STRUCTURE AND TWINNING OF SR3CUPTO6

Sr3CuPtO6 was obtained during the investigation of the Bi-Sr-Cu-O system in Pt crucibles. The reaction with Pt gives rise to large platelets of mixed phases. The new phase Sr3CuPtO6 reported here has a well defined composition and its structure is related to that of other Pt4+ compounds. The solution and refinement of the structure were based on single-crystal X-ray diffraction data at room temperature. The symmetry is monoclinic, space group C2/c with Z = 4, a = 9.317 (4), b = 9.720 (4), c = 6.685 (3) angstrom, beta = 91.95 (2)-degrees, V = 605.1 (8) angstrom 3, M(r) = 617.49, D(x) = 6.778 g cm-3, mu = 282.1 cm-1, F(000) = 1076, room temperature. Monochromatized Ag K-alpha radiation (lambda = 0.56083 angstrom) was used. The final refinement gave an R factor of 0.048 for 582 independent reflections. Owing to the presence of more than one pseudo-symmetry in the lattice of Sr3CuPtO6, all the crystals were heavily twinned. The possible twinning laws were investigated and a correction which eliminated the effect of twinning was applied. The structure contains chains alternating a PtO6 octahedron and a CuO4 'square' which share edges. They are connected by Sr sites in eightfold coordination. The Pt cations are surrounded by a nearly perfect O octahedron and Cu cations by six O atoms arranged as a disorted trigonal prism with four short and two long Cu-O distances. The Cu cations are displaced towards one of the rectangular faces of the prism and can be considered to have square coordination. The structure of Sr3CuPtO6 is related to that of trigonal Sr4PtO6. Chemical analysis of various platelets indicated the presence of intergrowth between Bi/Sr/Cu and Pt/Sr/Cu phases. However, no Bi/Pt substitution occurs in any phase. The microanalysis revealed that the surface of the crystal studied by X-rays did not contain any Bi. All Sr3CuPtO6 crystals are twinned either by reticular pseudomerohedry with a pseudo-orthorhombic cell and/or by pseudo-merohedry with a pseudo-rhombohedral cell. A detailed twinning analysis is given; it allows comparison of the symmetry of Sr3CuPtO6 with those occurring in other M4(2+) PtO6 compounds.