calculations, which provide a physical interpretation. The results reveal how the electrocaloric response of ferroelectrics can be optimized by defect engineering.

DOI: 10.1002/ente.201800140

On the origins of the inverse electrocaloric effect

Authors: Anna Grünebohm, Yang-Bin Ma, Madhura Marathe, Bai-Xiang Xu, Karsten Albe, Constanze Kalcher, Kai-Christian Meyer, Vladimir V. Shvartsman, Doru C. Lupascu, Claude Ederer

We discuss and analyze the occurrence of the inverse (or negative) electrocaloric effect, where the isothermal application of an electric field leads to an increase in entropy and the removal of the field decreases the entropy of the system under consideration. Inverse electrocaloric effects have been reported to occur in several cases, e.g., at transitions between ferroelectric phases with different polarization directions, in materials with certain polar defect configurations, and in antiferroelectrics. This counterintuitive relation between entropy and applied field is not only intriguing and thus of general scientific interest, but the combined application of normal and inverse effects has also been suggested as a means to achieve larger temperature differences between hot and cold reservoirs in future cooling devices. A good general understanding and the possibility to engineer inverse caloric effects in terms of temperature spans, required fields, and operating temperatures are thus of fundamental as well as technological importance. In this article, we review the known cases of inverse electrocaloric effects, discuss their physical origins, and compare the different cases in order to identify common aspects as well as potential differences. We show that in all these cases the inverse electrocaloric effect is related to the presence of competing phases or states which are close in energy and can easily be transformed with the applied field.

Publication cover image

DOI: 10.1002/ente.201800166

La2CoO4: a new intercalation based cathode material for fluoride ion batteries with improved cycling stability

Authors: Mohammad Ali Nowroozi, Sergei Ivlev, Jochen Rohrer and Oliver Clemens

In this study, we report on the electrochemical cycling behavior of La2CoO4 (against a composite of Pb/PbF2 as the anode material) for use as an intercalation-based cathode material for fluoride ion batteries (FIBs). The material can intercalate approximately 1.2 fluoride ions per formula under the formation of La2CoO4F1.2 resulting in a strong increase of the cell volume, confirmed by means of ex situ X-ray diffraction studies at various charging capacities and additional complementary chemical fluorination experiments. Furthermore, by only regulating the cut-off capacity we were able to remarkably avoid unwanted side reactions which were previously assumed to be a major problem for achieving high cycling numbers of LaSrMnO4. In this respect, electrochemical impedance spectroscopy was used to determine an initial critical specific charge capacity of ∼65 mA h g−1. By carefully designing the charging process, a discharge capacity as high as 32 mA h g−1 could be obtained, which is the highest capacity for an intercalation based cathode material reported so far, with a capacity retention of ∼25% of the initial discharge capacity after 50 cycles. In addition, we discuss why avoiding the cross-reactivity of the carbon additive is more difficult for fluoride ion batteries than for lithium ion batteries, showing that this is an important challenge for the successful implementation of high voltage cathode materials.

DOI: 10.1039/C7TA09427b

Developing intercalation based anode materials for fluoride-ion batteries: topochemical reduction of Sr2TiO3F2 via a hydride based defluorination process

Authors: Kerstin Wissel, Supratik Dasgupta, Alexander Benes, Roland Schoch, Matthias Bauer, Ralf Witte, Andrew Dominic Fortes, Emre Erdem, Jochen Rohrer and Oliver Clemens

Within this study, we demonstrate a newly developed reductive topochemical defluorination method which allows for a selective extraction/substitution of fluoride ions out of transition metal oxyfluorides using NaH as the reducing agent at temperatures as low as 300 °C, enabling the preparation and investigation of anode materials for fluoride-ion batteries in the charged state. A sequence of topochemical reactions, first substitutive fluorination of the K2NiF4 type precursor oxide Sr2TiO4 to Sr2TiO3F2 followed by the reductive defluorination/hydride-fluoride-substitution of Sr2TiO3F2, leads to the formation of compounds with approximate compositions of Sr2TiO3FH0.48 and Sr2TiO3H1.48. The evolution of the unit cell volumes upon extraction/substitution of fluoride ions has been monitored by an X-ray diffraction study. Strong structural changes were observed during the reaction, expressed by a decrease of the c-axis from ∼15.5 Å in Sr2TiO3F2 to ∼13.0 Å in Sr2TiO3FH0.48 and ∼12.7 Å in Sr2TiO3H1.48, with additional symmetry lowering for Sr2TiO3FH0.48, and confirmed by neutron powder diffraction. Changes of the Ti oxidation states and coordination environment were examined and confirmed by means of X-ray absorption spectroscopy, electron paramagnetic resonance and magnetic measurements. Furthermore, we found that the surface chemistry of such compounds differs significantly from the bulk properties by means of an X-ray photoelectron spectroscopy study, showing that surface compositions of Sr2TiO3F and Sr2TiO3 with Ti3+ and Ti2+ species can be obtained on the reaction, respectively. We further highlight that a ‘direct’ reduction of the oxide precursor Sr2TiO4 to Sr2TiO3 is not possible using the same method. Therefore, the two-step modification of Sr2TiO4 allows for the preparation of compounds with low oxidation states at the lowest temperatures used so far for hydride based reductions of titanium containing oxides (300 °C compared to ∼600 °C used previously). The observed differences in the reactivity of oxyfluorides compared to oxides are well supported by DFT based calculations of formation energies. They show that the formation of NaF is a strong driving force, resulting in exergonic reductions/hydride-fluoride substitution reactions as compared to the endergonic formation of Na2O for pure oxide compounds.

DOI: 10.1039/C8TA01012a

Influence of Na and Ga on the electrical properties of perfect 60° dislocations in Cu(In, Ga)Se2 thin-film photovoltaic absorbers

Authors: D. Barragan-Yani and K. Albe

The segregation of GaIn and NaCu to perfect 60° dislocations in CuIn1–xGaxSe2 is investigated by means of density functional theory calculations. We find that the segregation process is mainly driven by the elastic interaction of both defect types with the strain field of the dislocation. GaIn moves into the negatively strained region, while NaCu is found in the positively strained region. We show that both defects affect the electronic defect levels induced by the dislocation core and GaIn is able to passivate the β-core in CuInSe2. This result indicates that β-cores are inactive in CuIn1–xGaxSe2. NaCu; however, they do not have a significant effect on the electrical properties of the studied dislocation cores. Therefore, the experimentally observed sodium segregation to dislocation cores in CuIn1–xGaxSe2 cannot be considered as the passivation mechanism of the electrically active cores in that material.

DOI: 10.1063/1.5026483

Stochastic multistep polarization switching in ferroelectrics

Authors:Y. A. Genenko, R. Khachaturyan, J. Schultheiß, A. Ossipov, J. E. Daniels, and J. Koruza

Consecutive stochastic 90° polarization switching events, clearly resolved in recent experiments, are described by a nucleation and growth multistep model. It extends the classical Kolmogorov-Avrami-Ishibashi approach and includes possible consecutive 90°- and parallel 180° switching events. The model predicts the results of simultaneous time-resolved macroscopic measurements of polarization and strain, performed on a tetragonal Pb (Zr,Ti)O3 ceramic in a wide range of electric fields over a time domain of seven orders of magnitude. It allows the determination of the fractions of individual switching processes, their characteristic switching times, activation fields, and respective Avrami indices.

DOI: 10.1103/PhysRevB.97.144101

Intrinsic point defects in β-In2S3 studied by means of hybrid density-functional theory

Authors: Elaheh Ghorbani and Karsten Albe

We have employed first principles total energy calculations in the framework of density functional theory, with plane wave basis sets and screened exchange hybrid functionals to study the incorporation of intrinsic defects in bulk β-In2S3. The results are obtained for In-rich and S-rich experimental growth conditions. The charge transition level is discussed for all native defects, including VIn, VS, Ini, Si, SIn, and InS, and a comparison between the theoretically calculated charge transition levels and the available experimental findings is presented. The results imply that β-In2S3 shows n-type conductivity under both In-rich and S-rich growth conditions. The indium antiisite (InS), the indium interstitial (Ini), and the sulfur vacancy (VS′) are found to be the leading sources of sample's n-type conductivity. When going from the In-rich to the S-rich condition, the conductivity of the material decreases; however, the type of conductivity remains unchanged.

DOI: 10.1063/1.5020376

Diffusion mechanism in the superionic conductor Li4PS4I studied by first-principles calculations

Authors: Sabrina Sicolo, Constanze Kalcher, Stefan J. Sedlmaier, Jürgen Janek, Karsten Albe

Li4PS4I was recently discovered as a novel crystalline lithium ion conductor by applying a soft chemistry approach. It adopts a tetragonal structure type suggesting a three-dimensional migration pathway favorable for a high conductivity [S.J. Sedlmaier, Chem. Mater. 29 (2017) 1830]. Especially in view of the highly connected pathways for Li motion within a simple anion sublattice formed by PS 3-4and iodine anions, the conductivity measured from both impedance spectroscopy and NMR data appears to be rather small, and a negative influence of impurities and of grain boundaries effects was assumed. In order to shed light on the full potential of this material, we performed a theoretical study of Li4PS4I in the framework of density functional theory. After creating a structural model that accurately accounts for the partial occupancies determined by diffraction experiments, we performed molecular dynamics simulations, unraveled the diffusion mechanisms and calculated diffusion coefficients and the activation barrier for diffusion. The results of the theoretical study on both a crystalline and a glassy supercell imply that Li4PS4I is in fact a superionic conductor with a much higher conductivity than reported so far.

DOI: 10.1016/j.ssi.2018.01.046

Influence of elastic strain on the thermodynamics and kinetics of lithium vacancy in bulk LiCoO2

Authors: Ashkan Moradabadi, Payam Kaghazchi, Jochen Rohrer, and Karsten Albe

The influence of elastic strain on the lithium vacancy formation and migration in bulk LiCoO2 is evaluated by means of first-principles calculations within density functional theory (DFT). Strain dependent energies are determined directly from defective cells and also within linear elasticity theory from the elastic dipole tensor (Gij) for ground state and saddle point configurations. We analyze finite size effects in the calculation of Gij, compare the predictions of the linear elastic model with those obtained from direct calculations of defective cells under strain, and discuss the differences. Based on our data, we calculate the variations in vacancy concentration and mobility due to the presence of external strain in bulk LiCoO2 cathodes. Our results reveal that elastic in-plane and out-of-plane strains can significantly change the ionic conductivity of bulk LiCoO2 by up to several orders of magnitude and thus strongly affect the performance of Li-secondary batteries.

DOI: 10.1103/PhysRevMaterials.2.015402

Articles of Materials Modelling Division in the MSME Highlights 2017

The following articles of our Materials Modelling Division have been selected for inclusion in MSMSE’s “Highlights of 2017” collection:

“Anisotropic solid–liquid interface kinetics in silicon: an atomistically informed phase-field model”
„Atomicrex—a general purpose tool for the construction of atomic interaction models“
published in the journal Modelling and Simulation in Materials Science and Engineering.

The Highlights collection includes the top articles published in MSMSE each year, selected on the basis of novelty, scientific impact and broadness of appeal.

You can view a complete list of the Highlights of 2017 on the journal web page at

All articles included in the Highlights of 2017 collection will be free to access for all readers throughout 2018, and IOP Publishing will advertise the collection in an upcoming marketing campaign.

Defect thermodynamics and interfacial instability of crystalline Li4P2S6

Authors: Marcel Sadowski, Sabrina Sicolo, Karsten Albe

The defect chemistry of various lithium and sulfur point defects in the solid electrolyte Li4P2S6 in its planar arrangement in the P 3̅1m space group is studied by means of total energy calculations within density functional theory. We show that the formation of Li Frenkel-pairs is the dominant internal defect reaction which provides the charge carriers for ionic conductivity. External sulfur related defect equilibria can be excluded as compensation mechanisms for lithium defects.
Moreover, we find that charged lithium interstitials exhibit negative formation energies, pointing to the instability of the electrolyte against metallic lithium. This tendency is supported by total energy calculations that predict a highly exothermic reaction between Li4P2S6 and metallic lithium to give Li2S and Li3P. The extent of this interfacial instability is such that we observed substantial reactivity even within static calculations on representative interface models. Depending on the surface orientation, a more or less ordered interphase is formed. The formed interphase shows similarities to Li2S, which has been suggested to act as a passivating layer that inhibits further reaction between the thiophosphate and metallic lithium.

