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

Abstract

Room-temperature Raman scattering has been measured in barium tungstate $(\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4})$ up to $16\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. We report the pressure dependence of all the Raman active first-order phonons of the tetragonal scheelite phase ($\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}I$, space group $I{4}_{1}∕a$), which is stable at normal conditions. As pressure increases the Raman spectrum undergoes significant changes around $6.9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ due to the onset of the structural phase transition to the monoclinic $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}II$ phase (space group $P{2}_{1}∕n$). This transition is only completed above $9.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. A further change in the spectrum is observed at $7.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ related to a scheelite-to-fergusonite transition. The scheelite, $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}II$, and fergusonite phases coexist up to $9.0\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ due to the sluggishness of the $I\ensuremath{\rightarrow}II$ 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.