Crystal structure and magnetic properties of two new cobalt selenite halides:: Co5(SeO3)4X2 (X = Cl, Br)

Two new isostructural cobalt selenite halides Co-5(SeO3)(4)Cl-2 and Co-5(SeO3)(4)Br-2 have been synthesized. They crystallize in the triclinic system space group P-1 with the following lattice parameters for Co-5(SeO3)(4)Cl-2: a = 6.4935(8) angstrom, h = 7.7288(8)angstrom, c = 7.7443(10) angstrom, alpha = 66.051(11)degrees, beta = 73.610(11)degrees, gamma = 81.268(9)degrees, and Z = 1. The crystal structures were solved from single-crystal X-ray data, R1 = 3.73 and 4.03 for Co-5(SeO3)(4)Cl-2 and Co-5(SeO3)(4)Br-2, respectively. The new compounds are isostructural to Ni-5(SeO3)(4)Br-2. Magnetic susceptibility measurements on oriented single-crystalline samples show anisotropic response in a broad temperature range. The anisotropic susceptibility is quantitatively interpreted within the zero-field splitting schemes for Co2+ and Ni2+ ions. Sharp low-temperature susceptibility features, at T-N = 18 and 20 K for Co-5(SeO3)(4)Cl-2 and Co-5(SeO3)(4)Br-2, respectively, are ascribed to antiferromagnetic ordering in a minority magnetic subsystem. In isostructural Ni-5(SeO3)(4)Br-2 magnetically ordered subsystem represents a majority fraction (T-N = 46 K). Nevertheless, anisotropic susceptibility of Ni-5(SeO3)(4)Br-2 is dominated at low temperatures by a minority fraction, subject to single-ion anisotropy effects and increasing population of S-z = 0 (singlet) ground state of octahedrally coordinated Ni2+. (c) 2007 Elsevier Inc. All rights reserved.