Journal of physics: condensed matter vol:15 issue:6 pages:L139-L145
date:Katholieke Univ Leuven, Lab Vaste Stoffysica & Magnetisme, B-3001 Heverlee, Belgium; Univ Tokyo, Inst Solid State Phys, Mat Design & Characterizat Lab, Kashiwa, Chiba 2778581, Japan
Polarized Raman spectra of the calcium vanadium oxide bronze beta-Ca0.33V2O5 are measured in the temperature range between 300 and 20 K. The charge ordering phase transition at about 150 K is characterized by the appearance of new Raman-active modes in the spectra, and by anomalies in the electronic background scattering. The high-temperature Raman scattering spectra of beta-Ca0.33V2O5 show an apparent resemblance to those of alpha'-NaV2O5, which suggests that the charge-phonon dynamics of the two compounds are similar. A study of the dynamical properties and a symmetry analysis of the Raman modes show that in the mixed-valence state of beta-Ca-0.33 V2O5 the electrons are delocalized into V-1-O-5-V-3 orbitals. We propose that in the charge ordered state below 150 K the d electrons localize within V-1-V-3 ladders, either in a 'zigzag' fashion as in alpha'-NaV2O5 or in the form of double chains as in gamma-LiV2O5.