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

Abstract

In this manuscript we report crystal structure of a new complex binary phosphate K₂Ce⁴⁺(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) ų. The unit cell contains two distinguishable PO₄ tetrahedra and one CeO₈ distorted square anti-prism. Raman spectroscopy confirmed the presence of isolated PO₄³⁻ 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 K₄Ce₂ P₄O₁₆ 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.