Royal Society of Chemistry Achary, S.N.; Errandonea, D.; Muñoz, A.; Rodríguez Hernández, P.;Manjón, F.J.; Kishna, P. S. R.; Patwe, S. J.; Grover, V. ; Tyagi, A. (2013) Lattice-dynamic calculations of the phonon modes were performed at the zone center (Γ point) of the BZ. We used a direct force-constant approach (or supercell method) 43 as it is conceptually simple. These calculations provide information about the symmetry of the modes and their polarization vectors which allowed us to identify the irreducible representations and the character of the phonon modes at the Γ point. However, this method did not include the contribution from dipole-dipole interactions resulting the LO-TO splitting of infrared-active modes, due to the non-analyticity of this term at the Γ point. Results and discussion X-ray and neutron diffraction studies at ambient conditionsThe powder XRD patterns of the three phases of BiPO 4 , namely BiPO4-I, II and III (shown in Fig. 1

The high-pressure crystal structure, lattice-vibrations, and electronic band structure of BiSbO 4 were studied by ab initio simulations. We also performed Raman spectroscopy, infrared spectroscopy, and diffuse-reflectance measurements, as well as synchrotron powder x-ray diffraction. High-pressure x-ray diffraction measurements show that the crystal structure of BiSbO 4 remains stable up to at least 70 GPa, unlike other known MTO 4 -type ternary oxides. These experiments also give information on the pressure dependence of the unit-cell parameters. Calculations properly describe the crystal structure of BiSbO 4 and the changes induced by pressure on it. They also predict a possible high-pressure phase. A roomtemperature pressure-volume equation of state is determined and the effect of pressure on the coordination polyhedron of Bi and Sb is discussed. Raman-and infrared-active phonons have been measured and calculated. In particular, calculations provide assignments for all the vibrational modes as well as their pressure dependence. In addition, the band structure and electronic density of states under pressure were also calculated. The calculations combined with the optical measurements allow us to conclude that BiSbO 4 is an indirect-gap semiconductor, with an electronic band gap of 2.9(1) eV. Finally, the isothermal compressibility tensor for BiSbO 4 is given at 1.8 GPa. The experimental (theoretical) data revealed that the direction of maximum compressibility is in the (0 1 0) plane at approximately 33º (38º) to the c-axis and 47º (42º) to the a-axis. The reliability of the reported results is supported by the consistency between experiments and calculations.

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In this manuscript we report crystal structure of a new complex binary phosphate K2Ce(4+)(PO4)2 in K2O-P2O5-CeO2 system prepared by solid state reaction at moderate temperature conditions. The prepared material was characterized by powder X-ray diffraction using lab source and synchrotron radiation as well as thermal analyses, Raman scattering, FTIR, and X-ray photoelectron spectroscopic studies. The crystal structure of the compound has been determined from powder XRD data by ab initio structure solution in direct space followed by Rietveld refinements. K2Ce(PO4)2 crystallizes in a monoclinic (P21/n) lattice with unit cell parameters: a = 9.1060(4), b = 10.8160(5), c = 7.6263(4) Å, β = 111.155(2)°, V = 700.50(6) Å(3). The unit cell contains two distinguishable PO4 tetrahedra and one CeO8 distorted square anti-prism. Raman spectroscopy confirmed the presence of isolated PO4(3-) groups in the structure. These PO4 tetrahedra are connected to one CeO8 polyhedra by sharing one edge and three other CeO8 polyhedra by sharing corners to form the three dimensional structure and empty channels parallel to a-axis. The channels are occupied by two crystallographically distinguishable K(+) ions which maintain the charge neutrality. Contrast to the earlier reported composition K4Ce2P4O15, this study revealed the composition in actual is K4Ce2P4O16 with Ce in 4+ oxidation state and is also supported by X-ray photoelectron spectroscopic and X-ray absorption near edge structure studies. Differential scanning calorimetric studies revealed a structural transition around 525 °C which reverts on cooling with a large thermal hysteresis. At higher temperature it undergoes a loss of oxygen atom and subsequently loss of phosphorus as P2O5. These thermal effects are also supported by in situ high temperature XRD studies. Finally the crystal chemistry of complex phosphates with tetravalent cations is also discussed.

The high-pressure behavior of the crystalline structure FeVO has been studied by means of X-ray diffraction using a diamond-anvil cell and first-principles calculations. The experiments were carried out up to a pressure of 12.3 GPa, until now the highest pressure reached to study an FeVO compound. High-pressure X-ray diffraction measurements show that the triclinic P1̅ (FeVO-I) phase remains stable up to ≈3 GPa; then a first-order phase transition to a new monoclinic polymorph of FeVO (FeVO-II') with space group C2/ m is observed, having an α-MnMoO-type structure. A second first-order phase transition is observed around 5 GPa toward the monoclinic ( P2/ c) wolframite-type FeVO-IV structure, which is stable up to 12.3 GPa in coexistence with FeVO-II'. The unit cell volume reductions for the first and second phase transitions are Δ V = -8.5% and -13.1%. It was observed that phase transitions are irreversible and both high-pressure phases remain stable once the pressure is released. Calculations were performed at the level of the generalized gradient approximation plus Hubbard correction (GGA+ U) and with the hybrid Heyd-Scuseria-Ernzerhof (HSE06) exchange-correlation functional in order to have a good representation of the pressure behavior of FeVO. We found that theoretical results follow the pressure evolution of structural parameters of FeVO, in good agreement with the experimental results. Also, we analyze FeVO-II (orthorhombic Cmcm, CrVO-type structure) and -III (orthorhombic Pbcn, α-PbO-type structure) phases and compare our results with the literature. Going beyond the experimental results, we study some possible post-wolframite phases reported for other compounds and we found a phase transition for FeVO-IV to raspite (monoclinic P2/ c) type structure (FeVO-V) at 36 GPa (Δ V = -8.1%) and a further phase transition to the AgMnO-type (monoclinic P2/ c) structure (FeVO-VI) at 66.5 GPa (Δ V = -3.7%), similar to the phase transition sequence reported for InVO.