Whitlockite-related phosphates Sr9A(PO4)7 (A=Sc, Cr, Fe, Ga, and In):: Structure refinement of Sr9In(PO4)7 with synchrotron X-ray powder diffraction data

Five phosphates, Sr(9)A(PO4)(7) (A = Sc, Cr, Fe, Ga, and In), were synthesized by the solid state method at (1270 to 1420) K and characterized by X-ray powder diffraction, infrared spectroscopy, differential scanning calorimetry, thermogravimetry, and second-harmonic generation (SHG). SHG showed Sr(9)A(PO4)7 to be centrosymmetric. Sr9In(PO4)(7) is structurally related to beta-Ca-3(PO4)(2) (space group R3c; a = 10.439 Angstrom and c = 37.375 Angstrom; Z = 21) and Sr(9+x)A'(1.5-x)(PO4)(7) (A' = Mn, F, Co, Ni, Cu, and Cd; space group R (3) over barm; a approximate to 10.6 Angstrom and c approximate to 19.7 Angstrom; Z = 3), having a new type of a monoclinic superstructure. Structure parameters of Sr9In(PO4)(7) were refined by the Rietveld method from synchrotron X-ray diffraction data on the basis of space. group I-2/a to afford lattice parameters of a = 18.0425(2) Angstrom, b = 10.66307(4) Angstrom, c = 18.3714(2) Angstrom, and beta = 132.9263(5)degrees (Z = 4). The asymmetric unit of Sr9In(PO4)(7) contains five Sr, one In, four P, and 14 O sites. Sr2+ ions are coordinated to either eight or nine oxide ions. Sr2+ ions at an Sr4 site are disordered over two positions near a center of symmetry. In3+ ions occupy an octahedral site. 012 atoms belonging to P1O(4) tetrahedra are highly disordered because they are bonded to the Sr4 atoms. (C) 2002 Elsevier Science (USA).