Anharmonicity and negative thermal expansion in zirconium tungstate

In order to investigate the anharmonicity of different phonons in the negative thermal expansion material zirconium tungstate, Raman spectroscopic measurements are carried out as a function of temperature over the complete range of stability of α phase. The temperature dependence of phonon frequencies and their line widths are analyzed based on known anharmonicity models. The reported pressure dependence of phonon frequencies is used to separate the ``true anharmonic'' (the explicit temperature effect at constant volume) and ``quasiharmonic'' (the implicit effect caused by volume change) contributions to the total anharmonicities. In addition to the 41-cm^-1 librational mode, several other lattice and bending modes of tungstate ion are found to be strongly anharmonic.