We have studied the structural behavior of bismuth phosphate under compression. We performed x-ray powder diffraction measurements up to 31.5 GPa and ab initio calculations. Experiments were carried out on different polymorphs: trigonal (phase I) and monoclinic (phases II and III). Phases I and III, at low pressure (P < 0.2 -0.8 GPa), transform into phase II, which has a monazite-type structure. At room temperature, this polymorph is stable up to 31.5 GPa. Calculations support these findings and predict the occurrence of an additional transition from the monoclinic monazite-type to a tetragonal scheelite-type structure (phase IV). This transition was experimentally found after the simultaneous application of pressure (28 GPa) and temperature (1500 K), suggesting that at room temperature the transition might by hindered by kinetic barriers.Calculations also predict an additional phase transition at 52 GPa, which exceeds the 2 maximum pressure achieved in the experiments. This transition is from phase IV to an orthorhombic barite-type structure (phase V). We also studied the axial and bulk compressibility of BiPO 4 . Room-temperature pressure-volume equations of state are reported. BiPO 4 was found to be more compressible than isomorphic rare-earth phosphates. The discovered phase IV was determined to be the less compressible polymorph of BiPO 4 . On the other hand, the theoretically predicted phase V has a bulk modulus comparable with that of monazite-type BiPO 4 . Finally, the isothermal compressibility tensor for the monazite-type structure is reported at 2.4 GPa showing that the direction of maximum compressibility is in the (0 1 0) plane at approximately 15º (21º) to the a axis for the case of our experimental (theoretical) study.

We have studied the electronic properties at ambient pressure and under high pressure of InVO 4 , InNbO 4 , and InTaO 4 powders, three candidate materials for hydrogen production by means of photocatalytic water splitting using solar energy. A combination of optical absorption and resistivity measurements and band structure calculations have allowed us to determine that these materials are wide band-gap semiconductors with a band-gap energy of 3.62(5), 3.63(5), and 3.79(5) eV for InVO 4 , InNbO 4 , and InTaO 4 , respectively. The last two compounds are indirect band-gap materials, and InVO 4 is a direct band-gap material. The pressure dependence of the band-gap energy and the electrical resistivity have been determined too. In the three compounds, the band gap opens under compression until reaching a critical pressure, where a phase transition occurs. The structural transition triggers a band-gap collapse larger than 1.2 eV in the three materials, being the abrupt decrease in the band-gap energy related to an increase in the pentavalent cation coordination number. The phase transitions also cause changes in the electrical resistivity, which can be correlated with changes induced by pressure in the band structure. An explanation to the reported results is provided based upon ab initio calculations. The conclusions attained are of significance for technological applications of the studied oxides.

: Raman-scattering measurements on NdVO 4 suggest a pressure-induced zircon to monazite phase transition beyond 5.9 GPa. The monazite phase undergoes a second phase transition to a yet unknown phase at 18.1 GPa. Lattice-dynamics calculations well support the experimental findings and predict a possible orthorhombic structure for the post-monazite structure of NdVO 4 .

Polycrystalline samples of La 2 MMnO 6 (M = Co and Mg) were prepared by a combined gel-combustion and high temperature reaction method. The samples were annealed in different oxygen partial pressure (p O2 ) and characterized by powder XRD, SQUID magnetometry, ac impedance spectroscopy, and electron paramagnetic resonance techniques. Monoclinic (P2 1 /n) and rhombohedral (R3) lattices were observed for La 2 CoMnO 6 and La 2 MgMnO 6 , respectively. On annealing in inert atmosphere, La 2 MgMnO 6 partially converted to monoclinic La 2 CoMnO 6 type structure, whereas no structural change was observed in La 2 CoMnO 6 .

P09.-A high-pressure, low temperature, synchrotron single crystal x-ray diffraction study of the ordering of the confined water in microporous AlPO4-54•xH2O