Investigation of (Sr4-delta Ca delta)PtO6 using X-ray Rietveld refinement

The structures of the solid solution series (Sr4-deltaCadelta)PtO6, with de lta = 0, 0.85(1), 2, and 3, have been investigated using the Rietveld refin ement technique with laboratory X-ray powder diffraction data. A complete s olid solution between Sr and Ca was confirmed to exist. These compounds cry stallize in the rhombohedral space group R (3) over bar c. The cell paramet ers of the series range from a of 9.4780(3) to 9.7477(1) Angstrom, and c fr om 11.3301(4) to 11.8791(1)Angstrom for delta from 3 to 0, respectively. Th e structure consists of chains of alternating trigonal prismatic (Sr, Ca)O- 6 and octahedral PtO6 units running parallel to the c axis. These chains ar e connected to each other via a second type of (Sr, Ca) ions, which are sur rounded by eight oxygens, in a distorted square antiprismatic geometry. As Ca replaced Sr in Sr4PtO6, it was found to substitute preferentially in the smaller octahedral (Sr, Ca)1 site (6a) rather than at the eight-coordinate (Sr, Ca)2 site (18e). There appears to be an anomaly of cell parameters a and c at the compound Sr3.15Ca0.85PtO6 Their dependence on Ca content chang es at delta approximate to 1.00, where the Ca has fully replaced Sr in the 6a site. The substitution of Sr by Ca reduced the average (Sr, Ca)1-0 lengt h from 2.411 to 2.311 Angstrom and (Sr, Ca)2-0 from 2.659 to 2.570 Angstrom as the composition varied from Sr4PtO6 to SrCa3PtO6. Reference X-ray powde r diffraction patterns were prepared from the Rietveld refinement:results f or these members of the solid solution series. Magnetic susceptibility meas urements of three of the samples (delta = 0, 0.85, 2) show electronic trans itions at low temperatures. (C) 1999 International Centre for Diffraction D ata. [S0885-7156(98)00304-2].