Determination of the high-pressure crystal structure of BaWO{sub 4} and PbWO{sub 4}

We report the results of both angle-dispersive x-ray diffraction and x-ray absorption near-edge structure studies in BaWO{sub 4} and PbWO{sub 4} at pressures of up to 56 GPa and 24 GPa, respectively. BaWO{sub 4} is found to undergo a pressure-driven phase transition at 7.1 GPa from the tetragonal scheelite structure (which is stable under normal conditions) to the monoclinic fergusonite structure whereas the same transition takes place in PbWO{sub 4} at 9 GPa. We observe a second transition to another monoclinic structure which we identify as that of the isostructural phases BaWO{sub 4}-II and PbWO{sub 4}-III (space group P2{sub 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 P2{sub 1}/n phases were kinetically inhibited. Our experiments in BaWO{sub 4} indicate that it becomes amorphous beyond 47 GPa.