The oxidation states of the dopant elements were studied by X-ray photoelectron spectroscopy (XPS) on a Kratos AXIS Ultra DLD spectrometer. Deconvolution of the spectra was performed in CasaXPS program package. There were two series of experiments: in the first one as-recovered discs were used, in the second one Ar + -beam etching was applied in order to study deeper layers of the material. The spectra from the surface of the samples are shown in Fig. .