Equations of State and Crystal Structures of KCaPO4, KSrPO4, and K2Ce(PO4)2 under High Pressure: Discovery of a New Polymorph of KCaPO4

We have studied by means of angle-dispersive powder synchrotron X-ray diffraction the structural behavior of KCaPO₄, SrKPO₄, and K₂Ce(PO₄)₂ under high pressure up to 26, 25, and 22 GPa, respectively. For KCaPO₄, we have also accurately determined the crystal structure under ambient conditions, which differs from the structure previously reported. Arguments supporting our structural determination will be discussed. We have found that KCaPO₄ undergoes a reversible phase transition. The onset of the transition is at 5.6 GPa. It involves a symmetry decrease. The low-pressure phase is described by space group P3̅m1 and the high-pressure phase by space group Pnma. For KSrPO₄ and K₂Ce(PO₄)₂, no evidence of phase transitions has been found up to the highest pressure covered by the experiments. For the three compounds, the linear compressibility for the different crystallographic axes and the pressure-volume equation of states are reported and compared with those of other phosphates. The three studied compounds are among the most compressible phosphates. The results of the study improve the knowledge about the high-pressure behavior of complex phosphates.

Temperature dependent structural, vibrational and magnetic properties of K3Gd5(PO4)6

Herein we report the evolution of the crystal structure of K₃Gd₅(PO₄)₆ in the temperature range from 20 K to 1073 K, as observed from variable temperature X-ray diffraction and Raman spectroscopic studies. K₃Gd₅(PO₄)₆ has an open tunnel containing a three dimensional structure built by [Gd₅(PO₄)₆]^3- ions which in turn are formed of PO₄ tetrahedra and GdO_n (n = 8 and 9) polyhedra. The empty tunnels in the structure are occupied by K⁺ ions and maintain charge neutrality in the lattice. Evolution of unit cell parameters with temperature shows a systematic increase with temperature. The average axial thermal expansion coefficients between 20 K and 1073 K are: α_a = 10.6 × 10^-6 K^-1, α_b = 5.5 × 10^-6 K^-1 and α_c = 16.4 × 10^-6 K^-1. The evolution of distortion indices of the various coordination polyhedra with temperature indicates a gradual decrease with increasing temperature, while those of Gd2O₉ and K2O₈ polyhedra show opposite trends. The overall anisotropy of the lattice thermal expansion is found to be controlled largely by the effect of temperature on GdO_n polyhedra and their linkages. Temperature dependent Raman spectroscopic studies indicated that the intensities and wavenumbers of most of the Raman modes decrease continuously with increasing temperature. Anharmonic analyses of Raman modes indicated that the lattice, rigid translation and librational modes have larger contributions towards thermal expansion of K₃Gd₅(PO₄)₆ compared to high frequency internal modes. The temperature and field dependent magnetic measurements indicated no long range ordering down to 2 K and the observed effective magnetic moment per Gd³⁺ ion and the Weiss constant are 7.91 μ_B and 0.38 K, respectively.

Preparation and crystal structure of K2Ce(PO4)2: a new complex phosphate of Ce(iv) having structure with one-dimensional channels

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.

Cs6RE2(PO4)4 (RE = Y and Gd): two new members of the alkali rare-earth double phosphates

Two double phosphates with new structural types were prepared and demonstrated to be suitable hosts for luminescence materials.

Structural, Raman, FT-IR and optical properties of Rb3Y2(PO4)3 and Rb3La(PO4)2 doped with Eu3+ ions

Rb₃La(PO₄)₂ and Rb₃Y₂(PO₄)₃ doped with Eu³⁺ were obtained through two different routes. The synthesis technique influences the optical properties of compounds.