CRYSTAL-STRUCTURES AND CHARACTERIZATION OF CA9FE(PO4)(7) AND CA9FEH0.9(PO4)(7)

Whitlockite-like compounds s- and o-Ca9Fe(PO4)(7) and r-Ca-9 FeH0.9(PO 4)(7) were prepared and studied by XRD, IR, Mossbauer technique, and d iffuse reflective spectroscopy. The crystal structures of the three ph ases were determined by the Rietveld analysis. The unit cell is hexago nal with a = 10.3391(2) Angstrom c = 37.130(1) Angstrom, s-Ca9Fe(PO4)( 7); a = 10.3543(2) Angstrom, c = 37.168(1) Angstrom, r-Ca9FeH0.9(PO4)( 7); and a = 10.3406(2) Angstrom, c = 37.157(1) Angstrom, o-Ca9Fe(PO4)( 7); Z = 6; R3c space group. The redox reactions proceed reversibly wit hout destruction of the crystal lattice in double calcium-iron phospha tes and the oxygen stoichiometry is not variable. The introduction of hydrogen into the lattice causes a change of the P(1)-O(11) bond lengt h and the formation of an O(11)-H ... O(34) hydrogen bond. The redox r eactions are accompanied by a position change of oxygen atoms O(24) an d O(33). The main differences between the three phases are discussed o n the basis of the crystal structures. (C) 1996 Academic Press, Inc.