Evidence of disorder in cubic zirconium tungstate from the temperature dependence ofRaman spectra

Abstract

The cubic phase of zirconium tungstate exhibits a larger number of vibrational Ramanmodes in the frequency region of the symmetric stretching mode of tungstate ions thanpermitted by group-theoretical analysis for a perfectly ordered lattice. This suggeststhe existence of disorder in the tungstate sublattice. The additional modes areidentified and assigned on the basis of their relative intensities and temperaturedependences of line-widths. Using a real-space model for misoriented tetrahedral ions,the occurrence of four disorder modes is explained. From the evolution of theintensities of different components of the symmetric stretching mode as a function oftemperature, the disorder is found to be largely static (frozen-in) in nature. As eachtungstate ion carries a net dipole moment, randomly misoriented tetrahedral unitscan result in a dipole glass, explaining a recently reported glass-like behaviourof thermal conductivity. The proposed disordered configurations of tungstateions are indeed found to be energetically stable from first-principles simulationstudies.