We have studied BaWO 4 under compression at room temperature by means of x-ray diffraction and Raman spectroscopy. When compressed with neon as pressuretransmitting medium (quasi-hydrostatic conditions), we found that BaWO 4 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 BaWO 4 -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 non-hydrostaticity may be responsible for previously reported rich polymorphism in BaWO 4 . 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 is discussed.2 Finally, the pressure evolution of the Raman modes of different phases is reported and compared with previous studies.

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Angle-dispersive X-ray diffraction measurements have been performed in acanthite, Ag(2)S, up to 18 GPa in order to investigate its high-pressure structural behavior. They have been complemented by ab initio electronic structure calculations. From our experimental data, we have determined that two different high-pressure phase transitions take place at 5 and 10.5 GPa. The first pressure-induced transition is from the initial anti-PbCl(2)-like monoclinic structure (space group P2(1)/n) to an orthorhombic Ag(2)Se-type structure (space group P2(1)2(1)2(1)). The compressibility of the lattice parameters and the equation of state of both phases have been determined. A second phase transition to a P2(1)/n phase has been found, which is a slight modification of the low-pressure structure (Co(2)Si-related structure). The initial monoclinic phase was fully recovered after decompression. Density functional and, in particular, GGA+U calculations present an overall good agreement with the experimental results in terms of the high-pressure sequence, cell parameters, and their evolution with pressure.

We have performed pseudopotential calculations and x-ray-diffraction and x-ray-absorption measurements on the CuGaO 2 delafossite under high pressure. We have completely characterized the structural behavior of the low pressure phase. We have found out that the a axis is more compressible than the c axis, and as a consequence the oxygen octahedra defined by the gallium environment tend to become more regular under high pressure. We have determined the internal parameter describing the oxygen position inside the unit cell, and seen that it is nearly constant when pressure is applied. We have observed an irreversible phase transition affecting the copper environment but not the gallium one. Such phase transition begins at 24Ϯ2 GPa and is completed at 28Ϯ2 GPa.