X-ray photoelectron spectroscopy study of mixed-valence effects and charge fluctuation in Na_xV₂O₅

X-ray photoelectron spectra of vanadium 2p states in α^′-NaV₂O₅ and γ-LiV₂O₅, single crystals show distinct splitting of both j=1∕2 and j=3∕2 spin-orbit lines due to the existence of V⁴⁺ and V⁵⁺ ions. In contrast to γ-LiV₂O₅, the α^′-NaV₂O₅ exhibits the disproportional intensities and larger binding energy difference of (V⁴⁺)2p^3∕2 and (V⁵⁺)2p^3∕2 lines, that we attribute to a charge delocalization effect. By increasing the sodium deficiency, both effects reduce in a very good agreement with a model that takes into account the relative abundance of V⁴⁺ and V⁵⁺ ions, and the change of degree of delocalization. We estimate that sodium deficiency of about 15% increases electron localization by about 0.4%.