High-Spin Cobalt(II) Ions in Square Planar Coordination: Structures and Magnetism of the Oxysulfides Sr2CoO2Cu2S2 and Ba2CoO2Cu2<

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• 作者：Catherine F. Smura ; Dinah R. Parker ; Mohamed Zbiri ; Mark R. Johnson ; Zolt谩n A. G谩l ; Simon J. Clarke
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：March 2, 2011
• 年：2011
• 卷：133
• 期：8
• 页码：2691-2705
• 全文大小：1312K
• 年卷期：v.133,no.8(March 2, 2011)
• ISSN：1520-5126

文摘

The antiferromagnetic structures of the layered oxychalcogenides (Sr_1鈭?i>xBa_x)₂CoO₂Cu₂S₂ (0 鈮?x 鈮?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 鈭?a 脳 鈭?a 脳 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) 渭_B 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 渭_B; there is a strong correlation between the size of the long-range ordered moment and the occupancy of the Cu site. Both the tetragonal elongation of the CoO₄S₂ polyhedron and the ordered moment in (Sr_1鈭?i>xBa_x)₂CoO₂Cu₂S₂ increase with increasing Ba content, and in Ba₂CoO₂Cu₂S₂, which has Co²⁺ in an environment that is close to purely square planar, the ordered moment of 4.5(1) 渭_B at 5 K is over 0.7 渭_B larger than that in Sr₂CoO₂Cu₂S₂, so the unquenched orbital component in this case is even larger than that observed in octahedral Co²⁺ systems such as CoO. The experimental observations of antiferromagnetic ground states and the changes in properties resulting from replacement of Sr by Ba are supported by ab initio calculations on Sr₂CoO₂Cu₂S₂ and Ba₂CoO₂Cu₂S₂. The large orbital moments in these systems apparently result from spin鈭抩rbit mixing of the unequally populated d_xz, d_yz, and d_z² orbitals, which are reckoned to be almost degenerate when the CoO₄S₂ polyhedron reaches its maximum elongation. The magnitudes of the ordered moments in high-spin Co²⁺ oxide, oxychalcogenide, and oxyhalide systems are shown to correlate well with the tetragonal elongation of the coordination environment. The large orbital moments lead to an apparently magnetostrictive distortion of the crystal structures below the Ne茅l temperature, with the symmetry lowered from tetragonal I4/mmm to orthorhombic Immm and the size of the distortion correlating well with the size of the long-range ordered moment for all compositions and for temperature-dependent data gathered on Ba₂CoO₂Cu₂S₂.