Hydrothermal synthesis, structural characterization, and physical properties of a new mixed valence iron phosphate, SrFe3(PO4)3

A new mixed valence Fe(II/III) phosphate, SrFe3(PO4)(3), has been synthesized hydrothermally and structurally characterized by single-crystal X-ray diffraction, EDAX, magnetic susceptibility, Mossbauer, infrared and diffuse reflectance spectroscopies, DTA, and bond valence sum calculations. The compound crystallizes in the orthorhombic space group mma (No, 74) with a = 10.452(3), b = 13.429(3), c = 6.528(2) Angstrom, V = 916.3(4) Angstrom(3) and Z=4 with R/R-w = 0.0207/0.0307. The structure consists of a complex low-dimensional framework constructed of FeO6 octahedra and PO4 tetrahedra that form 2D channels in which the strontium ions are located. The framework contains two simple building blocks: rigid columns of edge-sharing Fe2P2O14 units and zigzag chains of alternating PO4-FeO6-PO4 units, both of which run parallel to the b direction. This linkage forms intersecting channels which run parallel to the a and b axes. The divalent strontium cations reside within these channels and create openings in the ab plane with dimensions of 6.903(4) and 7.469(4) Angstrom, respectively. These openings can easily accommodate both small and large monovalent or divalent cations, as demonstrated by several cation exchange reactions. The Mossbauer spectral parameters are consistent with the structure and reveal the presence of mixed valent octahedral iron(II) and iron(III) sites in the expected two to one ratio. The quadrupole splittings observed for both iron sites are very large as is expected for the very asymmetric coordination environments found for each iron site. (C) 1999 Academic Press.