Synthesis, structure, electrical conductivity, and band structure of the rare-earth copper oxychalcogenide La5Cu6O4S7

The compound La5Cu6O4S7 has been synthesized by the reaction of La2S3 and CuO in a KI flux at 1173 K. La5Cu6O4S7 crystallizes in a new structure type in a cell of dimensions a = 5,667(1), b = 15,666(3), c = 17,534(3) Angstrom with four formula units in space group D-2h(28)-Imma of the orthorhombic system at T = 153 K, The structure comprises [Cu2S2] antifluorite-like and [La10S2O8] fluorite-like layers, and these two kinds of layers stack in turn along the [001] direction. The La atom is coordinated by a distorted square antiprism of four O and four S atoms, and the Cu atom is coordinated by a tetrahedron of four S atoms. The Cu atom in the [Cu2S2] layer is bonded to neighboring Cu atoms; the Cu-Cu distances vary from 2.5864(7) to 2.965(1) Angstrom. The average oxidation state of the Cu atoms is +7/6. La5Cu6O4S7 is metallic in the [100] direction, consistent with the results of extended Huckel band structure calculation. (C) 2000 Academic Press.