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) ? with four formula units in space group D282h-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) ?. The average oxidation state of the Cu atoms is +7/6. La5Cu6O4S7 is metallic in the [100] direction, consistent with the results of extended Hückel band structure calculation.