DOI: 10.1016/j.ssi.2018.01.047

Comment on “Incipient plasticity of diamond during nanoindentation” by C. Xu, C. Liu and H. Wang, RSC Advances, 2017, 7, 36093

Authors: Saurav Goel and Alexander Stukowski

A recent molecular dynamics simulation study on nanoindentation of diamond carried out by Xu et al. 1 has reported observation of the presence of a controversial hexagonal lonsdaleite phase of carbon in the indentation area. In this comment, we question the reported observation and attribute this anomaly to shortcomings of the long range bond order potential (LCBOP) employed in the nanoindentation study.

DOI: 10.1039/c7ra12219e

Strain Distribution Across an Individual Shear Band in Real and Simulated Metallic Glasses

Authors: Sergio Scudino and Daniel Şopu

Because of the fast dynamics of shear band formation and propagation along with the small size and transient character of the shear transformation zones (STZs), the elementary units of plasticity in metallic glasses, the description of the nanoscale mechanism of shear banding often relies on molecular dynamics (MD) simulations. However, the unrealistic parameters used in the simulations related to time constraints may raise questions about whether quantitative comparison between results from experimental and computational analyses is possible. Here, we have experimentally analyzed the strain field arising across an individual shear band by nanobeam X-ray diffraction and compared the results with the strain characterizing a shear band generated by MD simulations. Despite their largely different spatiotemporal scales, the characteristic features of real and simulated shear bands are strikingly similar: the magnitude of the strain across the shear band is discontinuous in both cases and the direction of the principal strain axes exhibits the same antisymmetric profile. This behavior can be explained by considering the mechanism of STZ activation and percolation at the nanoscale, indicating that the nanoscale effects of shear banding are not limited to the area within the band but they extend well into the surrounding elastic matrix. These findings not only demonstrate the reliability of MD simulations for explaining (also quantitatively) experimental observations of shear banding but also suggest that designed experiments can be used the other way around to verify numerical predictions of the atomic rearrangements occurring within a band.

DOI: 10.1021/acs.nanolett.7b04816

Defect-driven flexochemical coupling in thin ferroelectric films

Authors: Eugene A. Eliseev, Ivan S. Vorotiahin, Yevhen M. Fomichov, Maya D. Glinchuk, Sergei V. Kalinin, Yuri A. Genenko, and Anna N. Morozovska

Using the Landau-Ginzburg-Devonshire theory, we considered the impact of the flexoelectrochemical coupling on the size effects in polar properties and phase transitions of thin ferroelectric films with a layer of elastic defects. We investigated a typical case, when defects fill a thin layer below the top film surface with a constant concentration creating an additional gradient of elastic fields. The defective surface of the film is not covered with an electrode, but instead with an ultrathin layer of ambient screening charges, characterized by a surface screening length. Obtained results revealed an unexpectedly strong effect of the joint action of Vegard stresses and flexoelectric effect (shortly flexochemical coupling) on the ferroelectric transition temperature, distribution of the spontaneous polarization and elastic fields, domain wall structure and period in thin PbTiO3 films containing a layer of elastic defects. A nontrivial result is the persistence of ferroelectricity at film thicknesses below 4 nm, temperatures lower than 350 K, and relatively high surface screening length (∼0.1nm). The origin of this phenomenon is the flexoelectric coupling leading to the rebuilding of the domain structure in the film (namely the cross-over from c-domain stripes to a-type closure domains) when its thickness decreases below 4 nm. The ferroelectricity persistence is facilitated by negative Vegard effect. For positive Vegard effect, thicker films exhibit the appearance of pronounced maxima on the thickness dependence of the transition temperature, whose position and height can be controlled by the defect type and concentration. The revealed features may have important implications for miniaturization of ferroelectric-based devices.

DOI: 10.1103/PhysRevB.97.024102

Microstructure formation of metallic nanoglasses: Insights from molecular dynamics simulations

Authors: Omar Adjaoud, Karsten Albe

We investigate the microstructure formation of Pd80Si20 and Cu64Zr36 nanoglasses by molecular dynamics simulations of the consolidation process of nanometer-sized metallic glassy spheres. Our results reveal that during cold compaction most of the glassy spheres deform by homogeneous plastic flow and in some glassy spheres strain localization occurs in a shear band which traverses the whole glassy sphere. Moreover, the porosity is closed if hydrostatic pressures exceed 4GPa. The resulting nanoglasses are composed of glassy regions connected by glass-glass interfaces. The results reveal that the width of these interfaces is significantly larger than estimated in previous atomistic models based on planar interfaces. Moreover, structural changes occur not only in the interfaces but also in the glassy regions. In addition, thermodynamics analysis show that surface segregation is favorable in the primary glassy spheres but not always in the nanoglass. The present findings shed light on the process of the microstructure formation of metallic nanoglasses and can serve to interpret the experimental results.

DOI: 10.1016/j.actamat.2017.12.014

Structural modification through pressurized sub-Tg annealing of metallic glasses

Authors: A. Foroughi, H. Ashuri, R. Tavakoli, M. Stoica, D. Şopu, and J. Eckert

The atomic structure of metallic glasses (MGs) plays an important role in their physical and mechanical properties. Numerous molecular dynamics (MD) simulations have been performed to reveal the structure of MGs at the atomic scale. However, the cooling rates utilized in most of the MD simulations (usually on the order of 109–1012 K/s) are too high to allow the structure to relax into the actual structures. In this study, we performed long-term pressurized sub-Tg annealing for up to 1 μs using MD simulation to systematically study the structure evolution of Cu50Zr50 MG. We find that from relaxation to rejuvenation, structural excitation of MGs and transition during sub-Tg annealing depend on the level of hydrostatic pressure. At low hydrostatic pressures, up to 2 GPa in this alloy, the relaxation rate increases with the increasing pressure. The lowest equivalent cooling rate reaches 3.3 × 106 K/s in the sample annealed at 2 GPa hydrostatic pressure, which is in the order of the cooling rate in melt spinning experiments. Higher pressures retard the relaxation rate or even rejuvenate the sample. Structural relaxation at low hydrostatic pressure during sub-Tg annealing is governed by short-range atomic rearrangements through annihilation of free volume and anti-free volume defects. In contrast, at high hydrostatic pressures, most of the atoms just experience thermal vibration rather than real atomic jumps. The formation of anti-free volume defects is the main source of structural instability at the high pressure region.

DOI: 10.1063/1.5004058

Thermochemical stability of Li–Cu–O ternary compounds stable at room temperature analyzed by experimental and theoretical methods

Authors: Maren Lepple, Jochen Rohrer, Robert Adam, Damian M. Cupida, David Rafaja, Karsten Albe and Hans J. Seifert

Compounds in the Li–Cu–O system are of technological interest due to their electrochemical properties which make them attractive as electrode materials, i. e., in future lithium ion batteries. In order to select promising compositions for such applications reliable thermochemical data are a prerequisite. Although various groups have investigated individual ternary phases using different experimental setups, up to now, no systematic study of all relevant phases is available in the literature. In this study, we combine drop solution calorimetry with density function theory calculations to systematically investigate the thermodynamic properties of ternary Li–Cu–O phases. In particular, we present a consistently determined set of enthalpies of formation, Gibbs energies and heat capacities for LiCuO, Li2CuO2 and LiCu2O2 and compare our results with existing literature.

DOI: 10.3139/146.111560

Ionic conductivity of acceptor doped sodium bismuth titanate: influence of dopants, phase transitions and defect associates

Authors: Leonie Koch, Sebastian Steiner, Kai-Christian Meyer, In-Tae Seo, Karsten Albe and Till Frömling

We investigate both, experimentally and theoretically, the electrical conductivity of Mg- and Fe-doped polycrystalline Na0.5Bi0.5TiO3. Samples with up to 4% of acceptor dopants are studied by means of impedance spectroscopy, scanning electron microscopy, and X-ray diffraction, while an analytical defect chemical model is developed for describing the measured conductivities. Within the framework of defect chemistry, we demonstrate that the experimentally measured conductivities can only be reproduced, if the formation of dopant–vacancy defect complexes is considered and the phase transition from a rhombohedral to a tetragonal symmetry is taken into account, affecting the dissociation of the dopant–vacancy complex. By using migration energies from density functional theory calculations, we obtain a good agreement between the data obtained from the analytical model and the experimental results, if we assume that the association energy is strongly affected by the dopant concentration.

DOI: 10.1039/C7TC03031B

Diffusion of yttrium in bcc-iron studied by kinetic Monte Carlo simulations

Authors: Markus Mock, Karsten Albe

The formation of oxide nanoclusters in oxide dispersion strengthened steels is controlled by the diffusion of yttrium. Yttrium atoms and other oversized solutes show a high binding energy to vacancies and a considerable relaxation from their lattice site towards a neighboring vacancy. In the case of yttrium the relaxation is so prominent, that the resulting situation may also be considered as an interstitial atom sitting in between two vacancies. We calculated the yttrium-vacancy binding energy and the migration barriers of vacancy jumps in the vicinity of a yttrium atom by means of nudged-elastic band calculations using density functional theory calculations. These barriers were used in a kinetic Monte Carlo code to calculate the diffusivity of yttrium and investigate the diffusion mechanism of yttrium in bcc iron with focus on correlation effects. The results reveal that the diffusion of yttrium is due to a sequence of vacancy jumps between the nearest and third nearest neighbor shell of the yttrium atom.

DOI: 10.1016/j.jnucmat.2017.07.021

Highly Porous Silicon Embedded in a Ceramic Matrix: A Stable High-Capacity Electrode for Li-Ion Batteries

Authors: Dragoljub Vrankovic, Magdalena Graczyk-Zajac, Constanze Kalcher, Jochen Rohrer, Malin Becker, Christina Stabler, Grzegorz Trykowski, Karsten Albe, and Ralf Riedel

We demonstrate a cost-effective synthesis route that provides Si-based anode materials with capacities between 2000 and 3000 mAh·gSi–1 (400 and 600 mAh·gcomposite–1), Coulombic efficiencies above 99.5%, and almost 100% capacity retention over more than 100 cycles. The Si-based composite is prepared from highly porous silicon (obtained by reduction of silica) by encapsulation in an organic carbon and polymer-derived silicon oxycarbide (C/SiOC) matrix. Molecular dynamics simulations show that the highly porous silicon morphology delivers free volume for the accommodation of strain leading to no macroscopic changes during initial Li–Si alloying. In addition, a carbon layer provides an electrical contact, whereas the SiOC matrix significantly diminishes the interface between the electrolyte and the electrode material and thus suppresses the formation of a solid–electrolyte interphase on Si. Electrochemical tests of the micrometer-sized, glass-fiber-derived silicon demonstrate the up-scaling potential of the presented approach.

DOI: 10.1021/acsnano.7b06031

Bond length deviation in CuZr metallic glasses

Authors: Chuan-Xiao Peng, Daniel Şopu, Kai-Kai Song, Zhen-Ting Zhang, Li Wang, and Jürgen Eckert

We define a structural parameter, called atomic bond length deviation (BLDi), to characterize structural heterogeneity of CuZr melt and metallic glass (MG). Molecular dynamics simulations have been performed to explore the average BLDi of the system evolution with temperature during Cu64Zr36 and Cu50Zr50 MGs formation and the correlation between BLDi and thermal relaxation/local atomic shear strain upon compressive loading. The results indicate that BLDi contains both symmetrical characteristic and volumetric information of the short-range order clusters while symmetry seems to play a more important role in relaxation and deformation events; the fast decreasing of average BLDi near above the glass transition temperature Tg with decreasing temperature corresponds to the sharp increase of the number of full icosahedra while the shear transformation zones or single jump events have a high propensity to originate from those regions with the higher BLDi clusters. Additionally, the system average BLDi can also be accessed experimentally, through the radial distribution function.

