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 characterized by X-ray powder diffraction, thermal analysis in air, magnetic susceptibility, and the IR and Mössbauer spectroscopy methods. Mössbauer spectroscopy confirmed the presence of only Fe2+ cations in both compounds. SrFe2(PO4)2 crystallizes in a monoclinic system with a=10.5376(7) Å, b=6.8529(4) Å, c=9.3658(6) Å, β= 109.498(5)°. The crystal structure of Sr9Fe1.5(PO4)7 was refined by the Rietveld method: space group R–3m; Z=3; a=10.6102(1) Å, c=19.7135(1) Å with Rwp=3.00% (S=1.51), Rp=2.35%, RB=3.44%, RF=1.72%. A second-harmonic-generation study showed that the structures of these two compounds have a center of symmetry. The crystal structure of Sr9Fe1.5(PO4)7 is related to that of β-Ca3(PO4)2 and α-Sr3(PO4)2, but is topologically more similar to that of β-Ca3(PO4)2. Tetrahedra P1O4 and strontium 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 quarter-occupied by iron cations. SrFe2(PO4)2 and Sr9Fe1.5(PO4)7 are stable in air up to 750 and 810 K, respectively.