Abstract
Five phosphates, Sr 9 A(PO 4) 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(PO 4) 7 to be centrosymmetric. Sr 9In(PO 4) 7 is structurally related to β-Ca 3(PO 4) 2 (space group R3 c; a=10.439Å and c=37.375Å; Z=21) and Sr 9+ x A′ 1.5− x (PO 4) 7 ( A′=Mn, Fe, Co, Ni, Cu, and Cd; space group R3m;a≈10.6Å and c≈19.7Å Z=3), having a new type of a monoclinic superstructure. Structure parameters of Sr 9In(PO 4) 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) Å, b=10.66307(4) Å, c=18.3714(2) Å;, and β=132.9263(5)° ( Z=4). The asymmetric unit of Sr 9In(PO 4) 7 contains five Sr, one In, four P, and 14 O sites. Sr 2+ ions are coordinated to either eight or nine oxide ions. Sr 2+ ions at an Sr4 site are disordered over two positions near a center of symmetry. In 3+ ions occupy an octahedral site. O12 atoms belonging to P1O 4 tetrahedra are highly disordered because they are bonded to the Sr4 atoms.
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