NEUTRON-DIFFRACTION STRUCTURE DETERMINATION OF THE SUPERCONDUCTING COPPER OXYCHROMATE TL-3(CRO4)SR8CU4O16

The structure of the superconducting copper oxychromate Tl-3(CrO4)Sr8C u4O16 has been determined from powder neutron diffraction data. This c ompound derives from the ''1201'' classical structure by an ordered su bstitution of CrO4 groups for thallium oxygen polyhedra showing defini tively that chromium is hexavalent, The main features that characteriz e this original structure deal with the waving character of the [CuO2] (infinity) layers, and the absolute planar character of the [(TlCr),O] (infinity), layers. In the [(TlCr),O](infinity) layers, the thallium a nd oxygen atoms form triple ribbons of edge-sharing TIO4 tetrahedra an d TIO6 octahedra running along alpha, whereas the CrO4 tetrahedra are displayed in rows running along alpha, but are disconnected from each other and from the other polyhedra in the layer. Two kinds of CuO6 oct ahedra are observed, whose distortions originate from their connection with different polyhedra along c. The great flexibility of the two ki nds of layers, and the structural analogy with copper oxycarbonates ar e emphasized.