DOI: 10.1103/PhysRevB.96.174112

Extended Tersoff potential for boron nitride: Energetics and elastic properties of pristine and defective h-BN

Authors: J. H. Los, J. M. H. Kroes, K. Albe, R. M. Gordillo, M. I. Katsnelson, and A. Fasolino

We present an extended Tersoff potential for boron nitride (BN-ExTeP) for application in large scale atomistic simulations. BN-ExTeP accurately describes the main low energy B, N, and BN structures and yields quantitatively correct trends in the bonding as a function of coordination. The proposed extension of the bond order, added to improve the dependence of bonding on the chemical environment, leads to an accurate description of point defects in hexagonal BN (h-BN) and cubic BN (c-BN). We have implemented this potential in the molecular dynamics LAMMPS code and used it to determine some basic properties of pristine 2D h-BN and the elastic properties of defective h-BN as a function of defect density at zero temperature. Our results show that there is a strong correlation between the size of the static corrugation induced by the defects and the weakening of the in-plane elastic moduli.

DOI: 10.1103/PhysRevB.96.184108

Relaxation of dynamically disordered tetragonal platelets in the relaxor ferroelectric 0.964Na1/2Bi1/2TiO3−0.036BaTiO3

Authors: Florian Pforr, Kai-Christian Meyer, Márton Major, Karsten Albe, Wolfgang Donner, Uwe Stuhr, and Alexandre Ivanov

The local dynamics of the lead-free relaxor 0.964Na1/2Bi1/2TiO3−0.036BaTiO3 (NBT-3.6BT) have been investigated by a combination of quasielastic neutron-scattering (QENS) and ab initio molecular dynamics simulations. In a previous paper, we were able to show that the tetragonal platelets in the microstructure are crucial for understanding the dielectric properties of NBT-3.6BT [Pforr et al., Phys. Rev. B 94, 014105 (2016)]. To investigate their dynamics, ab initio molecular dynamics simulations were carried out using Na1/2Bi1/2TiO3 with 001 cation order as a simple model system for the tetragonal platelets in NBT-3.6BT. Similarly, 111-ordered Na1/2Bi1/2TiO3 was used as a model for the rhombohedral matrix. The measured single-crystal QENS spectra could be reproduced by a linear combination of calculated spectra. We find that the relaxational dynamics of NBT-3.6BT are concentrated in the tetragonal platelets. Chaotic stages, during which the local tilt order changes incessantly on the time scale of several picoseconds, cause the most significant contribution to the quasielastic intensity. They can be regarded as an excited state of tetragonal platelets, whose relaxation back into a quasistable state might explain the frequency dependence of the dielectric properties of NBT-3.6BT in the 100 GHz to THz range. This substantiates the assumption that the relaxor properties of NBT-3.6BT originate from the tetragonal platelets.

DOI: 10.1103/PhysRevB.96.184107

Former Ovito publication of MM appears in a special edition of the journal „Modelling and Simulation in Materials Science“ (MSMSE)

The journal is celebrating its 25th year of publication and its serving in the multidisciplinary materials community. To mark the occasion they put together an anniversary collection featuring the top articles and contributors over the years.

The journal informed that the Co-Editors-in-Chief have selected at least one article from each volume that has had a high impact and advanced the state of the art, capturing the multidisciplinary character of the journal and the magnitude of significant methods and research that have advanced the understanding of fundamental materials issues.

We are delighted that our former Ovito publication is among these articles.

Atomic-Level Processes of Shear Band Nucleation in Metallic Glasses

Authors: D. Şopu, A. Stukowski, M. Stoica, and S. Scudino

The ability to control the plastic deformation of amorphous metals is based on the capacity to influence the percolation of the shear transformation zones (STZs). Despite the recent research progress, the mechanism of STZ self-assembly has so far remained elusive. Here, we identify the structural perturbation generated by an STZ in the surrounding material and show how such a perturbation triggers the activation of the neighboring STZ. The mechanism is based on the autocatalytic generation of successive strong strain and rotation fields, leading to STZ percolation and, ultimately, to the formation of a shear band.

DOI: 10.1103/PhysRevLett.119.195503

Interface-controlled creep in metallic glass composites

Authors: Constanze Kalcher, Tobias Brink, Jochen Rohrer, Alexander Stukowski, Karsten Albe

In this work we present molecular dynamics simulations on the creep behavior of Cu64Zr36 metallic glass composites. Our results reveal that all composites exhibit much higher creep rates than the homogeneous glass. This is because the glass–crystal interface acts like a weak interphase, where the activation of shear transformation zones is easier than in the surrounding glass. We observe that the creep behavior of the composites does not only depend on the interface area but also on the orientation of the interface with respect to the loading axis. We propose an explanation in terms of different mean Schmid factors of the interfaces, with the amorphous interface regions acting as preferential slip sites.

DOI: 10.1016/j.actamat.2017.08.058

Probing the limits of metal plasticity with molecular dynamics simulations

Authors: Luis A. Zepeda-Ruiz, Alexander Stukowski,Tomas Oppelstrup & Vasily V. Bulatov

Ordinarily, the strength and plasticity properties of a metal are defined by dislocations—line defects in the crystal lattice whose motion results in material slippage along lattice planes1. Dislocation dynamics models are usually used as mesoscale proxies for true atomistic dynamics, which are computationally expensive to perform routinely2. However, atomistic simulations accurately capture every possible mechanism of material response, resolving every “jiggle and wiggle”3 of atomic motion, whereas dislocation dynamics models do not. Here we present fully dynamic atomistic simulations of bulk single-crystal plasticity in the body-centred-cubic metal tantalum. Our goal is to quantify the conditions under which the limits of dislocation-mediated plasticity are reached and to understand what happens to the metal beyond any such limit. In our simulations, the metal is compressed at ultrahigh strain rates along its [001] crystal axis under conditions of constant pressure, temperature and strain rate. To address the complexity of crystal plasticity processes on the length scales (85–340 nm) and timescales (1 ns–1μs) that we examine, we use recently developed methods of in situ computational microscopy4,5 to recast the enormous amount of transient trajectory data generated in our simulations into a form that can be analysed by a human. Our simulations predict that, on reaching certain limiting conditions of strain, dislocations alone can no longer relieve mechanical loads; instead, another mechanism, known as deformation twinning (the sudden re-orientation of the crystal lattice6), takes over as the dominant mode of dynamic response. Below this limit, the metal assumes a strain-path-independent steady state of plastic flow in which the flow stress and the dislocation density remain constant as long as the conditions of straining thereafter remain unchanged. In this distinct state, tantalum flows like a viscous fluid while retaining its crystal lattice and remaining a strong and stiff metal.

DOI: 10.1038/nature23472

Reaction and Space Charge Layer Formation at the LiCoO2–LiPON Interface: Insights on Defect Formation and Ion Energy Level Alignment by a Combined Surface Science–Simulation Approach

Authors: Mathias Fingerle , Roman Buchheit, Sabrina Sicolo, Karsten Albe, and René Hausbrand

In this contribution, we investigate the formation and evolution of LiCoO2–LiPON interfaces upon annealing using photoelectron spectroscopy. We identify interlayer compounds related to the deposition process and study the chemical reactions leading to interlayer formation. Based on the structure of the pristine interface as well as on its evolution upon annealing, we relate reaction layer and space charge layer formation to chemical potential differences between the two materials. The results are discussed in terms of a combined Li-ion and electron interface energy level scheme providing insights into fundamental charge transfer processes. In constructing the energy level alignment, we take into account calculated defect formation energies of lithium in the cathode and solid electrolyte.

DOI: 10.1021/acs.chemmater.7b00890

Phase transformations in the relaxor Na1/2Bi1/2TiO3 studied by means of density functional theory calculations

Authors: Kai-Christian Meyer, Leonie Koch and Karsten Albe

The relaxor material Na1/2Bi1/2TiO3 (NBT) is an important basis for the development of lead-free piezoceramics, but still many features of this material are not well understood. Here, we study the kinetics of phase transformations by octahedral tilts and A-cation displacements in NBT by means of density functional theory calculations, employing ab initio molecular dynamics and nudged elastic band calculations. Our results show that the energetic differences between the low temperature rhombohedral, intermediate orthorhombic and other metastable phases are close to the room temperature thermal energy. Therefore, it is likely that above room temperature, several octahedral tilt patterns are present simultaneously on the local scale, just because of thermal vibration of the oxygen ions. Octahedral tilt transformations and A-cation displacements show similarly high energy barriers, however, since the vibrational frequency of oxygen is higher, tilt transformations occur more frequently. Further, tilt transformations in which the oxygen octahedra get deformed the least are more probable to occur. We also find that the chemical A-cation order affects energy barriers, influences the coupling between rotational and displacive modes and determines the stability of certain octahedral tilt orders. We conclude that the so-called polar nanoregions in this material result from local octahedral tilt transformations and subsequent A-cation displacements, which are driven by thermal vibration and are mediated by the underlying chemical order.

DOI: 10.1111/jace.15207

Local segregation versus irradiation effects in high-entropy alloys: Steady-state conditions in a driven system

Authors: Leonie Koch, Fredric Granberg, Tobias Brink, Daniel Utt, Karsten Albe, Flyura Djurabekova and Kai Nordlund

We study order transitions and defect formation in a model high-entropy alloy (CuNiCoFe) under ion irradiation by means of molecular dynamics simulations. Using a hybrid Monte-Carlo/molecular dynamics scheme, a model alloy is generated which is thermodynamically stabilized by configurational entropy at elevated temperatures, but partly decomposes at lower temperatures by copper precipitation. Both the high-entropy and the multiphase sample are then subjected to simulated particle irradiation. The damage accumulation is analyzed and compared to an elemental Ni reference system. The results reveal that the high-entropy alloy—independent of the initial configuration—installs a certain fraction of short-range order even under particle irradiation. Moreover, the results provide evidence that defect accumulation is reduced in the high-entropy alloy. This is because the reduced mobility of point defects leads to a steady state of defect creation and annihilation. The lattice defects generated by irradiation are shown to act as sinks for Cu segregation.

DOI: 10.1063/1.4990950

Correlated polarization-switching kinetics in bulk polycrystalline ferroelectrics: A self-consistent mesoscopic switching model

Authors: Ruben Khachaturyan, Jens Wehner and Yuri A. Genenko

Analysis of statistical distributions and auto- and cross correlations of polarization and electric field during the field-driven polarization reversal in a bulk polycrystalline ferroelectric is performed. A mesoscopic switching model is used which accounts self-consistently for the development of depolarization fields. Correlations mediated by electrostatic fields are shown to be mostly isotropic and short range at the typical scale of the grain size which is explained by an effective screening via adapting bound charges. The short-range screening clarifies the paradoxical ability of common statistical concepts neglecting the feedback effect of depolarization fields to adequately describe the polarization switching kinetics. The statistical distribution of the local electric field magnitudes is continuously spreading in the course of the global polarization reversal due to mismatching of both dielectric tensor and spontaneous polarization at grain boundaries. The increasing field dispersion substantially contributes to the well-known deceleration of the polarization reversal at long times.

DOI: 10.1103/PhysRevB.96.054113

Effect of Ti content and nitrogen on the high-temperature oxidation behavior of (Mo,Ti)5Si3

Authors: M.A. Azim, B. Gorr, H.-J. Christ, O. Lenchuk, K. Albe, D. Schliephake, M. Heilmaier

The binary intermetallic compounds Mo5Si3 (T1) and Ti5Si3 are prone to rapid oxidation below 1000 °C. Recent investigations on (Mo,Ti)5Si3, however, revealed that macro-alloying with 40 at.% Ti can result in a very good oxidation resistance in a wide temperature range (750–1300 °C) due to the formation of a duplex layer composed of a silica matrix with dispersed titania. Additionally, Ti decreases density making (Mo,Ti)5Si3 a promising key constituent of quaternary Mo-Si-B-Ti alloys considered for ultrahigh temperature structural applications. The aim of this study is to obtain an in-depth understanding of the influence of different Ti concentrations as well as of nitrogen on the oxidation behavior of (Mo,Ti)5Si3 at intermediate and elevated temperatures. The microstructure and oxidation mechanisms were analyzed using various experimental techniques. The experimental results were supported by ab initio and thermodynamic calculations.

