Optical properties ofα′−NaxV2O5

Abstract

The optical properties of sodium-deficient ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}{\mathrm{Na}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ $(0.85<~x<~1.00)$ single crystals are analyzed in the wide energy range, from 0.012 to 4.5 eV, using ellipsometry, infrared reflectivity, and Raman-scattering techniques. The material remains insulating up to the maximal achieved hole concentration of about 15%. In sodium-deficient samples the optical absorption peak associated to the fundamental electronic gap develops at $\ensuremath{\sim}0.44 \mathrm{eV}.$ It corresponds to the transition between vanadium ${d}_{\mathrm{xy}}$ and the impurity band, which forms in the middle of the pure ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}{\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$ gap. Raman spectra measured with incident photon energy larger than 2 eV show strong resonant behavior, due to the presence of the hole-doping activated optical transitions, peaked at $\ensuremath{\sim}2.8 \mathrm{eV}.$