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(PO4)(7) were prepared and studied by XRD, IR, Mossbauer technique, and diffuse reflective spectroscopy. The crystal structures of the three phases were determined by the Rietveld analysis. The unit cell is hexagonal 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 without destruction of the crystal lattice in double calcium-iron phosphates and the oxygen stoichiometry is not variable. The introduction of hydrogen into the lattice causes a change of the P(1)-O(11) bond length and the formation of an O(11)-H ... O(34) hydrogen bond. The redox reactions are accompanied by a position change of oxygen atoms O(24) and O(33). The main differences between the three phases are discussed on the basis of the crystal structures. (C) 1996 Academic Press, Inc.