LiNiO2, a dynamic Jahn−Teller System

In this joined work with researchers from BASF we studied the dynamics and cooperativity of JT distortions in LiNiO2. While static DFT calculations unequivocally advocate the stability of a monoclinic JT-distorted LNO, this is elusive to experiments: JT distortions are observed on a local scale, but the global crystallographic symmetry is systematically identified as a perfectly undistorted rhombohedral structure isostructural with LiCoO2. Thanks to molecular dynamics simulations, we monitored individual Ni−O bond lengths as a function of time and assessed that JT distortions indeed take place, but reorient dynamically. The statistical correlation analysis revealed that, although some extent of cooperativity might survive within one plane, correlation between different layers is for all practical purposesnonexistent. The continuous reorientation of the JT direction appears on average as though all Ni−O bonds were equal, whereas non-cooperativity between layers prevents the system from undergoing a macroscopic monoclinic shear; the combination of these two effects attributes to LNO the commonly accepted R3̅m symmetry. This result rationalizes the conundrum that, for nearly two decades, juxtaposed the clear JT activity of Ni3+ to the lack of macroscopic monoclinic shear. Quoting John Hasbrouck Van Vleck, who extended the JT theorem to ions in crystals, “it is a great merit of the JT effect that it disappears when not needed”. R3̅m is neither a local minimum nor a saddle point: it is the tip of the Mexican-hat-shaped potential, whereas the reorientation path goes along the warped trough around the maximum. Computational studies of LNO and directly related compounds should therefore avoid using R3̅m as the reference compound and use the true ground state (P21/c)instead; not only is using R3̅m conceptually wrong, but it also leads to non-negligible quantitative errors. R3̅m should still be used in the analysis of diffraction data on LNO.

A comparative, computational study of size effects in nanoglasses and homogeneous bulk glasses

We have investigated the influence of structure size on the mechanical properties of NG and HG nanopillars with 7 nm grain size and diameters ranging from 4.5 up to 54 nm by means of MD simulations. Simulations were done for two different glasses, namely, Cu 64 Zr 36 and Pd 80 Si 20 , as representatives of metal–metal and metal–metalloid systems, respectively. Different from previous studies, the NGs were produced by consolidation rather than Voronoi tessellation and thus have a more realistic microstructure. Our results show a clear difference in the deformation mode between NG and HG for the 36 and 54 nm nanopillars, independent of the glass type. While HG nanopillars exhibit a stress drop and strain localization developing in a shear band, NG nanopillars show ductile deformation behavior with softening at larger engineering strains and deformation by necking. The tensile ductility of about 13–15% found in our simulations is in agreement with 15% plastic strain observed for a 400 nm Sc 75 Fe 25 NG nanopillar using in situ tensile tests in a transmission electron microscope (Wang et al., 2015). HG and NG nanopillars with D = 4.5 nm, where the pillar diameter is smaller than the average grain size d = 7 nm of the NG, deform by necking since the nucleation of STZs on the surface is dominating. In the HG nanopillars with D = 9 and 18 nm, shear banding is more obvious in Cu 64 Zr 36 than in Pd 80 Si 20 . When reducing the NG nanopillar diameter to near or double the average grain size, strain softening appears at the larger engineering strain (>20%). Moreover, structural relaxation after a cyclic loading leads to local recovery, and the stress increases upon reloading. We determined Young’s modulus and yield strengths from stress–strain curves of tensile deformations. We find that both properties are smaller in the NG nanopillars as compared with their homogeneous counterparts in both glasses. From Young’s modulus values and the shear band energy, the critical stress for shear band formation is estimated. We find that the predicted critical stress values are quite consistent with the observed deformation modes.

Molecular dynamics simulations reveal the influence of solute segregation

Dislocation evolution in alloyed and pure nanoporous foams is studied by molecular-dynamics simulations. We find that the yielding behavior is dependent on surface compositions and the corresponding capillary forces, while elastic behavior is dependent on bulk composition. Trends in the yield response of this material as a function of size and surface composition are effectively predicted with a modelthat accounts for surface effects.

Atomistic insights into metal hardening

In contrast with conventional views, ultra-large-scale atomistic simulations show that the staged character of strain hardening of metals originates from crystal rotation, whereas the dislocation behaviours remain the same across all the stages.

New investigations on the exfoliabiltiy of maxenes

Ali Malik investigated the exfoliability of maxenes using DFT calculations. The results were published together with the C. Birkels group. The paper appeared on the front page of Dalton Transactions.