Synthesis, Crystal Structure, and Magnetic Properties of the Copper Selenite Chloride Cu₅(SeO₃)₄Cl₂

A new copper selenite chloride Cu5(SeO3)4Cl2 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 P21/c with the unit cell parameters a = 10.9104(8) Å, b = 8.3134(6) Å, c = 7.5490(6) Å, β = 90.715(6)°, Z = 2, and R1 = 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 4s2 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 < Tc ∼45 K, Néel 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.