Combined Raman scattering andab initioinvestigation of pressure-induced structural phase transitions in the scintillatorZnWO4

The room-temperature Raman scattering was measured in ${\text{ZnWO}}_{4}$ up to 45 GPa. We report the pressure dependence of all the Raman-active phonons of the low-pressure wolframite phase. As pressure increases additional Raman peaks appear at 30.6 GPa due to the onset of a reversible structural phase transition to a distorted monoclinic $\ensuremath{\beta}$-fergusonite-type phase. The low-pressure and high-pressure phases coexist from 30.6 to 36.5 GPa. In addition to the Raman measurements we also report ab initio total-energy and lattice-dynamics calculations for the two phases. These calculations helped us to determine the crystalline structure of the high-pressure phase and to assign the observed Raman modes in both the wolframite and $\ensuremath{\beta}$-fergusonite phases. Based upon the ab initio calculations we propose the occurrence of a second phase transition at 57.6 GPa from the $\ensuremath{\beta}$-fergusonite phase to an orthorhombic $Cmca$ phase. The pressure evolution of the lattice parameters and the atomic positions of wolframite ${\text{ZnWO}}_{4}$ are also theoretically calculated, and an equation of state reported.