On the structure and properties of hexagonal perovskite Sr₄Ru₂O₉

Materials design requires detailed knowledge of structure-property relationships over a variety of systems. Such knowledge can only be gained from detailed chemical, structural and physical property characterisation of samples. This Thesis aims to characterise structure-property relationships in Sr₄Ru₂O₉. This material is a quasi-1D hexagonal perovskite, first reported in 1995 to crystallise in the P¯62c space group. The first high quality single crystals of Sr₄Ru₂O₉ have been successfully grown using a molten-salt flux. Magnetic property measurements on these new single crystals have allowed us to correct a previous misidentification of the physical properties of Sr₄Ru₂O₉. We show this material has a previously unreported antiferromagnetic transition at T(N) ≈ 100 K, with easy-axis anisotropic susceptibility along the crystallographic c-axis, consistent with S = 3/2 localised magnetic moments on the Ru sites. The presence of a transition at this temperature is confirmed by heat capacity measurements, and the 5+ oxidation state of ruthenium consistent with S = 3/2 moments is corroborated by thermogravimetric analysis and magnetic susceptibility. Based on resistivity measurements on single crystals, we are able to classify Sr₄Ru₂O₉ as a semiconductor, in agreement with previous reports. Using high resolution neutron powder diffraction we have found previously unreported magnetoelastic coupling in this material, and establish a minimal model for the ground state as a long-range ordered k = 0 antiferromagnet. Through both lab-based single crystal X-ray diffraction and specialised synchrotron measurements focused on diffuse scattering, we have discovered previously unreported supercell reflections and diffuse scattering streaks originating in correlated disorder. We show the two phenomena are linked and establish a framework for a detailed structural description of Sr₄Ru₂O₉ based on discrete shifts of quasi-1D chains, including an orthorhombic distortion of the original hexagonal structural model which is corroborated by high resolution neutron powder diffraction.