Complex high-pressure polymorphism of barium tungstate

We have studied BaWO4 under compression at room temperature by means of x-ray diffraction and Raman spectroscopy. When compressed with neon as a pressure-transmitting medium (quasihydrostatic conditions), we found that BaWO4 transforms from its low-pressure tetragonal structure into a much denser monoclinic structure. This result confirms our previous theoretical prediction based on ab initio calculations that the scheelite to BaWO4-II transition occurs at room temperature if kinetic barriers are suppressed by pressure. However, our experiment without any pressure-transmitting medium has resulted in a phase transition to a completely different structure, suggesting nonhydrostaticity may be responsible for previously reported rich polymorphism in BaWO4. The crystal structure of the low-and high-pressure phases from the quasihydrostatic experiments has been Rietveld refined. Additionally, for the tetragonal phase the effects of pressure on the unit-cell volume and lattice parameters are discussed. Finally, the pressure evolution of the Raman modes of different phases is reported and compared with previous studies.