High-Spin Cobalt(II) Ions in Square Planar Coordination: Structures and Magnetism of the Oxysulfides Sr2CoO2Cu2S2 and Ba2CoO2Cu2S2 and Their Solid Solution

The antiferromagnetic structures of the layered oxychalcogenides (Sr1−xBax)2CoO2Cu2S2 (0 ≤ x ≤ 1) have been determined by powder neutron diffraction. In these compounds Co2+ is coordinated by four oxide ions in a square plane and two sulfide ions at the apexes of an extremely tetragonally elongated octahedron; the polyhedra share oxide vertexes. The magnetic reflections present in the diffraction patterns can in all cases be indexed using a √2a × √2a × c expansion of the nuclear cell, and nearest-neighbor Co2+ moments couple antiferromagnetically within the CoO2 planes. The ordered magnetic moment of Co2+ in Sr2CoO2Cu2S2 (x = 0) is 3.8(1) μB at 5 K, consistent with high-spin Co2+ ions carrying three unpaired electrons and with an additional significant unquenched orbital component. Exposure of this compound to moist air is shown to result in copper deficiency and a decrease in the size of the ordered moment to about 2.5 μB; there is a strong correlation between the size of the long-range ordered moment an...