Synthesis, Crystal Structure, and Magnetic Properties of the Copper Selenite Chloride Cu-5(SeO3)(4)Cl-2

A new copper selenite chloride Cu-5(SeO3)(4)Cl-2 has been prepared by chemical vapor transport reactions. Its crystal structure was determined by single-crystal X-ray diffraction. The title compound crystallizes in the monoclinic space group P2(1)/c with the unit cell parameters a = 10.9104(8) angstrom, b = 8.3134(6) angstrom, c = 7.5490(6) angstrom, beta = 90.715(6)degrees, Z = 2, and R-1 = 0.0383. Bond valence sum calculations indicate that the cations have the oxidation state Cu(II) and Se(IV), respectively. Three crystallographic different copper atoms, having different coordination polyhedra, [CuO5], [CuO6], and [CuO3Cl2], are connected by corner and edge sharing to form a framework that can be described as metal oxygen slabs connected by Cl atoms via edge sharing [CuO3Cl2] polyhedra. The two crystallographic different selenium atoms both have [SeO3E] coordination, where E is the 4s(2) lone pair on Se(IV); they are isolated from each other and do bond to the Cu-coordination polyhedra only. The magnetic properties of the Cu2+ ions with effective spin S = 1/2 moments are dominated by antiferromagnetic interactions. For temperatures T < T-c similar to 45 K, Neel magnetic ordering is observed with small ferromagnetic canted moments. We attribute these to antisymmetric Dzyaloshinskii-Moriya (DM) spin exchange which is allowed by the low symmetry spin exchange paths along the distorted transition metal oxyhalide coordinations.