Synthesis, structure and magnetic properties of complex metal oxides

Knee, Christopher Sebastian (1999) Synthesis, structure and magnetic properties of complex metal oxides. University of Southampton, Doctoral Thesis.

Abstract

Complex metal oxides with structures that are related to high temperature superconducting cuprate phases 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 vibrating sample magnetometry.

TlSr₂NiO_4+δ was synthesised by reaction of Tl₂O₃, Sr₂Ni₂O₅ and SrO in a sealed gold capsule at 950^oC. It was found to be isostructural with the orthorhombic form of TlSr₂CuO_y, the end member of a series of high temperature superconductors, TlSr₂Ca_n-1O_3.5+2n, n = 1, 2, 3, 4. Structural characterisation of the materials using low temperature neutron diffraction indicated a high level of structural similarity between the nickelate and cuprate. Refinement of the occupation of the basal oxygen sites revealed ordering of vacancies along the b cell direction, producing a 2b superlattice and the structure of TlSr₂NiO_4+δ contains alternating NiO₆ octahedra and NiO₄ square planes. Both phases were found to display a Curie-Weiss temperature dependence. The related tetragonal cobalt phase, TlSr₂CoO₅, was also synthesised and studied, and differences between it and the orthorhombic nickelate and cuprate were ascribed to the marked oxygen deficiency present in the latter two compounds.

An investigation of the Tl-Sr-Ni-La-O system resulted in the isolation of a new family of nickelates described by the formula Tl(Sr₂Ln₂)Ni₂O₉, with Ln = La, Pr, Nd, Sm, Eu and Gd. The materials were found to be direct structural analogues of the superconductor Tl(Ba_1.6La_2.4)Cu₂O₉ and crystallise with the 1201-0201 intergrowth structure, consisting of layers of apex-sharing NiO₆ octahedra separated by a TlO layer. The phases are closely related to a series of nickelates, that adopt the K₂NiF₄ structure, and contain stoichiometric NiO₂ layers within the ab-plane. However, the new materials exhibit partial cation ordering with the segregation of the lanthanide and larger strontium ion on two similar crystallographic positions with the structure. A decrease in cell dimensions consistent with the lanthanide contraction was observed for the series. Members of the family containing the smaller lanthanides, Eu and Gd, possess particularly short in-plane nickel-oxygen bonds which are accommodated by the 1201-0201 structure. No evidence of long range magnetic order was found for the lanthanum or gadolinium containing phases. A strontium analogue, Tl(Sr₃La)Cu₂O₉, of the original high T_c compound was synthesised as the largest component of a multiphase system and its structure determined from Rietveld analysis of X-ray and neutron diffraction data.

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