Raman scattering study of charge ordering in beta-Ca0.33V2O5

TL;DR

This study uses polarized Raman spectroscopy to investigate charge ordering in beta-Ca0.33V2O5, revealing a phase transition at 150 K characterized by new modes and electronic anomalies, and comparing its charge dynamics to similar compounds.

Contribution

It provides the first detailed Raman analysis of charge ordering in beta-Ca0.33V2O5 and suggests the nature of electron localization below the transition temperature.

Findings

Charge ordering transition at ~150 K identified by new Raman modes.

Electrons are delocalized in V1-O5-V3 orbitals at high temperature.

Electrons localize within V1-V3 ladders in the charge-ordered state.

Abstract

Polarized Raman spectra of the calcium vanadium oxide bronze beta-Ca0.33V2O5 are measured in the temperature range between 300 K and 20 K. The charge ordering phase transition at about 150 K is characterized by the appearance of the 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 are in apparent resemblance with those of alpha'-NaV2O5, which suggests that there is a similar charge-phonon dynamics in both compounds. The study of dynamical properties and the symmetry analysis of the Raman modes show that in the mixed valence state of beta-Ca0.33V2O5 the electrons are delocalized into V1-O5-V3 orbitals. We propose that in the charge ordered state below 150 K the d-electrons localize within V1-V3 ladders, either in "zig-zag" fashion like in alpha'-NaV2O5 or in the forms of the double chains like in gamma-LiV2O5.