Charge-ordering signatures in the optical properties of beta-Na0.33V2O5.

ABSTRACT

Temperature dependent optical spectra are reported for beta-Na0.33V2O5. The sodium ordering transition at T(Na)=240 K and, in particular, the charge ordering transition at T(MI)=136 K strongly influence the optical spectra. The metal-insulator transition at T(MI) leads to the opening of a pseudogap ( variant Planck's over 2pi omega=1700 cm(-1)) and to the appearance of a large number of optical phonons. These observations and the presence of a midinfrared band (typical for low dimensional metals) strongly suggest that the charge carriers in beta-Na0.33V2O5 are small polarons.