Lattice dynamics study of scheelite tungstates under high pressure I.: BaWO4

Room-temperature Raman scattering has been measured in barium tungstate (BaWO4) up to 16 GPa. We report the pressure dependence of all the Raman active first-order phonons of the tetragonal scheelite phase (BaWO4-I, space group I4(1)/a), which is stable at normal conditions. As pressure increases the Raman spectrum undergoes significant changes around 6.9 GPa due to the onset of the structural phase transition to the monoclinic BaWO4-II phase (space group P2(1)/n). This transition is only completed above 9.5 GPa. A further change in the spectrum is observed at 7.5 GPa related to a scheelite-to-fergusonite transition. The scheelite, BaWO4-II, and fergusonite phases coexist up to 9.0 GPa due to the sluggishness of the I -> II phase transition. Further to the experimental study, we have performed ab initio lattice dynamics calculations that have greatly helped us in assigning and discussing the pressure behavior of the observed Raman modes of the three phases.