Diffraction and DAFS studies of a Ba4(BaxPt1-x2+)Pt24+O9 twinned crystal, a member of the Bap(BaxPt1-x2+)Ptp-24+O3p-3 series

Crystallographic studies of the Ba-Pt-O system have been undertaken using 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 trigonal prismatic sites. The structure of a Ba-4(Ba0.04Pt0.962+)Pt24+O9 crystal has been solved by using 5548 X-ray difraction reflections collected on a twinned crystal, Refinements were performed with two distinct 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 along the c axis, consisting of two face-sharing Pt4+O6 octahedra connected with one Pt2+O6 trigonal prism. A lattice misfit occurs between the rigid barium lattice and the PtO3 chains, giving rise to a composite structure. Twinning and domain configurations are described and taken into account in the refinement. This twinning is related to the presence 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 prismataic site and the Pt4+ valence of the octahedral sites. Reflection overlaps, due to twinning, flatten the DAFS sensitivity to Pt atoms in the prismatic sites and did not allow their clear valence determination, but Pt-O bond lengths agree with the presence of Pt2+ cations at the center of prismatic faces. Electron diffraction patterns of powders having slightly different composition show a continuous evolution of incommensurate Bragg peaks and a weak correlation between the PtO3 chains. They also confirm the composite nature and the one-dimensionality of the Ba-p(BaxPt1-x2+)Pp-24+O3p-3 series, which can produce highly anisotropic physical properties. (C) 1998 Academic Press.