POLYMORPHISM AND SUPERSTRUCTURE IN BACOS2-DELTA

BaCoS2-delta, consists of at least three different polymorphs: two tha t are stoichiometric and one that is potentially sulfur deficient with 0.0 less than or equal to delta less than or equal to 0.2. Two of the se polymorphs were investigated here: a tetragonal single crystal P4/n mm, a 4.568(1) Angstrom, c = 8.942(2) Angstrom (delta approximate to 0 ), and polycrystalline powder Cmma, a = 6.4390(3) Angstrom, b = 6.4909 (3) Angstrom, c = 8.9379(4) Angstrom (delta approximate to 0.2). Room temperature TEM studies show that several classes of superstructures e xist in the stoichiometric single crystals of this phase; on cooling t he TEM diffraction patterns change and become more complex. Temperatur e-dependent lattice constants of polycrystalline BaCoS2-delta show ano malies near -60 degrees C. BaCoS2-delta is a metastable compound that is prepared above 850 degrees C and subsequently quenched. Synthetic s tudies show that (1) at the preparation temperature BaCoS2 is in equil ibrium with S vapor at pressures on the order of an atmosphere, produc ing a nonstoichiometric phase BaCoS2-delta; (2) adding some excess sul fur can reduce delta to zero; and (3) too much excess sulfur results i n partial or complete melting of the system (however, stoichiometric B aCoS2 crystals can be obtained from those melts). Magnetic susceptibil ity measurements verify that the Co moments order antiferromagneticall y at 310 K. At higher temperatures a Curie-Weiss fit to the data gives mu(eff) = 2.14(4) mu(B)/Co and theta = 150(20) K; these data are cons istent with a low spin state for Co2+ (d(7)). (C) 1996 Academic Press.