DOI: 10.1016/j.intermet.2017.05.023

Reinforcement of nanoglasses by interface strengthening

Authors: Constanze Kalcher, Omar Adjaoud, Jochen Rohrer, Alexander Stukowski, KarstenAlbe

Nanoglasses consist of glassy grains connected by an amorphous interface. While internal interfaces in nanoglasses help to prevent brittle failure, they are usually not beneficial to the glasses overall strength. In this molecular dynamics study, we manipulate the glass–glass interfaces of a Cu–Zr nanoglass, such that they are replaced by stronger crystalline interphases. Analogous to grain boundary strengthening in crystalline materials, we show that it is possible to reinforce the nanoglass without compromising its ductility.


Experimental and theoretical study of AC losses in variable asymmetrical magnetic environments

Authors: S.T. Ranecky, H. Watanabe, J. Ogawa, D. Gölden, L. Alff and Yuki A. Genenko

Measurements of AC losses in a HTS-tape placed in between two bulk magnetic shields of high permeability were performed by applying calorimetric techniques for various asymmetrical shielding arrangements. The experiment was supported by analytical calculations and finite-element simulations of the field and current distributions, based on the Bean model of the critical state. The simulated current and field profiles perfectly reproduce the analytic solutions known for certain shielding geometries. The evaluation of the consequent AC losses exhibits good agreement with measurements for the central position of the tape between the magnets but have increasing discrepancy when the tape is approaching the shields. This can be explained by the increasing contribution of the eddy currents and magnetic hysteresis losses in the conducting shields.

DOI: 10.1088/1361-6668/aa73bc

Tuning the polar states of ferroelectric films via surface charges and flexoelectricity

Authors: Ivan S.Vorotiahin, Eugene A. Eliseev, Qian Li, Sergei V. Kalinin, Yuri A.Genenko, Anna N. Morozovska

Using the self-consistent Landau-Ginzburg-Devonshire approach we simulate and analyze the spontaneous formation of the domain structure in thin ferroelectric films covered with the surface screening charge represented by the Bardeen-type surface states. Hence we consider the competition between the screening and the domain formation as alternative ways to reduce the electrostatic energy and reveal unusual peculiarities of distributions of polarization, electric and elastic fields conditioned by the surface screening length and the flexocoupling strength. We have established that the critical thickness of the film and its transition temperature to a paraelectric phase strongly depend on the Bardeen screening length, while the flexocoupling affects the polarization rotation and closure domain structure. Furthermore the flexocoupling induces ribbon-like nano-scale domains in the film depth far from the top open surface, which might be related to the enigmatic polar nanoregions in relaxor ferroelectrics. Thus the joint action of the surface screening (originating from e.g. the adsorption of ambient ions or surface states) and flexocoupling may remarkably modify polar and electromechanical properties of thin ferroelectric films.

DOI: 10.1016/j.actamat.2017.07.033

Anisotropic solid–liquid interface kinetics in silicon: an atomistically informed phasefield model

Authors: S. Bergmann, K. Albe, E. Flegel, D. A. Barragan-Yani and B. Wagner

We present an atomistically informed parametrization of a phase-field model for describing the anisotropic mobility of liquid–solid interfaces in silicon. The model is derived from a consistent set of atomistic data and thus allows to directly link molecular dynamics and phase field simulations. Expressions for the free energy density, the interfacial energy and the temperature and orientation dependent interface mobility are systematically fitted to data from molecular dynamics simulations based on the Stillinger–Weber interatomic potential. The temperature-dependent interface velocity follows a Vogel–Fulcher type behavior and allows to properly account for the dynamics in the undercooled melt.


Si- and Sn-containing SiOCN-based nanocomposites as anode materials for lithium ion batteries: synthesis, thermodynamic characterization and modeling

Authors: Jochen Rohrer, Dragolyub Vrankovic, Damian Cupid, Ralf Riedel, Hans J. Seifert, Karsten Albe and Magdalena Graczyk-Zajac

Novel nanocomposites consisting of silicon/tin nanoparticles (n-Si/n-Sn) embedded in silicon carbonitride (SiCN) or silicon oxycarbide (SiOC) ceramic matrices are investigated as possible anode materials for Li-ion batteries. The goal of our study is to exploit the large mass specific capacity of Si/Sn (3579 mAh g−1/994 mAh g−1), while avoiding rapid capacity fading due to the large volume changes of Si/Sn during Li insertion. We show that a large amount (∼30–40 wt.%) of disordered carbon phase is dispersed within the SiOC/SiCN matrix and stabilizes the Si/Sn nanoparticles with respect to extended reversible lithium ion storage. Silicon nanocomposites are prepared by mixing of a polymeric precursor with commercial and “home-synthesized” crystalline and amorphous silicon. Tin nanocomposites, in contrast, are prepared using a single precursor approach, which allows the in-situ generation of Sn nanoparticles homogeneously dispersed within the SiOC host. The best electrochemical stability along with capacities of 600–700 mAh g−1 is obtained when amorphous/porous silicon is used. Mechanisms contributing to the increase of storage capacity and the cycle stability are clarified by analyzing elemental composition, local solid-state structures, intercalation hosts and Li-ion mobility. Our work is supplemented by first-principles based atomistic modeling and thermochemical measurements.


Excursion 27.06.2017 – 28.06.2017

Students of the 4th semester in Materials Science visited the steel production in Salzgitter and the automotive manufacturing at Volkswagen in Wolfsburg.

PhD-graduation in the MM Group

We are happy to congratulate our dear colleague Kai Meyer to his defense
„Phase Transformation Kinetics and Oxygen Transport in the Relaxor Ferroelectric Na1/2Bi1/2TiO3 studied by First-Principles Calculations “. The defense was followed by a buffet and an after-defense barbecue in the evening.

We thank Kai Meyer for all the work he has done within the Materials Modelling group and wish him all the best in the further course of her life.

PhD-graduation in the MM Group

We are happy to congratulate Ms Olena Lenchuk to her defense
„Density-Functional Theory Calculations of Solutes in Molybdenum Grain Boundaries“. The defense was followed by a buffet and an after-defense barbecue in the evening.

We thank Ms Olena Lenchuk not only for this memorable day but also for all the work she has done within the Materials Modelling group and wish her all the best in the further course of her life.

Cohesive strength of zirconia/molybdenum interfaces and grain boundaries in molybdenum: A comparative study

Authors: Olena Lenchuk, , Jochen Rohrer, Karsten Albe

We present calculations within density functional theory on the thermodynamic stability and mechanical properties of t-ZrO2(001)/Mo(001) interfaces. The interfacial strength is evaluated by applying energy-based (work of separation) and stress-based (theoretical strength) criteria for different cleavage planes. The lowest energy for crack propagation is obtained for a cut creating a stoichiometric ZrO2(001) surface. Our results reveal that molybdenum grain boundaries contaminated with oxygen are less stable against brittle fracture than pure Mo GBs. Addition of Zr to Mo-based alloys, however, can strengthen Mo grain boundaries that contain oxygen by forming an ultrathin zirconia film between Mo grains. The stress required to cleave an ultrathin zirconia film is equal to that required for a pure Mo GB.


Point defect segregation and its role in the detrimental nature of Frank partials in Cu(In,Ga)Se2 thin-film absorbers

Authors: E. Simsek Sanli, D. Barragan-Yani, Q. M. Ramasse, K. Albe, R. Mainz, D. Abou-Ras, A. Weber, H.-J. Kleebe, and P. A. van Aken

The interaction of point defects with extrinsic Frank loops in the photovoltaic absorber material Cu(In,Ga)Se2 was studied by aberration-corrected scanning transmission electron microscopy in combination with electron energy-loss spectroscopy and calculations based on density-functional theory. We find that Cu accumulation occurs outside of the dislocation cores bounding the stacking fault due to strain-induced preferential formation of Cu−2In, which can be considered a harmful hole trap in Cu(In,Ga)Se2. In the core region of the cation-containing α-core, Cu is found in excess. The calculations reveal that this is because Cu on In-sites is lowering the energy of this dislocation core. Within the Se-containing β-core, in contrast, only a small excess of Cu is observed, which is explained by the fact that CuIn and Cui are the preferred defects inside this core, but their formation energies are positive. The decoration of both cores induces deep defect states, which enhance nonradiative recombination. Thus, the annihilation of Frank loops during the Cu(In,Ga)Se2 growth is essential in order to obtain absorbers with high conversion efficiencies.


On the origin of the anomalous compliance of dealloying-derived nanoporous gold

Authors: B.-N. D.Ngô, B. Roschning, K. Albe, J. Weissmüller, J. Markmann

The origin of the anomalously compliant behavior of nanoporous gold is studied by comparing the elasticity obtained from molecular dynamics (MD) and finite element method (FEM) simulations. Both models yield a compliance, which is much higher than the predictions of the Gibson-Ashby scaling relation for metal foams and thus confirm the influence of other microstructural features besides the porosity. The linear elastic FEM simulation also yields a substantially stiffer response than the MD simulation, which reveals that nonlinear elastic behavior contributes decisively to the anomalous compliance of nanoporous gold at small structure size.

DOI: 10.1016/j.scriptamat.2016.11.006

Atomicrex—a general purpose tool for the construction of atomic interaction models

Authors: Alexander Stukowski, Erik Fransson, Markus Mock and Paul Erhart

We introduce atomicrex, an open-source code for constructing interatomic potentials as well as more general types of atomic-scale models. Such effective models are required to simulate extended materials structures comprising many thousands of atoms or more, because electronic structure methods become computationally too expensive at this scale. atomicrex covers a wide range of interatomic potential types and fulfills many needs in atomistic model development. As inputs, it supports experimental property values as well as ab initio energies and forces, to which models can be fitted using various optimization algorithms. The open architecture of atomicrex allows it to be used in custom model development scenarios beyond classical interatomic potentials while thanks to its Python interface it can be readily integrated e.g., with electronic structure calculations or machine learning algorithms.


Interfacial instability of amorphous LiPON against lithium: A combined Density Functional Theory and spectroscopic study

Authors: Sabrina Sicolo, Mathias Fingerle, René Hausbrand, Karsten Albe

The chemical instability of the glassy solid electrolyte LiPON against metallic lithium and the occurrence of side reactions at their interface is investigated by combining a surface science approach and quantum-mechanical calculations. Using an evolutionary structure search followed by a melt-quenching protocol, a model for the disordered structure of LiPON is generated and put into contact with lithium. Even the static optimization of a simple model interface suggests that the diffusion of lithium into LiPON is driven by a considerable driving force that could easily take place under experimental conditions. Calculated reaction energies indicate that the reduction and decomposition of LiPON is thermodynamically favorable. By monitoring the evolution of the LiPON core levels as a function of lithium exposure, the disruption of the LiPON network alongside the occurrence of new phases is observed. The direct comparison between UV photoelectron spectroscopy measurements and calculated electronic densities of states for increasing stages of lithiation univocally identifies the new phases as Li2O, Li3P and Li3N. These products are stable against Li metal and form a passivation layer which shields the electrolyte from further decomposition while allowing for the diffusion of Li ions.


Atomic and electronic structure of perfect dislocations in the solar absorber materials CuInSe2 and CuGaSe2 studied by first-principles calculations

Authors: Daniel Barragan-Yani and Karsten Albe

Structural and electronic properties of screw and 60∘-mixed glide and shuffle dislocations in the solar absorber materials CuInSe2 and CuGaSe2 are investigated by means of electronic structure calculations within density functional theory (DFT). Screw dislocations present distorted bonds but remain fully coordinated after structural relaxation. Relaxed 60∘-mixed dislocations, in contrast, exhibit dangling and “wrong,” cation-cation or anion-anion bonds, which induce deep charge transition levels and are electrically active. Analysis of Bader charges and local density of states (LDOS) reveals that acceptor and donor levels are induced by α and β cores, respectively. Moreover, there is local charge accumulation in the surrounding of those cores which contain dangling or “wrong” bonds. Thus the apparently harmless nature of dislocations is not because they are electrically inactive, but can only be a result of passivation by segregating defects.


