High-pressure Raman spectroscopic study of zirconiumtungstate

Abstract

Raman spectroscopic measurements on negative-thermal-expansion (NTE) material zirconium tungstate Zr(WO4)2at 20 K over the complete range of phonon frequencies yield39 out of the predicted 54 optical phonons. The modes are assignedas lattice modes, and translational, librational and internalmodes of the WO4 ion. High-pressure measurements in a diamondanvil 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 exhibitnegative Grüneisen parameter in the cubic phase. In thehigh-pressure orthorhombic phase above 0.3 GPa splitting ofphonon modes is found to be consistent with the lowering of symmetry. Pressure-induced amorphization in this system at2.2±0.3 GPa is argued to arise because a pressure-induceddecomposition of the compound into a mixture of ZrO2 andWO3 is kinetically constrained. The temperature dependence ofspecific heat and thermal expansion coefficient are calculatedand compared with reported results. In contrast to earlier models and calculations, which considered only the phonons below8 meV (64 cm-1) to explain the NTE, it is shown thatmodes much higher than 8 meV also contribute significantly toNTE in this material.