Direct determination of the Fe3+/2+ charge-transition level in BaTiO3 and isovalently substituted Ba0.82Ca0.18Ti0.92Zr0.08O3 by x-ray photoelectron spectroscopy

Charge-transition levels of dopants in oxides and other semiconductors are key factors affecting a wide range of material properties. Despite their importance, only very few charge-transition levels are known quantitatively. This work aims to validate the direct experimental determination of charge-transition levels of dopants in oxides by means of x-ray photoelectron spectroscopy (XPS). The approach is used to derive the energy level associated with the Fe3+/2+ transition, which is determined as 2.45±0.05 eV and 2.65±0.05eV above the valence band maximum of BaTiO3 and Ba0.82Ca0.18Ti0.92Zr0.08O3, respectively. The former agrees with thermogravimetric and electric measurements. The results consolidate that XPS is a versatile and reliable technique to experimentally determine charge-transition levels, which can be used to reveal systematic dependencies on concentration, temperature, and host material. It is further demonstrated that high-temperature near-ambient pressure XPS performed at a synchrotron is ideally suited for the determination of charge-transition levels.