To whom correspondence should be addressed. Journal of Solid State Chemistry 155, 366 } 371 (2000) doi:10.1006/jssc.2000.8926, available online at http://www.idealibrary.com on Synthesis, Structure, Electrical Conductivity, and Band Structure of the Rare-Earth Copper Oxychalcogenide La 5 Cu 6 O 4 S 7 Fu Qiang Huang,* Paul Brazis,- Carl R. Kannewurf,- and James A. Ibers* *Department of Chemistry and -Department of Electrical and Computer Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113 Received March 31, 2000; in revised form July 25, 2000; accepted August 9, 2000; published online November 29, 2000 The compound La 5 Cu 6 O 4 S 7 has been synthesized by the reac- tion of La 2 S 3 and CuO in a KI 6ux at 1173 K. La 5 Cu 6 O 4 S 7 crystallizes in a new structure type in a cell of dimensions a  5.667(1), b  15.666(3), c  17.534(3) A  with four formula units in space group D 28 2h -Imma of the orthorhombic system at T  153 K. The structure comprises [Cu 2 S 2 ] anti6uorite-like and [La 10 S 2 O 8 ] 6uorite-like layers, and these two kinds of layers stack in turn along the [001] direction. The La atom is coor- dinated 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 [Cu 2 S 2 ] layer is bonded to neigh- boring Cu atoms; the Cu}Cu distances vary from 2.5864(7) to 2.965(1) A  . The average oxidation state of the Cu atoms is 7/6. La 5 Cu 6 O 4 S 7 is metallic in the [100] direction, consistent with the results of extended Hu K ckel band structure calculation.  2000 Academic Press INTRODUCTION Interest in high-temperature superconductors has led to numerous studies of layered lanthanide copper oxides (1}3). However, there are very few reports of related oxychal- cogenides. The compounds ¸nCuOQ (¸n"rare earth ele- ment; Q"S, Se) (4}7) are known. These belong to the LaAgOS structure type (4) and contain the intergrowth of #uorite-like [¸n  O  ] and anti#uorite-like [Cu  Q  ] layers. The formal oxidation state of Cu is #1 in these com- pounds, and they should all be semiconductors (8). Given the ¸nCuOQ structure, if the [Cu  Q  ] layer were kept unchanged and some Q atoms were to replace some O atoms in the [¸n  O  ] layer, then the dimensions of the #uorite-like layer might expand with the result that the ¸n : Cu ratio would decrease and the average oxidation state of Cu would be greater than one. This could result in more interesting physical properties. We have successfully replaced some O atoms with S atoms in the [¸n  O  ] layer to form the compound La  Cu  O  S  . This compound belongs to a new structure type, but it is closely related to the ¸nCuOS structure. The Cu atoms in La  Cu  O  S  have an average oxidation state of #7/6 and the compound shows metallic conductivity in one direction. Here we present the synthesis, structure, elec- trical conductivity, and band structure calculations on La  Cu  O  S  . EXPERIMENTAL Synthesis La  S  (1.0 mmol; Strem, 97%), CuO (2.0 mmol; Alfa, 99.99%), and KI (2.4 mmol, Aldrich, 99.99#%) were loaded into a fused silica tube under an argon atmosphere in a glove box. This tube was sealed under a 10-Torr atmo- sphere and then placed in a computer-controlled furnace. The sample was heated to 1173 K at 1 K/min, kept at 1173 K for 4 days, and cooled at 0.05 K/min to 573 K; then the furnace was turned o!. The reaction mixture was washed free of iodide salts with water and then dried with acetone. There were two major products: orange needles and light yellow powder. The yield of needles was about 50}60%. Analysis of the needles with an EDX-equipped Hitachi S-4500 SEM showed the presence of La, Cu, and S approxi- mately in the molar ratio of 5:6:7; O was observed but could not be quanti"ed. No evidence of K or I was found. The compound is La  Cu  O  S  , as determined by an X-ray di!raction study. It is very stable in air. Analysis of the light yellow powder gave a molar ratio La:S approximately 2:3 with no trace of O corresponding to the starting material La  S  . Structure Determination Single-crystal X-ray di!raction data were collected on a 0.0320.0580.380-mm needle with the use of graphite- monochromatized MoK radiation ("0.71073 A  ) at 366 0022-4596/00 $35.00 Copyright  2000 by Academic Press All rights of reproduction in any form reserved.