Crystal Structure and Magnetic Properties of Two New Cobalt Selenite Halides: Co5(SeO3)4X2 (X: Cl, Br).

Two new isostructural cobalt selenite halides Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} have been synthesized. They crystallize in the triclinic system space group P-1 with the following lattice parameters for Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2}: a=6.4935(8) A, b=7.7288(8) A, c=7.7443(10) A, {alpha}=66.051(11){sup o}, {beta}=73.610(11){sup o}, {gamma}=81.268(9){sup o}, and Z=1. The crystal structures were solved from single-crystal X-ray data, R1=3.73 and 4.03 for Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2}, respectively. The new compounds are isostructural to Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 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 Co{sup 2+} and Ni{sup 2+} ions. Sharp low-temperature susceptibility features, at T {sub N}=18 and 20 K for Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2}, respectively, are ascribed to antiferromagnetic ordering in a minority magnetic subsystem. In isostructural Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} magnetically ordered subsystem represents a majority fraction (T {sub N}=46 K). Nevertheless, anisotropic susceptibility of Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} is dominated at low temperatures by a minority fraction, subject to single-ion anisotropy effects and increasingmore » population of S{sub z} =0 (singlet) ground state of octahedrally coordinated Ni{sup 2+}. - Graphical abstract: Two new iso-structural cobalt selenite halides Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} have been synthesized which are iso-structural to Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 2}. Magnetic susceptibility measurements on oriented single-crystalline samples show anisotropic response in a broad temperature range, revealing significant single-ion anisotropy effects.« less