REFINED STRUCTURE AND PROPERTIES OF THE LAYERED MOTT INSULATOR BACOS2

BaCoS2 is orthorhombic Cmma a = 6.4413(3) angstrom, b = 6.4926(3) angs trom, c = 8.9406(3) angstrom, and is closely related to BaNiS2, which contains Ni-S layers separated by rock salt BaS sheets. Crystals were grown from the melt by slow cooling from 1050 to 950-degrees-C. The su lfur atoms in the Co-S layers have very anisotropic thermal displaceme nt parameters, possibly indicating that these sulfurs are actually in a double well potential centered at a high symmetry position in the sp ace group Cmma. BaCoS2 is found to be a paramagnetic semiconductor (Mo tt insulator). The magnetic properties indicate that BaCoS2 undergoes a transformation at 300 K, which is likely due to the onset of antifer romagnetic order. (C) 1994 Academic Press, Inc.