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

Two new isostructural cobalt selenite halides Co5(SeO3)4Cl2 and Co5(SeO3)4Br2 have been synthesized. They crystallize in the triclinic system space group P-1 with the following lattice parameters for Co5(SeO3)4Cl2: a=6.4935(8)A , b=7.7288(8)A , c=7.7443(10)A, alpha=66.051(11)1, beta=73.610(11)1, gama=81.268(9)1, and Z=1. The crystal structures were solved from single-crystal X-ray data, R1=3.73 and 4.03 for Co5(SeO3)4Cl2 and Co5(SeO3)4Br2, respectively. The new compounds are isostructural to Ni5(SeO3)4Br2. 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 TN=18 and 20K for Co5(SeO3)4Cl2 and Co5(SeO3)4Br2, respectively, are ascribed to antiferromagnetic ordering in a minority magnetic subsystem. In isostructural Ni5(SeO3)4Br2 magnetically ordered subsystem represents a majority fraction (TN= 46 K). Nevertheless, anisotropic susceptibility of Ni5(SeO3)4Br2 is dominated at low temperatures by a minority fraction, subject to single-ion anisotropy effects and increasing population of Sz=0 (singlet) ground state of octahedrally coordinated Ni2+.

Oxo-halide; Lone-pair elements; Crystal structure determination; Magnetic susceptibility; Zero-field splitting; Single-ion anisotropy

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