Local Structural Investigations, Defect Formation, and Ionic Conductivity of the Lithium Ionic Conductor Li4P2S6

Authors: Christian Dietrich, Marcel Sadowski, Sabrina Sicolo, Dominik A. Weber, Stefan J. Sedlmaier, Kai S. Weldert, Sylvio Indris, Karsten Albe, Jürgen Janek, and Wolfgang G. Zeier

Glassy, glass–ceramic, and crystalline lithium thiophosphates have attracted interest in their use as solid electrolytes in all-solid-state batteries. Despite similar structural motifs, including PS43–, P2S64–, and P2S74– polyhedra, these materials exhibit a wide range of possible compositions, crystal structures, and ionic conductivities. Here, we present a combined approach of Bragg diffraction, pair distribution function analysis, Raman spectroscopy, and 31P magic angle spinning nuclear magnetic resonance spectroscopy to study the underlying crystal structure of Li4P2S6. In this work, we show that the material crystallizes in a planar structural arrangement as a glass ceramic composite, explaining the observed relatively low ionic conductivity, depending on the fraction of glass content. Calculations based on density functional theory provide an understanding of occurring diffusion pathways and ionic conductivity of this Li+ ionic conductor.

DOI: 10.1021/acs.chemmater.6b04175

Polarization-switching dynamics in bulk ferroelectrics with isometric and oriented anisometric pores

Authors: R. Khachaturyan, S Zhukov, J Schultheiß, C Galassi, C Reimuth, J Koruza, H von Seggern and Y A Genenko

Highly porous ferroelectric ceramics possess remarkably less polarizability than dense ceramics; instead they display high tunability of various physical properties. Particularly, the shape and orientation of pores as well as the total porosity exhibit a great effect on the polarization-switching dynamics. In the present work, finite-element simulations of the electric-field distributions and related statistical distributions of local switching times are analysed and compared with the switching characteristics of porous lead zirconate titanate ceramics, extracted from the experiment by means of the inhomogeneous field mechanism model of polarization switching. Surprisingly, the simulated statistical field-distributions turn out to be virtually independent of the pore-size distribution; however, they are sensitive to the anisometric shape and orientation of the pores. Additionally, they exhibit notable broadening with increasing porosity; an effect confirmed by experimental observations.


LaSrMnO4: Reversible Electrochemical Intercalation of Fluoride Ions in the Context of Fluoride Ion Batteries

Authors: Mohammad Ali Nowroozi, Kerstin Wissel†, Jochen Rohrer, Anji Reddy Munnangi, and Oliver Clemens

This article reports on the investigation of LaSrMnO4 with K2NiF4 type structure for use as an intercalation based high voltage cathode material with high capacity for fluoride ion batteries (FIBs). Charging was performed against PbF2 based anodes and shows that fluoride intercalation proceeds stepwise to form LaSrMnO4F and LaSrMnO4F2–x. Ex-situ X-ray diffraction experiments were recorded for different cutoff voltages for a deeper understanding of the charging process, highlighting additional potential of the method to be used to adjust fluorine contents more easily than using conventional fluorination methods. A discharging capacity of approximately 20–25 mAh/g was found, which is ∼4–5 times higher compared to what was reported previously on the discharging of BaFeO2.5/BaFeO2.5F0.5, approaching discharge capacities for conversion based fluoride ion batteries. Density functional theory based calculations confirm the observed potential steps of approximately 1 and 2 V for the first (LaSrMnO4 → LaSrMnO4F) and second (LaSrMnO4F → LaSrMnO4F2–x) intercalation steps against Pb-PbF2, respectively. Additionally, a detailed structure analysis was performed for chemically prepared LaSrMnO4F2–x (x ∼ 0.2), showing strong similarity to the product which is obtained after charging the batteries to voltages above 2 V against Pb-PbF2. It was observed that charging and discharging kinetics as well as coulomb efficiencies are limited for the batteries in the current state, which can be partly assigned to overpotentials arising from the use of conversion based anode composites and the stability of the charged sample toward carbon black and the current collectors. Therefore, the structural stability of LaSrMnO4F2 on the deintercalation of fluoride ions was demonstrated by a galvanostatic discharging to −3 V against Pb-PbF2, which can be used to compensate such overpotentials, resulting in almost complete recovery of fluorine free LaSrMnO4 with a discharge capacity of ∼100 mAh/g. This is the first report showing that selective extraction of fluoride ions from an oxyfluoride matrix is possible, and it highlights that compounds with K2NiF4 type structure can be considered as interesting host lattices for the reversible intercalation/deintercalation of fluoride ions within intercalation based FIBs.

DOI: 10.1021/acs.chemmater.6b05075

Influence of phase transitions and defect associates on the oxygen migration in the ion conductor Na1/2Bi1/2TiO3

Authors: Kai-Christian Meyer and Karsten Albe

Doped or non-stoichiometric Na1/2Bi1/2TiO3 (NBT) exhibits an ion conductivity comparable to yttria stabilized zirconia (YSZ) [M. Li et al., Nat. Mater., 2014, 13, 31–35], with a temperature dependent activation energy. To understand the origin of this temperature dependence we calculated oxygen vacancy migration barriers for three different phases of NBT by means of nudged elastic band calculations within a density functional theory (DFT) approach. We find that for structures with rock-salt ordered A-cations (111-order), the room temperature rhombohedral phase, the intermediate orthorhombic phase and the high temperature tetragonal phase show different migration barriers, decreasing from the rhombohedral to the tetragonal phase. The change in migration barriers from the rhombohedral to tetragonal phase is, however, not large enough to explain the experimentally observed difference. At lower temperatures, the association of oxygen vacancies with either Mg dopants or Bi vacancies increases the activation energy for the migration of oxygen vacancies. Thus, a combination of phase dependent migration barriers and defect association can explain the temperature dependent change in activation energy. Further, when a layered A-cation order (001-order) is present, the oxygen vacancies prefer to be located within the Bi-layer and a fast diffusion along the Bi-layer can occur. Large migration barriers are due to electronic defect states of the migrating oxygen ion.

DOI: 10.1039/c6ta10566a

State transition and electrocaloric effect of BaZrxTi1−xO3: Simulation and experiment

Authors: Yang-Bin Ma, Christian Molin, Vladimir V. Shvartsman, Sylvia Gebhardt, Doru C. Lupascu, Karsten Albe, and Bai-Xiang Xu

We present a systematic study on the relation of the electrocaloric effect (ECE) and the relaxor state transition of BaZrxTi1−xO3 (BZT) using a combination of computer simulation and experiment. The results of canonical and microcanonical lattice-based Monte Carlo simulations with a Ginzburg-Landau-type Hamiltonian are compared with measurements of BaZrxTi1−xO3 (x = 0.12 and 0.2) samples. In particular, we study the ECE at various temperatures, domain patterns by piezoresponse force microscopy at room temperature, and the P-E loops at various temperatures. We find three distinct regimes depending on the Zr-concentration. In the compositional range 0≤x≤0.2, ferroelectric domains are visible, but the ECE peak drops considerably with increasing Zr-concentration. In the range 0.3≤x≤0.7, relaxor features become prominent, and the decrease in the ECE with Zr-concentration is moderate. In the range of high concentrations, x≥0.8, the material is almost nonpolar, and there is no ECE peak visible. Our results reveal that BZT with a Zr-concentration around x=0.12∼0.3 exhibits a relatively large ECE in a wide temperature range at rather low temperature.

DOI: 10.1063/1.4973574

Polarization-Mediated Modulation of Electronic and Transport Properties of Hybrid MoS2–BaTiO3–SrRuO3 Tunnel Junctions

Authors: Tao Li, Pankaj Sharma, Alexey Lipatov, Hyungwoo Lee, Jung-Woo Lee, Mikhail Y. Zhuravlev, Tula R. Paudel, Yuri A. Genenko, Chang-Beom Eom, Evgeny Y. Tsymbal, Alexander Sinitskii, and Alexei Gruverman

Hybrid structures composed of ferroelectric thin films and functional two-dimensional (2D) materials may exhibit unique characteristics and reveal new phenomena due to the cross-interface coupling between their intrinsic properties. In this report, we demonstrate a symbiotic interplay between spontaneous polarization of the ultrathin BaTiO3 ferroelectric film and conductivity of the adjacent molybdenum disulfide (MoS2) layer, a 2D narrow-bandgap semiconductor. Polarization-induced modulation of the electronic properties of MoS2 results in a giant tunneling electroresistance effect in the hybrid MoS2–BaTiO3–SrRuO3 ferroelectric tunnel junctions (FTJs) with an OFF-to-ON resistance ratio as high as 104, a 50-fold increase in comparison with the same type of FTJs with metal electrodes. The effect stems from the reversible accumulation-depletion of the majority carriers in the MoS2 electrode in response to ferroelectric switching, which alters the barrier at the MoS2–BaTiO3 interface. Continuous tunability of resistive states realized via stable sequential domain structures in BaTiO3 adds memristive functionality to the hybrid FTJs. The use of narrow band 2D semiconductors in conjunction with ferroelectric films provides a novel pathway for development of the electronic devices with enhanced performance.

DOI: 10.1021/acs.nanolett.6b04247

Flexocoupling impact on the kinetics of polarization reversal

Authors: Ivan S. Vorotiahin, Anna N. Morozovska, Eugene A. Eliseev, and Yuri A. Genenko

The impact of flexoelectric coupling on polarization reversal and space-charge variation in thin films of ferroelectric semiconductors has been studied theoretically. The relaxation-type Landau-Khalatnikov equation together with the Poisson equation and the theory of elasticity equations have been used to calculate in a self-consistent way the spatial-temporal development of ferroelectric polarization, electric potential, space charge, elastic stresses and strains. The analysis of the obtained results reveals a moderate increase in the flexocoupling influence on the polarization, elastic strain, electric potential, and space-charge development with a decrease in the ferroelectric film thickness. In contrast, the dependence of polarization switching time on the applied electric field is remarkably affected by the flexocoupling strength. The polarization reversal process consists typically of two stages; the first stage has no characteristic time, whereas the second one exhibits a switching time strongly dependent on the applied electric field.

DOI: 10.1103/PhysRevB.95.014104

Departments Award for the best Master Thesis

Our colleague, Ms Leonie Koch, completed her master thesis with distinction and received the Departments Award for the best master thesis with the topic „Computer simulations of ordering effects and dislocation structures in high entropy alloys“.

PhD-graduation in the MM Group

We congratulate our dear colleague Mr. Tobias Brink to his doctoral work on the „Heterogeneities in metallic glasses“.

We thank Mr. Tobias Brink not only for this memorable day with a nice after-defense ceremony but also for all the work he has done within the Materials Modelling group.


Structural origins of the boson peak in metals: From high-entropy alloys to metallic glasses

Authors: Tobias Brink, Leonie Koch and Karsten Albe

The boson peak appears in all amorphous solids and is an excess of vibrational states at low frequencies compared to the phonon spectrum of the corresponding crystal. Until recently, the consensus was that it originated from “defects” in the glass. The nature of these defects is still under discussion, but the picture of regions with locally disturbed short-range order and/or decreased elastic constants has gained some traction. Recently, a different theory was proposed: The boson peak was attributed to the first van Hove singularity of crystal lattices which is only smeared out by the disorder. This new viewpoint assumes that the van Hove singularity is simply shifted by the decreased density of the amorphous state and is therefore not a glass-specific anomaly. In order to resolve this issue, we use computer models of a four-component alloy, alternatively with chemical disorder (high-entropy alloy), structural disorder, and reduced density. Comparison to a reference glass of the same composition reveals that the boson peak consists of additional vibrational modes which can be induced solely by structural disorder. While chemical disorder introduces fluctuations of the elastic constants, we find that those do not lead to sufficient local softening to induce these modes. A boson peak due to a reduction of density could be excluded for the present metallic system.


