Synthesis and Structure of Two Phosphate Representatives Formed by Metal Cations in Oxidation State III, α-CaMg2(SO4)3 Analogues

Polycrystals of the BiFe2(PO4)3 compound and Bi1 – xSbхCr2(PO4)3 solid solution, which form a family isostructural to α-CaMg2(SO4)3 (sp. gr. P63/m), have been synthesized from an aqueous solution by the salt co-precipitation method. The samples are characterized by X-ray diffraction, IR spectroscopy, differential thermal analysis, and thermogravimetry, microprobe analysis. The BiFe2(PO4)3 single crystals have been grown by the flux method. The structure consists of polyhedra of three types: PO4 tetrahedra, two face-sharing FeO6 octahedra, and BiO6 octahedra. Isolated PO4 tetrahedra, which share oxygen vertices with doubled FeO6 octahedra, form a three-dimensional [Fe2(PO4)3]3∞ framework, whose voids are occupied by bismuth atoms. Structures of the BiFe2(PO4)3 and NASICON types are compared.