THE SUPERCONDUCTING COPPER OXYCHROMATE TL -3(CRO4)SR8CU4O16 - LONG-RANGE ORDERING BETWEEN THALLIUM AND CHROMIUM

Long range ordering between chromium and thallium has been evidenced f or the first time in copper oxychromate Tl-3(CrO4)Sr8Cu4O16 using HREM , XRD and ND. This compound, which crystallizes in an orthorhombic cel l with a = 3.7803(1) Angstrom, b = 15.2573(2) Angstrom and c = 17.6737 (3) Angstrom derives from the ''1201'' TlSr2CuO5 structure by replacin g one thallium oxygen polyhedra out of four by one CrO4 group in an or dered way. The main features that characterize this original structure deal with the waving character of the [CuO2](infinity) layers, and th e absolute planar character of the [(Tl,Cr)O](infinity) layers. In the [(Tl,Cr)O](infinity) layers, the thallium and oxygen atoms form tripl e ribbons of edge-sharing TlO4 tetrahedra and TlO6 octahedra running a long a, whereas the CrO4 tetrahedra are displayed in rows running alon g a, but are disconnected from each other and from the other polyhedra in the layer. Two kinds of CuO6 octahedra are observed, whose distort ions originate from their connection with different polyhedra along c. This superconductor, whose T-c can be raised to 34 K after argon-hydr ogen annealing, exhibits a rather high superconducting volume fraction (>40%) compared to other ''1201'' thallium cuprates.