Charge ordering signatures in the optical properties of beta-Na0.33V2O5
  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 \tmi leads to the opening of a psuedogap (\hbar\omega = 1700 cm-1), and to the appearance of a large number of optical phonons. These observations, and the presence of a mid-infrared band (typical for low dimensional metals) strongly suggests that the charge carriers in beta-Na0.33V2O5 are small polarons. 
C.Presura, M.Popinciuc, P.H.M. van Loosdrecht, D. van der Marel, M.   Mostovoy, T. Yamauchi, and Y. Ueda