HYDROTHERMAL SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF MIXED-VALENCE IRON PHOSPHATES - AFE5(PO4)5(OH).H2O (A = CA, SR)

Mixed-valence iron phosphates of the stoichiometry AFe5(PO4)5(OH).H2O (A = Ca, Sr) have been synthesized hydrothermally and structurally cha racterized by single-crystal X-ray diffraction. Crystal data: CaFe5(PO 4)5-(OH).H2O, triclinic, P1BAR, a = 6.404(1) angstrom, b = 7.866(1) an gstrom, c = 15.694(2) angstrom, alpha = 85.00(1)-degrees, beta = 88.79 (1)-degrees, gamma = 78.22(1)-degrees, V = 770.97(18) angstrom3, Z = 2 , R = 0.034 for 2544 unique reflections; SrFe5(PO4)5(OH).H2O, triclini c, P1BAR, a = 6.4303(2) angstrom, b = 7.8749(8) angstrom, c = 15.894(2 ) angstrom, alpha = 84.75(1)-degrees, beta = 87.96(1)-degrees, gamma = 78.114(9)-degrees, V = 784.16(16) angstrom3, Z = 2, R = 0.0469 for 18 54 unique reflections. The two compounds are isostructural. The comple x framework consists of dimers of corner-sharing and edge-sharing Fe(I II)O6 octahedra joined through Fe(II)O5 trigonal bipyramids by corner- and edge-sharing, and discrete Fe(III)O6 octahedra. Phosphate tetrahed ra further knit together these Fe-O polyhedra by corner-sharing. Schle gel diagrams are used to help describe the arrangement of the next-nea rest neighbors of the coordination polyhedra of Fe. The calcium ions a re 7-coordinated and are located at intersections of the tunnels along the [010] and [110] directions. The structures are further defined by thermogravimetric and Mossbauer studies. Mossbauer spectroscopy confi rms the presence of one Fe(II) and four Fe(III) ions.