Crystallographic studies of the Ba-Pt-O system were carried out by using X-ray and electron diffraction techniques. The material was described as: Bap(BaxPt1-x2+)Ptp-24+O3p-3. This corresponded to a BaO3 hexagon-based framework with Pt chains, where p represented the O deficiency and the presence of both Pt4+ and Pt2+ cations in the compound. Here, x represented the possible replacement of Pt2+ by Ba2+ in trigonal prismatic sites. The structure of a Ba4(Ba0.04Pt0.962+)Pt24+O9 crystal was solved by using 5548 X-ray diffraction reflections from a twinned crystal. Refinement was performed by using 2 distinct models. One was an average P321 space group, and the other was an orthorhombic space group with a = 1.7460, b = 1.0085 and c = 0.8614nm. In this structure, two Pt4+ cations and one Pt2+ cation were distributed over four Ba planes; and formed chains along the c-axis which consisted of 2 face-sharing Pt4+O6 octahedra which were linked to one Pt2+O6 trigonal prism. Lattice misfit occurred between the rigid Ba lattice and the PtO3 chains, and gave rise to a composite structure. Twinning and domain configurations were observed and were taken into account by the refinement. The twinning was related to the presence of Pt2+ cations, whose positions broke the 3-fold axis symmetry. A diffraction anomalous fine structure study was also made of the twinned crystals. The anomalous scattering factors confirmed a weak Ba occupancy of the trigonal prismatic site, and the Pt4+ valence of the octahedral sites. Reflection overlaps, due to twinning, flattened the diffraction anomalous fine structure sensitivity to Pt atoms in prismatic sites and did not permit their clear valence determination. The Pt-O bond-lengths were consistent with the location of Pt2+ cations at the centers of prismatic faces.

Diffraction and DAFS Studies of a Ba4(Ba,Pt2+)Pt4+O9 Twinned Crystal. J.Vacínová, J.L.Hodeau: Journal of Solid State Chemistry, 1998, 140[2], 201-18