Evidence of mixed valence Cr $$⁺³$$ + 3 /Cr $$⁺⁴$$ + 4 in Y $$_1-x$$ 1 - x Ca $$_x$$ x CrO $$_3$$ 3 polycrystalline ceramics by X-ray photoelectron spectroscopy

Y $$_{{1-x}}$$ Ca $$_x$$ CrO $$₃$$ polycrystalline ceramics with 0.000 $$\le $$ x $$\le $$ 0.100 compositions were studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results revealed that the $$\langle $$ Cr–O $$\rangle $$ average bond length decreases with increasing Ca content. This fact infers that the charge compensation is through the change from Cr $$⁺³$$ to Cr $$⁺⁴$$ instead of oxygen vacancies. To confirm this supposition, the high-resolution XPS study revealed that as the Ca is introduced in the YCrO $$₃$$ matrix, the Cr 2p $$_{{3/2}}$$ core-level is well fitted at 576.22 and 577.57 eV belonging at Cr $$⁺³$$ and Cr $$⁺⁴$$ core-level, respectively. Furthermore, we found an increase of the valence band XPS spectra at the Fermi energy as the Ca replaces the Y site. This fact suggests an increase of the Cr 3d-state at the Fermi energy, which is confirmed by the XPS valence band spectra and the electrical conductivity measurements.