Anisotropy of self-diffusion in forsterite grain boundaries derived from molecular dynamics simulations

Authors: Johannes Wagner, Omar Adjaoud, Katharina Marquardt and Sandro Jahn

Diffusion rates and associated deformation behaviour in olivine have been subjected to many studies, due to the major abundance of this mineral group in the Earth’s upper mantle. However, grain boundary (GB) transport studies yield controversial results. The relation between transport rate, energy, and geometry of individual GBs is the key to understand transport in aggregates with lattice preferred orientation that favours the presence and/or alignment of specific GBs over random ones in an undeformed rock. In this contribution, we perform classical molecular dynamics simulations of a series of symmetric and one asymmetric tilt GBs of Mg2SiO4 forsterite, ranging from 9.58° to 90° in misorientation and varying surface termination. Our emphasis lies on unravelling structural characteristics of high- and low-angle grain boundaries and how the atomic structure influences grain boundary excess volume and self-diffusion processes. To obtain diffusion rates for different GB geometries, we equilibrate the respective systems at ambient pressure and temperatures from 1900 to 2200 K and trace their evolution for run durations of at least 1000 ps. We then calculate the mean square displacement of the different atomic species within the GB interface to estimate self-diffusion coefficients in the individual systems. Grain boundary diffusion coefficients for Mg, Si and O range from 10−18 to 10−21m3/s, falling in line with extrapolations from lower temperature experimental data. Our data indicate that higher GB excess volumes enable faster diffusion within the GB. Finally, we discuss two types of transport mechanisms that may be distinguished in low- and high-angle GBs.


3D Dislocation Structure Evolution in Strontium Titanate: Spherical Indentation Experiments and MD Simulations

Authors: Farhan Javaid, Alexander Stukowski and Karsten Durst

In the present work, the dislocation structure evolution around and underneath the spherical indentations in (001) oriented single crystalline strontium titanate (STO) was revealed by using an etch-pit technique and molecular dynamics (MD) simulations. The 3D defect structure at various length scales and subsurface depths was resolved with the help of a sequential polishing, etching, and imaging technique. This analysis, combined with load–displacement data, shows that the incipient plasticity (manifested as sudden indenter displacement bursts) is strongly influenced by preexisting dislocations. In the early stage of plastic deformation, the dislocation pile-ups are all aligned in 〈100〉 directions, lying on {110}45 planes, inclined at 45° to the (001) surface. At higher mean contact pressure and larger indentation depth, however, dislocation pile-ups along 〈110〉 directions appear, lying on {110}90 planes, perpendicular to the (100) surface. MD simulations confirm the glide plane nature and provide further insights into the dislocation formation mechanisms by tracing the evolution of the complete dislocation line network as function of indentation depth.


Optimized electrocaloric effect by field reversal: Analytical model

Authors: Yang-Bin Ma, Nikola Novak, Karsten Albe and Bai-Xiang Xu

Applying a negative field on a positively poled ferroelectric sample can enhance the electrocaloric cooling and is a promising method to optimize the electrocaloric cycle. Experimental measurements show that the maximal cooling is not obtained, when the electric field is removed, but reversed to a value corresponding to the shoulder of the P-E loop. This phenomenon cannot be explained if a constant total entropy is assumed under adiabatic conditions. Thus, a direct analysis of entropy changes based on work loss is proposed in this work, which takes the entropy contribution of the irreversible process into account. The optimal reversed field determined by this approach agrees with the experimental observations. This study signifies the importance of considering irreversible process in the electrocaloric cycles.


Flexocoupling impact on size effects of piezoresponse and conductance in mixed-type ferroelectric semiconductors under applied pressure

Authors: Anna N. Morozovska, Eugene A. Eliseev, Yuri A. Genenko, Ivan S. Vorotiahin, Maxim V. Silibin, Ye Cao, Yunseok Kim, Maya D. Glinchuk, and Sergei V. Kalinin

We explore the role of flexoelectric effect in functional properties of nanoscale ferroelectric films with mixed electronic-ionic conductivity. Using a coupled Ginzburg-Landau model, we calculate spontaneous polarization, effective piezoresponse, elastic strain and compliance, carrier concentration, and piezoconductance as a function of thickness and applied pressure. In the absence of flexoelectric coupling, the studied physical quantities manifest well-explored size-induced phase transitions, including transition to paraelectric phase below critical thickness. Similarly, in the absence of external pressure flexoelectric coupling affects properties of these films only weakly. However, the combined effect of flexoelectric coupling and external pressure induces polarizations at the film surfaces, which cause the electric built-in field that destroys the thickness-induced phase transition to paraelectric phase and induces the electretlike state with irreversible spontaneous polarization below critical thickness. Interestingly, the built-in field leads to noticeable increase of the average strain and elastic compliance in this thickness range. We further illustrate that the changes of the electron concentration by several orders of magnitude under positive or negative pressures can lead to the occurrence of high- or low-conductivity states, i.e., the nonvolatile piezoresistive switching, in which the swing can be controlled by the film thickness and flexoelectric coupling. The obtained theoretical results can be of fundamental interest for ferroic systems, and can provide a theoretical model for explanation of a set of recent experimental results on resistive switching and transient polar states in these systems.


Enhanced electrocaloric cooling in ferroelectric single crystals by electric field reversal

Authors: Yang-Bin Ma, Nikola Novak, Jurij Koruza, Tongqing Yang, Karsten Albe and Bai-Xiang Xu

TAn improved thermodynamic cycle is validated in ferroelectric single crystals, where the cooling effect of an electrocaloric refrigerant is enhanced by applying a reversed electric field. In contrast to the conventional adiabatic heating or cooling by on-off cycles of the external electric field, applying a reversed field is significantly improving the cooling efficiency, since the variation in configurational entropy is increased. By comparing results from computer simulations using Monte Carlo algorithms and experiments using direct electrocaloric measurements, we show that the electrocaloric cooling efficiency can be enhanced by more than 20% in standard ferroelectrics and also relaxor ferroelectrics, like Pb(Mg1/3/Nb2/3)0.71Ti0.29O3.


* Geänderte Rufnummern im Fachgebiet Materialmodellierung seit 8. Februar 2016 *

Positive and negative electrocaloric effect in BaTiO3 in the presence of defect dipoles

Authors: Yang-Bin Ma, Anna Grünebohm, Kai-Christian Meyer, Karsten Albe and Bai-Xiang Xu

The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped BaTiO3 is studied by means of lattice-based Monte-Carlo simulations using a Ginzburg-Landau type effective Hamiltonian. Oxygen vacancy-acceptor associates are described by fixed local dipoles with orientation parallel or antiparallel to the external field. By a combination of canonical and microcanonical simulations the ECE is directly evaluated. Our results reveal that in the case of antiparallel defect dipoles the ECE can be positive or negative depending on the dipole density. Moreover, a transition from a negative to positive ECE can be observed when the external field increases. These transitions are due to the delicate interplay of internal and external fields and are explained by the domain structure evolution and related field-induced entropy changes. The results are in good qualitative agreement to those obtained by molecular dynamics simulations employing an ab initio based effective Hamiltonian. Finally, a modified electrocaloric cycle, which makes use of the negative ECE in the presence of defect dipoles, is proposed to enhance the cooling effect.


First-principles calculations on structure and properties of amorphous Li5P4O8N3 (LiPON)

Authors: Sabrina Sicolo, Karsten Albe

The structural, electronic and ion transport properties of an amorphous member of the LiPON family with non-trivial composition and cross-linking are studied by means of electronic structure calculations within Density Functional Theory. By a combination of an evolutionary algorithm followed by simulated annealing and alternatively by a rapid quenching protocol, structural models of disordered Li5P4O8N3 are generated, which are characterized by a local demixing in Li-rich and Li-poor layers. These structures have a composition close to what is found experimentally in thin films and contain all the expected diversely coordinated atoms, namely triply- and doubly-coordinated nitrogens and bridging and non-bridging oxygens. The issue of ionic conductivity is addressed by calculating defect formation energies and migration barriers of neutral and charged point defects. Li+ interstitials are energetically much preferred over vacancies, both when the lithium reservoir is metallic lithium and LiCoO2. The competitive formation of neutral Li interstitials when LiPON is contacted with metallic Li results in the chemical reduction of LiPON and the disruption of the network, as recently observed in experiments.


Visualization and Analysis Strategies for Atomistic Simulations
(book chapter)

Author: A. Stukowski

An important aspect of many molecular dynamics studies is the meaningful visualization of computed atomic configurations and trajectories, often contributing a lot to the understanding of the investigated phenomena. This chapter introduces visualization programs and analysis tools that have been developed for working with the output of classical molecular dynamics and other atomistic simulation models. Basic analysis techniques relevant for nanomechanics problems are described, which help to reveal structural phases, defects, and local deformations in materials. Furthermore, this chapter gives an overview of the dislocation extraction algorithm, which is a computational method for the automated detection and identification of dislocation lines in atomistic crystal models.


Bestes Poster beim Poster Award auf dem „International Symposium on Application of Ferroelectrics ISAF/ECAPD/PFM-2016“ für Ruben Khachaturyan.

Deutsch-Ukrainische Akademische Gesellschaft verleiht
Olena Lenchuk den 1. Platz beim PhD Thesis Presentation Contest

Magnetic invisibility of the magnetically coated type-II superconductor in partially penetrated state

Authors: J. Peña-Roche, Y. A. Genenko and A. Badía-Majós

The magnetic behavior of a cylindrical paramagnet/superconductor heterostructure has been studied by numerical simulations. Relying on the variational statement of the critical state model, our results show that magnetic invisibility is compatible with the partial penetration regime in the superconductor. This result accomplishes previous analytic studies that proved a possible perfect undetectability for the full penetration of magnetic flux. For a given geometry, invisibility may be realized only at a certain magnitude of the applied field. Such value decreases with increasing permeability of the magnetic sheath and eventually collapses to zero. This establishes a condition for obtaining realizable invisibility that extends previous expectations relying either on the full penetration ansatz or on perfect diamagnetism.


Fatigue effect on polarization switching dynamics in polycrystalline bulk ferroelectrics

Authors: S. Zhukov, J. Glaum, H. Kungl, E. Sapper, R. Dittmer, Y. A. Genenko and H. von Seggern

Statistical distribution of switching times is a key information necessary to describe the dynamic response of a polycrystalline bulk ferroelectric to an applied electric field. The Inhomogeneous Field Mechanism (IFM) model offers a useful tool which allows extraction of this information from polarization switching measurements over a large time window. In this paper, the model was further developed to account for the presence of non-switchable regions in fatigued materials. Application of the IFM-analysis to bipolar electric cycling induced fatigue process of various lead-based and lead-free ferroelectric ceramics reveals different scenarios of property degradation. Insight is gained into different underlying fatigue mechanisms inherent to the investigated systems.


Reconciling Local Structure Disorder and the Relaxor State in (Bi1/2Na1/2)TiO3-BaTiO3

Authors: Pedro B. Groszewicz, Melanie Gröting, Hergen Breitzke, Wook Jo, Karsten Albe, Gerd Buntkowsky & Jürgen Rödel

Lead-based relaxor ferroelectrics are key functional materials indispensable for the production of multilayer ceramic capacitors and piezoelectric transducers. Currently there are strong efforts to develop novel environmentally benign lead-free relaxor materials. The structural origins of the relaxor state and the role of composition modifications in these lead-free materials are still not well understood. In the present contribution, the solid-solution (100-x)(Bi1/2Na1/2)TiO3-xBaTiO3 (BNT-xBT), a prototypic lead-free relaxor is studied by the combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, dielectric measurements and ab-initio density functional theory (DFT). For the first time it is shown that the peculiar composition dependence of the EFG distribution width (ΔQISwidth) correlates strongly to the dispersion in dielectric permittivity, a fingerprint of the relaxor state. Significant disorder is found in the local structure of BNT-xBT, as indicated by the analysis of the electric field gradient (EFG) in 23Na 3QMAS NMR spectra. Aided by DFT calculations, this disorder is attributed to a continuous unimodal distribution of octahedral tilting. These results contrast strongly to the previously proposed coexistence of two octahedral tilt systems in BNT-xBT. Based on these results, we propose that considerable octahedral tilt disorder may be a general feature of these oxides and essential for their relaxor properties.


