6533b831fe1ef96bd1299748

RESEARCH PRODUCT

Determination of the high-pressure crystal structure ofBaWO4andPbWO4

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description

We report the results of both angle-dispersive x-ray diffraction and x-ray absorption near-edge structure studies in $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}$ and $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}$ at pressures of up to $56\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $24\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, respectively. $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}$ is found to undergo a pressure-driven phase transition at $7.1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ from the tetragonal scheelite structure (which is stable under normal conditions) to the monoclinic fergusonite structure whereas the same transition takes place in $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}$ at $9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. We observe a second transition to another monoclinic structure which we identify as that of the isostructural phases $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}II$ and $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}III$ (space group $P{2}_{1}∕n$). We have also performed ab initio total-energy calculations which support the stability of this structure at high pressures in both compounds. The theoretical calculations further find that upon increase of pressure the scheelite phases become locally unstable and transform displacively into the fergusonite structure. The fergusonite structure is, however, metastable and can only occur if the transition to the $P{2}_{1}∕n$ phases were kinetically inhibited. Our experiments in $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}$ indicate that it becomes amorphous beyond $47\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$.