Angle-dispersive x -ray diffraction (ADXRD) and x -ray absorption near-edge structure (XANES) measurements have been performed on CaWO 4 and SrWO 4 up to pressures of approximately 20 GPa. Both materials display similar behavior in the range of pressures investigated in our experiments. As in the previously reported case of CaWO 4 , under hydrostatic conditions SrWO 4 undergoes a pressure-induced scheelite-to-fergusonite transition around 10 GPa. Our experimental results are compared to those found in the literature and are further supported by ab initio total energy calculations, from which we also predict the instability at larger pressures of the fergusonite phases against an orthorhombic structure with space group Cmca. Finally, a linear relationship between the charge density in the AO 8 polyhedra of ABO 4 scheelite-related structures and their bulk modulus is discussed and used to predict the bulk modulus of other materials, like hafnon.

Abstract:We report the results of both angle-dispersive x -ray diffraction and x -ray absorption near-edge structure studies in BaWO 4 and PbWO 4 at pressures of up to 56 GPa and 24 GPa, respectively. BaWO 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 4 at 9 GPa. We observe a second transition to another monoclinic structure which we identify as that of the isostructural phases BaWO 4 -II and PbWO 4 -III (space group P2 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 1 /n phases were kinetically inhibited. Our experiments in BaWO 4 indicate that it becomes amorphous beyond 47 GPa.

In this paper, we report the angle-dispersive x-ray diffraction data of barite, BaSO 4 , measured in a diamond-anvil cell up to a pressure of 48 GPa, using three different fluid pressure-transmitting media (methanol-ethanol mixture, silicone oil, and He). Our results show that BaSO 4 exhibits a phase transition at pressures that range from 15 to 27 GPa, depending on the pressure media used. This indicates that nonhydrostatic stresses have a crucial role in the high-pressure behavior of this compound. The new high-pressure (HP) phase has been solved and refined from powder data, having an orthorhombic P2 1 2 1 2 1 structure. The pressure dependence of the structural parameters of both room-and HP phases of BaSO 4 is also discussed in light of our theoretical first-principles total-energy calculations. Finally, a comparison between the different equations of state obtained in our experiments is reported.

Abstract:We have studied by means of high-pressure x-ray diffraction the structural stability of Ni 2 Mo 3 N, Co 3 Mo 3 N, and Fe 3 Mo 3 N. We also report ab initio computing modeling of the high-pressure properties of these compounds, Pd 2 Mo 3 N, and Pt 2 Mo 3 N.We have found that the nitrides remain stable in the ambient-pressure cubic structure at least up to 50 GPa and determined their equation of state. All of them have a bulk modulus larger than 300 GPa. Single-crystal elastic constants have been calculated in order to quantify the stiffness of the investigated nitrides. We found that they should have a Vickers hardness similar to that of cubic spinel nitrides like γ-Si 3 N 4 .

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.