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

The cubic phase of zirconium tungstate exhibits a larger number of vibrational Raman modes in the frequency region of the symmetric stretching mode of tungstate ions than permitted by group-theoretical analysis for a perfectly ordered lattice. This suggests the existence of disorder in the tungstate sublattice. The additional modes are identified and assigned on the basis of their relative intensities and temperature dependences of line-widths. Using a real-space model for misoriented tetrahedral ions, the occurrence of four disorder modes is explained. From the evolution of the intensities of different components of the symmetric stretching mode as a function of temperature, the disorder is found to be largely static (frozen-in) in nature. As each tungstate ion carries a net dipole moment, randomly misoriented tetrahedral units can result in a dipole glass, explaining a recently reported glass-like behaviour of thermal conductivity. The proposed disordered configurations of tungstate ions are indeed found to be energetically stable from first-principles simulation studies.