X-ray photoelectron spectroscopy study of mixed-valence effects and charge fluctuation in NaxV2O5

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+)2p3/2 and (V5+)2p3/2 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%.