| Summary: | In this thesis the one-pot subcritical hydrothermal synthesis of several novel complex oxides with potential applications in catalysis is reported. The hydrothermal synthesis of nanocrystalline pentavalent ion-substituted cerium dioxide is reported. The materials were all characterised by powder diffraction and a range of spectroscopic techniques in order to elucidate their structures. Up to approximately 30% pentavalent ion-substitution the mixed metal oxides continue to adopt the fluorite unit cell, and the lattice parameter decreases as substitution increases. The choice of mineraliser in the hydrothermal reaction was found to affect the composition and structure of the oxide: when prepared in NaOH, Na is incorporated into the structure as well as the pentavalent ion, and the combination of these allows charge balance to be achieved without the need for Ce reduction or the incorporation of interstitial oxygen. The inclusion of Na greatly improves both the measured oxygen storage capacity and the stability of the fluorite phases, as measured by in situ powder X-ray diffraction. Precious metals Pt and Pd have also been successfully incorporated into the fluorite ceria structure by hydrothermal synthesis, with up to 20 – 25% precious metal substitution. A study of both the average structure and local environment of the precious metals leads to the conclusion that neither precious metal sits on the same crystallographic site as the Ce. Pd adopts a square-planar geometry by shifting ¼ of a unit cell in one direction, to sit in an oxide plane, and thereby expanding the unit cell. The Pt exists a distorted octahedral environment. These oxides were found to be inherently unstable, with precious metal extruded from the fluorite under even mildly reducing conditions. The hydrothermal reaction of KRuO4 with AO2 (A = Ca, Sr, Ba) peroxides at 200 °C yields a range of previously unreported ruthenium oxides. Ca1.5Ru2O7, an A-site deficient pyrochlore containing a mixed Ru(V/VI) B-site. SrRu2O6 adopts the PbSb2O6 structure and contains antiferromagnetically ordered Ru(V) moments. In situ powder neutron diffraction shows that this ordering persists up to 563 K, an extraordinarily high temperature. Ba2Ru3O9(OH) is an example of a new structure type, which is made up of puckered layers of quasi-trimeric edge-sharing Ru(V)O6 octahedra which are corner-linked together with layers of Ba atoms separating them. Ba4Ru3O12 is an 8H hexagonal perovskite with a previously unreported stacking order, containing Ru with an average oxidation state of +5.33. Structural refinements and magnetometry of all new materials are presented.
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