Structure and Magnetism of Sr_3Co_2O_4Cl_2-An Electronically Driven Lattice Distortion in an Oxychloride Containing Square Planar Co~Ⅱ Centers

摘要

Topochemical reduction of Sr_3Co_2O_5Cl_2 with NaH at 200 ℃ yields Sr_3Co_2O_5Cl_2, a phase consisting of infinite double sheets of corner-linked Co~(Ⅱ)O_4 square planes stacked with SrCl rocksalt layers. At 298 K, the structure of Sr_3Co_2O_5Cl_2 is described in the tetragonal space group 14/ mmm [a = 4.007(l) A, c = 22.282(1) A]; however, on cooling below 200 K, the structure undergoes a lattice distortion to adopt a structure with orthorhombic symmetry in space group Immm [a = 3.9757(5) A, b = 4.0294(5) A, and c = 22.147(3) A at 5 K]. The structural distortion can be considered Jahn-Teller-like as it lifts the orbital degeneracy of the square planar, d~7, Co~(Ⅱ) centers, demonstrating a strong coupling between the electronic configuration and the structural lattice of this oxychloride phase. On cooling below 50 K, Sr_3Co_2O_4Cl_2 adopts a canted antiferromagnetically ordered state. All magnetization data show that the local cobalt moment is much greater than would be expected for a simple spin-only S = 3/2 center, indicating a strong orbital contribution to the magnetic behavior.