Pressure and temperature-dependent Raman study of ZnWO4

The Raman spectra of monoclinic ZnWO4 (C-2h(4)-space group) are reported ov er wide ranges of pressure (0 - 24 GPa) and temperature (13 -970 K). All 18 Raman active phonons are observed throughout these ranges. Combining the p ressure and temperature Raman data, an identification scheme is suggested w hich makes possible to distinguish the internal modes (vibrations in the oc tahedral WO6 units) from the external ones (pure lattice modes). All phonon s harden with pressure, thus showing normal positive d omega /dP slopes, an d soften with temperature in the region 250-970 K (normal negative d omega /dr slopes). However, two phonons show a change of slope sign below 300 K, resulting in small positive slopes at low temperatures. Since the pressure dependence of these two phonons is normal and linear, it is concluded that. their anomalous, non-linear dependence with temperature is due to anharmon ic (temperature induced) phonon interactions rather that volume expansion. It is found that ZnWO4 remains stable throughout the pressure and temperatu re ranges.