Lattice dynamics study of scheelite tungstates under high pressure II.PbWO4

Abstract

Room-temperature Raman scattering has been measured in lead tungstate up to $17\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. We report the pressure dependence of all the Raman modes of the tetragonal scheelite phase ($\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}I$ or stolzite, space group $I{4}_{1}∕a$), which is stable at ambient conditions. Upon compression the Raman spectrum undergoes significant changes around $6.2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ due to the onset of a partial structural phase transition to the monoclinic $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}III$ phase (space group $P{2}_{1}∕n$). Further changes in the spectrum occur at $7.9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, related to a scheelite-to-fergusonite transition. This transition is observed due to the sluggishness and kinetic hindrance of the $I\ensuremath{\rightarrow}III$ transition. Consequently, we found the coexistence of the scheelite, $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}III$, and fergusonite phases from $7.9\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, and of the last two phases up to $14.6\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. 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. The Raman modes of the free $\mathrm{W}{\mathrm{O}}_{4}$ molecule are discussed.