  Room-temperature Raman scattering has been measured in lead tungstate up to
17 GPa. We report the pressure dependence of all the Raman modes of the
tetragonal scheelite phase (PbWO4-I, space group I41/a), which is stable at
ambient conditions. Upon compression the Raman spectrum undergoes significant
changes around 6.2 GPa due to the onset of a partial structural phase
transition to the monoclinic PbWO4-III phase (space group P21/n). Further
changes in the spectrum occur at 7.9 GPa, related to a scheelite-to-fergusonite
transition. This transition is observed due to the sluggishness and kinetic
hindrance of the I-to-III transition. Consequently, we found the coexistence of
the scheelite, PbWO4-III, and fergusonite phases from 7.9 to 9 GPa, and of the
last two phases up to 14.6 GPa. Further to the experiments, we have performed
ab initio lattice dynamics calculations which have greatly helped us in
assigning the Raman modes of the three phases and discussing their pressure
dependence.
