Synthesis and characterization of new strontium iron (II) phosphates, SrFe2 (PO4)(2) and Sr9Fe1.5 (PO4)(7)

New strontium iron(II) orthophosphates, SrFe2(PO4)(2) and Sr9Fe1.5(PO4)(7), were synthesized by the solid state method at 1170 K. They were characteri zed by X-ray powder diffraction, thermal analysis in air, magnetic suscepti bility, and the IR and Mossbauer spectroscopy methods. Mossbauer spectrosco py confirmed the presence of only Fe2+ cations in both compounds. SrFe2(PO4 )(2), crystallizes in a monoclinic system with a = 10.5376(7) Angstrom, b = 6.8529(4) Angstrom, c = 9.3658(6) Angstrom, beta = 109.498(5)degrees. The crystal structure of Sr9Fe1.5(PO4)(7) was refined by the Rietveld method: s pace group R-3m; Z = 3; a = 10.6102(1) Angstrom, c = 19.7135(1) Angstrom wi th R-wp = 3.00% (S = 1.51), R-p = 2.35%, R-B = 3.44%, R-F = 1.72%. A second -harmonic-generation study showed that the structures of these two compound s have a center of symmetry. The crystal structure of Sr9Fe1.5(PO4)(7) is r elated to that of beta -Ca-3(PO4)(2) and alpha -Sr-3(PO4)(2), but is topolo gically more similar to that of beta -Ca-3(PO4)(2). Tetrahedra P1O(4) and s trontium cations in the M3 sites are disordered in the structure of Sr9Fe1. 5(PO4)(7). The octahedral M5 site is fully occupied and the M4 site is quar ter-occupied by iron cations. SrFe2(PO4)(2) and Sr9Fe1.5(PO4)(7) are stable in air up to 750 and 810 K, respectively. (C) 2001 Academic Press.