DIFFRACTION AND DAFS STUDIES OF A BA-4(BAXPT1-X2-P(BAXPT1-X2+)PTP-24+O3P-3 SERIES()PT24+O9 TWINNED CRYSTAL, A MEMBER OF THE BA)

Crystallographic studies of the Ba-Pt-O system have been undertaken us ing X-ray and electron diffraction techniques. The system is described by means of a Ba-p(BaxPt1-x2+)Ptp-24+O3p-3 formula which corresponds to a BaO3 hexagonal based framework with Pt chains, where p represents the oxygen deficiency and the presence of both Pt4+ and Pt2+ cations in the compounds, and x a possible substitution of Pt4+ by Ba2+ in tri gonal prismatic sites. The structure of a Ba-4(Ba0.04Pt0.962+)Pt24+O9 crystal has been solved by using 5548 X-ray difraction reflections col lected on a twinned crystal, Refinements were performed with two disti nct models: an ''average'' P321 space group and an ''orthorhombic'' Ct space group with cell parameters a = 17.460(4) Angstrom, b = 10.085(2 ) Angstrom, c = 8.614(3) Angstrom. In this structure, two Pt4+ and one Pt2+ cations are distributed over four Ba planes and form chains alon g the c axis, consisting of two face-sharing Pt4+O6 octahedra connecte d with one Pt2+O6 trigonal prism. A lattice misfit occurs between the rigid barium lattice and the PtO3 chains, giving rise to a composite s tructure. Twinning and domain configurations are described and taken i nto account in the refinement. This twinning is related to the presenc e of Pt2+ cations, whose positions break the threefold axis symmetry. A diffraction anomalous fine structure (DAFS) study was also performed on this twinned single crystal. Anomalous scattering factors f' and f '' for platinum in this crystal were refined near the LmPt absorption edge. They confirm the weak barium occupancy of the trigonal prismata ic site and the Pt4+ valence of the octahedral sites. Reflection overl aps, due to twinning, flatten the DAFS sensitivity to Pt atoms in the prismatic sites and did not allow their clear valence determination, b ut Pt-O bond lengths agree with the presence of Pt2+ cations at the ce nter of prismatic faces. Electron diffraction patterns of powders havi ng slightly different composition show a continuous evolution of incom mensurate Bragg peaks and a weak correlation between the PtO3 chains. They also confirm the composite nature and the one-dimensionality of t he Ba-p(BaxPt1-x2+)Pp-24+O3p-3 series, which can produce highly anisot ropic physical properties. (C) 1998 Academic Press.