Direct determination of the charge transition level of dopants in oxides by x-ray photoelectron spectroscopy

Charge transition levels of dopants directly influence material properties, but experimental numbers are difficult to obtain. It will be demonstrated how x-ray photoelectron spectroscopy can be applied to directly determine charge transition levels of dopants in oxides. Oxidizing and reducing treatments induce changes of the Fermi level at the surface, accompanied by changes of the oxidation state of the dopant. The approach is exemplified for 0.2 mol % V-doped {BaTiO}₃ bulk ceramics, revealing a {V}^5+/3+ charge transition at a Fermi level ≈0.15 eV above the conduction-band minimum. Consequently, V is present as {V}⁵⁺ and acting as a donor in {BaTiO}₃ for most experimental conditions.