Thermodynamics and kinetics of defects in Li2S

Thermodynamics and kinetics of defects in Li2S

Authors: Ashkan Moradabadi and Payam Kaghazchi

Li2S is the final product of lithiation of sulfur cathodes in lithium-sulfur (Li-S) batteries. In this work, we study formation and diffusion of defects in Li2S. It is found that for a wide range of voltages (referenced to metal Li) between 0.17 V and 2.01 V, positively charged interstitial Li (Li+) is the most favorable defect type with a fixed formation energy of 1.02 eV. The formation energy of negatively charged Li vacancy (VLi-) is also constant, and it is only 0.13 eV higher than that of Li+. For a narrow range of voltages between 0.00 V and 0.17 V, the formation energy of neutral S vacancy is the lowest and it decreases with decreasing the cell voltage. The energy barrier for Li+ diffusion (0.45 eV), which takes place via an exchange mechanism, is 0.18 eV higher than that for VLi- (0.27 eV), which takes place via a single vacancy hopping. Considering formation energies and diffusion barriers, we find that ionic conductivity in Li2S is due to both Li+ and VLi-, but the latter mechanism being slightly more favorable.


Functional Interfaces for Transparent Organic Electronic Devices: Consistent Description of Charge Injection by Combining In Situ XPS and Current Voltage Measurements with Self-Consistent Modeling

Functional Interfaces for Transparent Organic Electronic Devices: Consistent Description of Charge Injection by Combining In Situ XPS and Current Voltage Measurements with Self-Consistent Modeling

Authors: Jürgen Gassmann, Sergey V. Yampolskii, Yuri A. Genenko, Thilo C.G. Reusch, and Andreas Klein

The interface properties between Sn-doped In2O3 (ITO) and the organic semiconductor α-NPD are studied using in situ X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) as well as with in situ current–voltage analysis in combination transport simulations using a self-consistent mean field model. In particular, ITO is sputtered onto α-NPD as required for transparent or inverted organic light-emitting diodes. We identify deposition conditions, which leave the organic molecules intact. The barrier heights determined by XPS/UPS for the inverted interfaces between undoped and doped α-NPD and ITO are 1.0 and 1.1 eV, respectively. These are in good agreement with barrier heights extracted from current–voltage simulations if the band width of the highest occupied molecular orbital (HOMO) is taken into account. The HOMO bandwidth determined by UPS is σUPS = 0.22 eV and that derived from simulations is σsim = 0.23 eV.


Interfaces and interphases in nanoglasses: Surface segregation effects and their implications on structural properties

Interfaces and interphases in nanoglasses: Surface segregation effects and their implications on structural properties

Authors: Omar Adjaoud, Karsten Albe

Metallic nanoglasses can be prepared by cold compaction of amorphous nanoparticles initially condensed in inert gas atmosphere. Experimentally, it has been found that a characteristic feature of nanoglasses is the occurrence of atomic density variations within the microstructure, that cannot be explained by interface induced topological variations in a chemically homogeneous material. Here we present molecular dynamics simulations, which reveal that compositional variations between glass-glass interfaces and the volume material can result from surface segregation effects already present in the primary particles. By comparing results for Pd80Si20 and Cu64Zr36 metallic glasses, we show that amorphous nanoparticles install an inhomogenous elemental equilibrium distribution in the gas phase before they undergo the glass transition into the solid state. A detailed analysis of planar interfaces generated by merging chemically equilibrated surfaces shows that glass-glass interfaces can be understood as interphases of different composition and short-range order, where the local topology, free volume and local composition are intimately linked.


Manipulating dislocation nucleation and shear resistance of bimetal interfaces by atomic steps

Manipulating dislocation nucleation and shear resistance of bimetal interfaces by atomic steps

Authors: R.F. Zhanga, I.J. Beyerlein, S.J. Zheng, S.H. Zhang, A. Stukowski, T.C. Germann

By means of atomistic simulations and interface dislocation theory, the mechanism of dislocation nucleation and shear resistance of various stepped fcc/bcc interfaces are comparatively studied using the Kurdjumov-Sachs (KS) Cu/Nb interface as a prototype. It is found that the introduction of atomic steps at the flat Cu{111}/{110}Nb KS interface does not change the most preferred slip systems, but influences the nucleation sites at the interface during tension loading, indicating that the flat and stepped interfaces possesses comparable energetic barriers for dislocation nucleation. During shear loading, the steps may significantly enhance the resistance to interface sliding by propagating partial dislocations that facilitate the emission and growth of parallel twins via cross slip. When the parallel twins are not favored or are hindered, the interface sliding will dominate in a “climbing peak-to-valley” manner. These results provide an effective pathway to solve the trade-off dilemma between dislocation nucleation and interface sliding by appropriately manipulating atomic steps at the flat interface in the design of high-strength metallic materials.


Influence of Crystalline Nanoprecipitates on Shear-Band Propagation in Cu-Zr-Based MetallicGlasses

Influence of Crystalline Nanoprecipitates on Shear-Band Propagation in Cu-Zr-Based Metallic Glasses

Authors: Tobias Brink, Martin Peterlechner, Harald Rösner, Karsten Albe and Gerhard Wilde

The interaction of shear bands with crystalline nanoprecipitates in Cu-Zr-based metallic glasses is investigated by a combination of high-resolution TEM imaging and molecular-dynamics computer simulations. Our results reveal different interaction mechanisms: Shear bands can dissolve precipitates, can wrap around crystalline obstacles, or can be blocked depending on the size and density of the precipitates. If the crystalline phase has a low yield strength, we also observe slip transfer through the precipitate. Based on the computational results and experimental findings, a qualitative mechanism map is proposed that categorizes the various processes as a function of the critical stress for dislocation nucleation, precipitate size, and distance.


Minimum energy path for the nucleation of misfit dislocations in Ge/Si(0 0 1) heteroepitaxy

Minimum energy path for the nucleation of misfit dislocations in Ge/Si(0 0 1) heteroepitaxy

Authors: O. Trushin, E. Maras, A. Stukowski, E. Granato, S. C. Ying, H Jónsson and T. Ala-Nissila

A possible mechanism for the formation of a 90° misfit dislocation at the Ge/Si(001) interface through homogeneous nucleation is identified from atomic scale calculations where a minimum energy path connecting the coherent epitaxial state and a final state with a 90° misfit dislocation is found using the nudged elastic band method. The initial path is generated using a repulsive bias activation procedure in a model system including 75 000 atoms. The energy along the path exhibits two maxima in the energy. The first maximum occurs as a 60° dislocation nucleates. The intermediate minimum corresponds to an extended 60° dislocation. The subsequent energy maximum occurs as a second 60° dislocation nucleates in a complementary, mirror glide plane, simultaneously starting from the surface and from the first 60° dislocation. The activation energy of the nucleation of the second dislocation is 30% lower than that of the first one showing that the formation of the second 60° dislocation is aided by the presence of the first one. The simulations represent a step towards unraveling the formation mechanism of 90° dislocations, an important issue in the design of growth procedures for strain released Ge overlayers on Si(1 0 0) surfaces, and more generally illustrate an approach that can be used to gain insight into the mechanism of complex nucleation paths of extended defects in solids.

DOI: 10.1088/0965-0393/24/3/035007

Interplay of dislocation-based plasticity and phase transformation during Si nanoindentation

Authors: Zhibo Zhang, Alexander Stukowski, Herbert M. Urbassek

Nanoindentation into single-crystalline Si is modeled by molecular dynamics simulation using a modified Tersoff potential. We observe that the high stress produced during indentation leads to three processes occurring consecutively in the substrate: (i) phase transformation of the original cubic diamond (cd) to the bct5 phase; (ii) generation of dislocations; and (iii) amorphization. The bct5 phase develops along {1 1 1} planes of the cd phase; when these meet, the enclosed volume of cd phase transforms to bct5. The particular role played by a stable tetrahedral structure formed by bct5 {1 1 1} planes and {1 1 1} intrinsic stacking faults in the cd structure is highlighted. The phase transformation to bct5 is partially reversed when dislocations nucleate in the cd phase and locally relieve stresses. The generation and reactions of the uncommon dislocations 1/4〈111〉 and 1/3〈112〉 are discussed.

DOI: 10.1016/j.commatsci.2016.03.039

Compositional and electrical properties of line and planar defects in Cu(In,Ga)Se2 thin films for solar cells – a review

Authors: Daniel Abou-Ras, Sebastian S. Schmidt, Norbert Schäfer, Jaison Kavalakkatt, Thorsten Rissom, Thomas Unold, Thomas Kirchartz, Ekin Simsek Sanli, Peter A. van Aken, Quentin M. Ramasse, Hans-Joachim Kleebe, Doron Azulay, Isaac Balberg, Oded Millo, Oana Cojocaru-Mirédin, Daniel Barragan-Yani, Karsten Albe, Jakob Haarstrich and Carsten Ronning

The present review gives an overview of the various reports on properties of line and planar defects in Cu(In,Ga)(S,Se)2 thin films for high-efficiency solar cells. We report results from various analysis techniques applied to characterize these defects at different length scales, which allow for drawing a consistent picture on structural and electronic defect properties. A key finding is atomic reconstruction detected at line and planar defects, which may be one mechanism to reduce excess charge densities and to relax deep-defect states from midgap to shallow energy levels. On the other hand, nonradiative Shockley–Read–Hall recombination is still enhanced with respect to defect-free grain interiors, which is correlated with substantial reduction of luminescence intensities. Comparison of the microscopic electrical properties of planar defects in Cu(In,Ga)(S,Se)2 thin films with two-dimensional device simulations suggest that these defects are one origin of the reduced open-circuit voltage of the photovoltaic devices.

DOI: 10.1002/pssr.201510440

Dislocation evolution and peak spall strengths in single crystal and nanocrystalline Cu

Authors: Karoon Mackenchery, Ramakrishna R. Valisetty, Raju R. Namburu, Alexander Stukowski, Arunachalam M. Rajendran and Avinash M. Dongare

The dynamic evolution and interaction of defects under the conditions of shock loading in single crystal and nanocrystalline Cu are investigated using a series of large-scale molecular dynamics simulations for an impact velocity of 1km/s. Four stages of defect evolution are identified during shock simulations that result in deformation and failure. These stages correspond to: the initial shock compression (I); the propagation of the compression wave (II); the propagation and interaction of the reflected tensile wave (III); and the nucleation, growth, and coalescence of voids (IV). The effect of the microstructure on the evolution of defect densities during these four stages is characterized and quantified for single crystal Cu as well as nanocrystalline Cu with an average grain size of 6nm, 10nm, 13nm, 16nm, 20nm, and 30nm. The evolution of twin densities during the shock propagation is observed to vary with the grain size of the system and affects the spall strength of the metal. The grain sizes of 6nm and 16nm are observed to have peak values for the twin densities and a spall strength that is comparable with the single crystal Cu.

Influence of microstructure on the cutting behaviour of silicon

Authors: Saurav Goel, Andrii Kovalchenko, Alexander Stukowski, Graham Cross

We use molecular dynamics simulation to study the mechanisms of plasticity during cutting of monocrystalline and polycrystalline silicon. Three scenarios are considered: (i) cutting a single crystal silicon workpiece with a single crystal diamond tool, (ii) cutting a polysilicon workpiece with a single crystal diamond tool, and (iii) cutting a single crystal silicon workpiece with a polycrystalline diamond tool. A long-range analytical bond order potential is used in the simulations, providing a more accurate picture of the atomic-scale mechanisms of brittle fracture, ductile plasticity, and structural changes in silicon. The MD simulation results show a unique phenomenon of brittle cracking typically inclined at an angle of 45°–55° to the cut surface, leading to the formation of periodic arrays of nanogrooves in monocrystalline silicon, which is a new insight into previously published results. Furthermore, during cutting, silicon is found to undergo solid-state directional amorphisation without prior Si–I to Si-II (beta tin) transformation, which is in direct contrast to many previously published MD studies on this topic. Our simulations also predict that the propensity for amorphisation is significantly higher in single crystal silicon than in polysilicon, signifying that grain boundaries eases the material removal process.

