The synthesis and characterisation of rare earth platinum group mixed metal oxide systems

• Main Author: Cruickshank, Karen M.
• Published: University of Aberdeen 1992
• Subjects: 546; Inorganic chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.304923

Rare earth-platinum group mixed metal ozide (RE-PG-O) systems comprise approximately 100 systems. The similarity of ionic radii and electronegativitis within constitutent cation groups (for comparable RE or PG oxidation states) is useful for examining trends in RE-PG-O phase development, and explaining structural stability in terms of tolerance factors. The refractory and electropositive influence of REs in the mixed metal oxide matrix enables less common oxidation states of PGs to be accessed via a novel synthesis technique. RE-PG-O systems are therefore selectively studied under 'neutral', 'reducing' and 'oxidising' conditions. X-ray diffraction methods are used for phase identification and the determination of stability relationships. PGII, PGIII, PGIV and PGV species are encountered in structures such as perovskite, pyrochlore, A3BO7 and Nd2CuO4 types. Re-PG-O systems can be divided into two main groups. The first comprises PG&61 Ru, Ir, Rh and includes structure types which can be described in terms of order-disorder transformations within a fluorite template. Phases in these systems are redox sensitive in both their synthesis and their defect chemistry, a feature which is reflected in their electrical properties; this is often a characteristic of fluorite-derived structures. The second group of RE-PG-O systems is characterised by formation of structures which permit square-planar coordination of PdII and PtII. The influence of REII, REIII and REIV species is considered in RE-PG-O phase development; REIII species predominate. The role of the RE is more subtle in determining the stable structure type since phase development and physical properties can be considered in terms of gradational change across the RE series. This behaviour can be used to detct the occasional presence of any REIV and to 'fine-tune' the physical properties of a given structure type. Generally, the half-filled 4f shell of Gd represents a transition point for polymorphism and RE-PG-O phase stability in the REIII series; the stabilisation of RE-PG-O phases is more difficult beyond Gd. Two novel systems are encountered, La-Ru-O and La-Ir-O. These sytems contain structures with mixed integral PG oxidation states, non-integral delocalised PG oxidation states, and/or metal-metal bonding. La is an unusual RE in that it lacks 4f electrons, in contrast to other REs, but since RE behaviour is dictated largely by 5d and 6s electrons, these novel systems should have features in common with their sister systems. Possible structural relationships are discussed in an attempt to rationalise the behaviour of the La-Ru-O system in particular. This thesis represents the first comprehensive overview of RE-PG-O systems, identifying areas of interest which merit further study.