Charge transition levels affect various properties of materials, such as electrical conductivity, as well as less obvious properties such as phase stability.

When the Fermi level crosses a charge transition level, the charge (or oxidation state) of the ions changes. This applies to ions from the host materials or or from dilute dopants.

However, despite their importance, experimental data on charge transition levels is very limited. This is mainly due to the lack of methods for their direct measurement.

Two recent publications in Physical Review Materials by Andreas Klein's research group at TU Darmstadt show how this gap can be filled using X-ray photoelectron spectroscopy (XPS).

The approach is based on a manipulation of the Fermi level at the surface of an oxide, which can, for example, be achieved through oxidizing or reducing sample treatments.

In collaboration with Elizabeth C. Dickey's group from Carnegie Mellon University, the first publication reveals that in BaTiO₃ doped with 0.2% vanadium (V), the charge state of the V ions switches from V⁵⁺ to V³⁺ when the Fermi level is raised into the conduction band. This shows that vanadium acts as a donor in BaTiO₃. (S. Chaoudhary et al., DOI: 10.1103/mg9m-hk3n).

The second study compared cobalt and iron in two similar materials: (La,Sr)CoO₃–δ and (La,Sr)FeO₃–δ. The charge transition level of Co²⁺/³⁺ is about 1.1 eV lower than the Fe²⁺/³⁺ level when their energy bands are aligned. Interestingly, this difference closely matches those observed in Bi(Fe,Co)O₃. (Y. Liu et al., DOI: 10.1103/tn7z-thjk)

These results show that XPS measurements enable the systematic study of defect energy levels and their dependence on factors such as the host material, concentration, and temperature. This could lead to generalized doping recipes, a major step towards predicting the effects of doping. Additionally, XPS can be used to experimentally validate defect predictions from density functional theory (DFT).