Optical properties of α′-Na_xV₂O₅

The optical properties of sodium-deficient α′-Na_xV₂O₅ (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 ∼0.44 eV. It corresponds to the transition between vanadium d_xy and the impurity band, which forms in the middle of the pure α′-NaV₂O₅ 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 ∼2.8 eV.