Synthesis, structure and magnetic properties of some low dimensional metal compounds

Abstract

A selection of low dimensional metal compounds have been synthesised; the structures and magnetic properties of these materials have been characterised using Rietveld analysis of powder X-ray and powder neutron data and magnetic susceptibility measurements. Refinements on the LnSrScO4 (Ln = La - Sm) were performed in several space groups accommodating octahedral tilts. Based on the statistics the Abma description was used to model the data and the orthorhombic distortion results from tilting of the ScC>6 octahedra within .the perovskite layer. Graphs of the b/a ratio against temperature show that the level of distortion increases between 25 - 300 CC and decreases between 300 - 1200 °C; for LaSrScO4 and PrSrScO4 the compounds revert to the ideal K2NiF4-type structure at approximately 900 and 1100 °C respectively. A similar analysis was undertaken for LnSrInO4 (Ln = La and Pr), these compounds adopt the Pbca space group at room temperature. A comprehensive study has been carried out to investigate the factors that lead to the choice of structure and drive distortions of the K2NiF4-type structure away from an Wmmrn description. An investigation into the LnSrCoO4 (Ln = La - Gd) series has been performed. As the smaller lanthanides are incorporated into the structure (La—>Er), the K2NiF4 structure is maintained until Tb. At which point the mismatch between the perovskite Co-0 layer and the rocksalt (Sr/ Ln)-0 layers become too great, resulting in the formation of SrLn2O4. Graphs of Co-O axial bond distances against temperature were plotted in order to determine whether a thermally induced spin transition occurs, anomalous behaviour for all compounds was observed. A comparison of the thermal expansion of the axial B-0 distances in the compounds; LaSrCoO4, LaSrFeO4, La, 8Sr02CuO4 and LaBaCoO4 enabled us to identify that the Co3+ compounds expand much faster in the apical direction. If low spin Co3+, (t2g 6, eg°) dxy2dzy2dxz2 can be thermally excited to 5eg'), the electron density would transfer to the dz2 orbital. This would an intermediate-spin, (t2g expand the axial distance faster than a normal thermal effect. Based on this there is strong evidence that a spin transition occurs in these compounds. Novel analogues of La2Ba6Co4Ois have been synthesised by replacing La3+ with Nd3+ and Sm3+. The structure of these phases and the previously reported La2Ba6Co4O15 have been determined from analysis of neutron diffraction data and adopt the ?63mc space group. The compound consists of one CoO6 octahedron and three CoO4 tetrahedra linked by corner sharing oxygen ions, forming a Co4Oi5 cluster. The clusters form a triangular based lattice that resembles a honeycomb. Magnetic data reveal an antiferromagnetic transition due to the long range ordering of clusters at 4, 8.5 and 17.5 K for Ln2Ba6Co4O15 (Ln = La, Nd and Sm) respectively. The ZFC and FC susceptibilities have been discussed in detail. Three complex transition metal triazole compounds have been synthesised and investigated by single crystal X-ray diffraction. 1) [Co2Cl(3-amino-l,2,4-triazole)3] is an intriguing magnetically coupled coordination network. The implications of the observed ferrimagnetic ordering are discussed in the context of the topological structural disorder. The compound was obtained from a hydrothermal synthesis, crystallising in a primitive hexagonal cell a good structural model for [Co2Cl(3-amino-l,2,4-triazole)3] was obtained in the non-polar space group P6Jmmc, revealing a 3-D coordination network. 2) the orthorhombic space group Vbcm and displays a zig-zag polymeric structure. 3) [Cu5Cl2(3,5- diamino-triazolate)3].H2O consists of unusual chains of trinuclear units with a 3,5-diamino- triazole linking the trimeric units. These trimeric entities are bridged by Cu coordinated to Cl forming a chain, which are in turn linked by Cu-Cl in a zig-zag formation to form layers. [CuCl(3-amino-l,2,4-triazole)2.Cl] crystallises in

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