Polymorphism and superstructure in BaCoS2-δ

Abstract

BaCoS2-δ consists of at least three different polymorphs: two that are stoichiometric and one that is potentially sulfur deficient with 0.0 ≤ δ ≤ 0.2. Two of these polymorphs were investigated here: a tetragonal single crystal P4/nmm, a = 4.568(1) Å, c = 8.942(2) Å (δ ≈ 0), and polycrystalline powder Cmma, a = 6.4390(3) Å, b = 6.4909(3) Å, c = 8.9379(4) Å (δ ≈ 0.2). Room temperature TEM studies show that several classes of superstructures exist in the stoichiometric single crystals of this phase; on cooling the TEM diffraction patterns change and become more complex. Temperature-dependent lattice constants of polycrystalline BaCoS2-δ show anomalies near -60°C. BaCoS2-δ is a metastable compound that is prepared above 850°C and subsequently quenched. Synthetic studies show that (1) at the preparation temperature BaCoS2 is in equilibrium with S vapor at pressures on the order of an atmosphere, producing a nonstoichiometric phase BaCoS2-δ; (2) adding some excess sulfur can reduce δ to zero; and (3) too much excess sulfur results in partial or complete melting of the system (however, stoichiometric BaCoS2 crystals can be obtained from those melts). Magnetic susceptibility measurements verify that the Co moments order antiferromagnetically at 310 K. At higher temperatures a Curie-Weiss fit to the data gives μeff = 2.14(4) μB/Co and θ = 150(20) K; these data are consistent with a low spin state for Co2+ (d7). © 1996 Academic Press.