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, Sr9A(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 Sr9A(PO4)7 to be centrosymmetric. Sr9In(PO4)7 is structurally related to beta-Ca3(PO4)2 (space group R3c; A=10.439? and C=37.375?; Z= 21) and Sr9+xAʹ1.5-x(PO4)7 (Aʹ=Mn, Fe, Co, Ni, Cu, and Cd; space group R3m;a~10.6? and c~19.7? 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 I2/a to afford lattice parameters of A=18.0425(2) ?, B=10.66307 (4) ?, C=18.3714(2) ?;, and beta=132.9263(5)° (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. O12 atoms belonging to P1O4 tetrahedra are highly disordered because they are bonded to the Sr4 atoms.