Optical Properties, Electronic Structure and Magnetism of α′-Na x V2O5

The optical properties of sodium-deficient α′-Na _x V₂O₅ (0.85 ≤ x ≤ 1.00) single crystals are analyzed using ellipsometry, and infrared reflectivity techniques. In sodium deficient samples, the optical absorption peak associated to the fundamental electronic gap develops in the middle of the pure α′-NaV₂O₅ gap at ～0.44 eV, and the material remains insulating up to the maximal achieved hole concentration of about 15%. Nonmetallic behavior under hole doping provoked reinterpretation of the α′-NaV₂O₅ optical spectra. We argue that the absorption peak at about 0.9 eV corresponds to the photoionization energy of a large polaron.