High-pressure Raman spectroscopic study of zirconium tungstate

Raman spectroscopic measurements on negative-thermal-expansion (NTE) material zirconium tungstate Zr(WO₄)₂ at 20 K over the complete range of phonon frequencies yield 39 out of the predicted 54 optical phonons. The modes are assigned as lattice modes, and translational, librational and internal modes of the WO₄ ion. High-pressure measurements in a diamond anvil cell (DAC) have revealed that in addition to the low-frequency rigid-unit modes (RUMs) several other phonons, including the bending modes of the WO₄ ion, also exhibit negative Grüneisen parameter in the cubic phase. In the high-pressure orthorhombic phase above 0.3 GPa splitting of phonon modes is found to be consistent with the lowering of symmetry. Pressure-induced amorphization in this system at 2.2+/-0.3 GPa is argued to arise because a pressure-induced decomposition of the compound into a mixture of ZrO₂ and WO₃ is kinetically constrained. The temperature dependence of specific heat and thermal expansion coefficient are calculated and compared with reported results. In contrast to earlier models and calculations, which considered only the phonons below 8 meV (64 cm^-1) to explain the NTE, it is shown that modes much higher than 8 meV also contribute significantly to NTE in this material.