Effect of texturing on polarization switching dynamics in ferroelectric ceramics

Authors: Sergey Zhukov, Yuri A. Genenko, Jurij Koruza, Jan Schultheiß, Heinz von Seggern, Wataru Sakamoto, Hiroki Ichikawa, Tatsuro Murata, Koichiro Hayashi and Toshinobu Yogo

Highly (100),(001)-oriented (Ba0.85Ca0.15)TiO3 (BCT) lead-free piezoelectric ceramics were fabricated by the reactive templated grain growth method using a mixture of plate-like CaTiO3 and BaTiO3 particles. Piezoelectric properties of the ceramics with a high degree of texture were found to be considerably enhanced compared with the BCT ceramics with a low degree of texture. With increasing the Lotgering factor from 26% up to 94%, the piezoelectric properties develop towards the properties of a single crystal. The dynamics of polarization switching was studied over a broad time domain of 8 orders of magnitude and was found to strongly depend on the degree of orientation of the ceramics. Samples with a high degree of texture exhibited 2–3 orders of magnitude faster polarization switching, as compared with the ones with a low degree of texture. This was rationalized by means of the Inhomogeneous Field Mechanism model as a result of the narrower statistical distribution of the local electric field values in textured media, which promotes a more coherent switching process. The extracted microscopic parameters of switching revealed a decrease of the critical nucleus energy in systems with a high degree of texture providing more favorable switching conditions related to the enhanced ferroelectric properties of the textured material.

Magnetic structures of the low temperature phase of Mn3(VO4)2 – towards understanding magnetic ordering between adjacent Kagomé layers

Authors: Oliver Clemens, Jochen Rohrer, Gwilherm Nénert

In this article we report on a detailed analysis of the magnetic structures of the magnetic phases of the low temperature (lt-) phase of Mn3(VO4)2 (vMn3V2O8) with a Kagomé staircase structure determined by means of powder neutron diffraction. Two magnetic transitions were found at ∼25 K (HT1 phase, Cmc’a’) and ∼17 K (LT1 phase, Pmc’a’), in excellent agreement with previous reports. The LT1 phase is characterized by commensurate magnetic ordering of the magnetic moments on two magnetic sites of the Mn1a/b (2a +2 d) and Mn2 (8i) ions of the nuclear structure (where for the latter site two different overall orientations of magnetic moments within the ab-plane (Mn2a and Mn2b) can be distinguished. This results in mainly antiferromagnetic interactions between edge-sharing Mn-octahedra within the Kagomé planes. The HT1 phase is characterised by strong spin frustration resulting from the loss of ordering of the magnetic moments of Mn2a/b ions along the b-axis. Both magnetic structures are in agreement with the previously reported ferrimagnetic properties of lt-Mn3(VO4)2 and shed light on the magnetic phase diagram of the compound reported previously. The magnetic structures are discussed with respect to superexchange interaction pathways within the Kagomé layers, which appear to be predominantly antiferromagnetic. The magnetic structures of Mn3(VO4)2 are different compared to those reported for Ni3(VO4)2 and Co3(VO4)2 and represent an unique commensurate way out of spin frustration for compounds with strong antiferromagnetic superexchange interactions within the Kagomé layers. Additionally, we derive a superexchange model, which will be called redox-mediated M–M’(d0)–M superexchange and which can help to understand the exclusively ferromagnetic ordering of adjacent Kagomé layers found only for lt-Mn3(VO4)2.!divAbstract

Atomistic modelling of zirconium and silicon segregation at twist and tilt grain boundaries in molybdenum

Authors: Olena Lenchuk, Jochen Rohrer, Karsten Albe

We investigate the influence of Zr and Si segregation on the cohesive strength of grain boundaries (GBs) in molybdenum using density functional theory calculations. A tilt Sigma5(310)[001] and twist Sigma5[001] GB in bicrystal geometry are chosen as structural models. We determine the site preference of Zr and Si for segregation in these GBs and define the segregation energy. We quantify the effect of solutes on the stability of the GBs against brittle fracture by means of the Griffith criterion (work of separation). Additionally, the intrinsic bond strength of the GB containing a solute is quantified by means of the theoretical strength. The results show that Zr and Si tend to segregate at the GBs if the low-energy insertion sites are available. However, the work of separation is decreased by the presence of Zr and Si and even in the presence of oxygen, there is no increase of the Griffith energy. Contributions of strain and chemical energy are analysed in order to explain our findings.

Investigation of partial discharge in piezoelectric ceramics

Investigation of partial discharge in piezoelectric ceramics

Authors: Tian Hanga, Julia Glauma, Yuri A. Genenko, Toan Phung, Mark Hoffman

Electrical partial discharges were studied in different piezoelectric ceramics. Epoxy material with micro sized cavities was also tested and compared to the piezoelectric ceramics. It is found that compared to epoxy, partial discharge (PD) occurs at relatively lower electric fields for piezoelectric ceramics. The PD inception voltage was found to be lower for materials with higher relative permittivity. This indicates that the intensification of the electric field within the defects is the main cause for the differences in inception field observed for epoxy compared to piezoelectric ceramics. Furthermore, phase resolved PD pattern analysis was carried out for all materials at elevated electric fields. A broad distribution of the discharge event was observed for both epoxy and hard PZT samples, whereas for soft PZT discharge occurs concentrated at electric fields slightly above the coercive field. This intensification of PDs close to the coercive field suggests that PDs may be enhanced due to an increase of the internal field and electron emission rate induced by the domain switching process.

Polarization dynamics variation across the temperature- and composition-driven phase transitions in the lead-free

Polarization dynamics variation across the temperature- and composition-driven phase transitions in the lead-free Ba(Zr02Ti0.8)O32x(Ba0.7Ca0.3)TiO3 ferroelectrics

Authors: Sergey Zhukov, Matias Acosta, Yuri A. Genenko and Heinz von Seggern

The method of thermally stimulated depolarization currents (TSDC) and polarization switching experiments over a large field, time, and temperature regime are used to refine the controversial phase diagram of Ba(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3 and comprehend its relation to ferroelectric and piezoelectric properties. TSDC results suggest the existence of three ferroelectric phases for the composition range of 0.30 ≤ x ≤ 0.60, which can be assigned to the rhombohedral (R), presumably orthorhombic (O), and tetragonal (T) symmetries. Spontaneous polarization is maximal all over the entire intermediate phase region, where the activation barrier for polarization switching is small, not just at R-O or O-T boundaries as might be deduced from previous observations.

Stabilization of the y-Sn phase in tin nanoparticles and nanowires

Stabilization of the γ-Sn phase in tin nanoparticles and nanowires

Authors: N. G. Hörmann, A. Gross, J. Rohrer and P. Kaghazchi

Structures of Sn nanoparticles and nanowires are studied using density functional theory in conjunction with thermodynamic considerations. Besides the low-temperature α and room-temperature β phases, the high-temperature γ phase is considered. Results show that at ambient temperatures for sizes smaller than 50 nm, metallic β- and γ-Sn nanoparticles are more stable than semimetallic α-Sn ones because of their lower surface energies. Moreover, very small Sn nanostructures, exemplified by nanowires, are expected to exhibit the γ phase even at 0 K.

Finite-element simulations of hysteretic alternating current losses in a magnetically coated superconducting tubular wire subject to an oscillating transverse magnetic field

Authors: Y. A. Genenko, H. Rauh and S. Kurdi

Numerical simulations of hysteretic ac losses in a tubular superconductor/paramagnet heterostructure subject to an oscillating transverse magnetic field are performed within the quasistatic approach, calling upon the COMSOL finite-element software package and exploiting magnetostaticelectrostatic analogues. It is shown that one-sided magnetic shielding of a thin, type-II superconducting tube by a coaxial paramagnetic support results in a slight increase of hysteretic ac losses as compared to those for a vacuum environment, when the support is placed inside; a spectacular shielding effect with a possible reduction of hysteretic ac losses by orders of magnitude, however, ensues, depending on the magnetic permeability and the amplitude of the applied magnetic field, when the support is placed outside.


On the origin of anisotropic lithiation of silicon

On the origin of anisotropic lithiation of silicon

Authors: Jochen Rohrer, Ashkan Moradabadi, Karsten Albe, Payam Kaghazchi

Silicon has the highest known theoretical capacity (∼4140 mAhg−1) to store lithium. Among different forms of Si, Si nanowires (SiNWs) are the most promising candidates for the next-generation lithium-ion batteries. Lithiation of SiNWs is a complex process, which is not very well understood. Here, we present density functional theory calculations on Li incorporation in SiNWs using surface and interface geometries. Our results show that, initially, Li intercalation proceeds through Si(110) facets, leading to the formation of an amorphous Li2Si shell. For interfaces between the lithiated (amorphous Li2Si) shell and unlithiated (pristine c-Si) core region we find that the Li intercalation barriers are independent of the actual interface orientation, while interface energies show an orientation dependence similar to surface energies. In particular, a-Li2Si/c-Si(111) is most favorable while a-Li2Si/c-Si(110) is least favorable. Since high-energy interfaces typically show a higher mobility than low-energy interfaces, the experimentally observed anisotropic swelling of SiNWs can be understood on the basis of interface energetics.


Lattice-based Monte Carlo simulations of the electrocaloric effect in ferroelectrics and relaxor ferroelectrics

Authors: Yang-Bin Ma, Karsten Albe, and Bai-Xiang Xu

Canonical and microcanonical Monte Carlo simulations are carried out to study the electrocaloric effect (ECE) in ferroelectrics and relaxor ferroelectrics (RFEs) by direct computation of field-induced temperature variations at the ferroelectric-to-paraelectric phase transition and the nonergodic-to-ergodic state transformation. A lattice-based Hamiltonian is introduced, which includes a thermal energy, a Landau-type term, a dipole-dipole interaction energy, a gradient term representing the domain-wall energy, and an electrostatic energy contribution describing the coupling to external and random fields. The model is first parametrized and studied for the case of BaTiO3. Then, the ECE in RFEs is investigated, with particular focus on the influence of random fields and domain-wall energies. If the strength or the density of the random fields increases, the ECE peak shifts to a lower temperature but the temperature variation is reduced. On the contrary, if the domain-wall energy increases, the peak shifts to a higher temperature and the ECE becomes stronger. In RFEs, the ECE is maximum at the freezing temperature where the nonergodic-to-ergodic transition takes place. Our results imply that the presence of random fields reduces the entropy variation in an ECE cycle by pinning local polarization.

Anomalous compliance and early yielding of nanoporous gold

Authors: Bao-Nam Dinh Ngô, Alexander Stukowski, Nadiia Mameka, Jürgen Markmann, Karsten Albe, Jörg Weissmüller

We present a study of the elastic and plastic behavior of nanoporous gold in compression, focusing on molecular dynamics simulation and inspecting experimental data for verification. Both approaches agree on an anomalously high elastic compliance in the early stages of deformation, along with a quasi immediate onset of plastic yielding even at the smallest load. Already before the first loading, the material undergoes spontaneous plastic deformation under the action of the capillary forces, requiring no external load. Plastic deformation under compressive load is accompanied by dislocation storage and dislocation interaction, along with strong strain hardening. Dislocation-starvation scenarios are not supported by our results. The stiffness increases during deformation, but never approaches the prediction by the relevant Gibson–Ashby scaling law. Microstructural disorder affects the plastic deformation behavior and surface excess elasticity might modify elastic response, yet we relate the anomalous compliance and the immediate yield onset to an atomistic origin: the large surface-induced prestress induces elastic shear that brings some regions in the material close to the shear instability of the generalized stacking fault energy curve. These regions are elastically highly compliant and plastically weak.

Publikation Brink, Sopu, Albe

Solid-state amorphization of Cu nanolayers embedded in a Cu64Zr36 glass

Authors: Tobias Brink*, Daniel Şopu†, and Karsten Albe

Solid-state amorphization of crystalline copper nanolayers embedded in a Cu64Zr36 metallic glass is studied by molecular dynamics simulations for different orientations of the crystalline layer. We show that solid-state amorphization is driven by a reduction of interface energy, which compensates the bulk excess energy of the amorphous nanolayer with respect to the crystalline phase up to a critical layer thickness. A simple thermodynamic model is derived, which describes the simulation results in terms of orientation-dependent interface energies. Detailed analysis reveals the structure of the amorphous nanolayer and allows a comparison to a quenched copper melt, providing further insights into the origin of excess and interface energy.