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) Å 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) Å. 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.