A Fundamental and Systematic Investigation into the Solid State Chemistry of Some Ternary Uranium Oxides

Abstract

This Ph.D. dissertation explores the solid state chemistry of the AUO4 family of oxides (A = divalent or trivalent cation), addressing the role uranyl bonding and 5f electron chemistry play in influencing their physicochemical properties using high resolution measurement methods and ab initio calculations. The irreversible phase transformation that occurs between the rhombohedral and orthorhombic variants of SrUO4 is examined and demonstrated to be first order and reconstructive. The transformation is shown to involve a sequential reduction and oxidation process related to reducing the activation energy barrier that can be traced to the respective ability and inability for the rhombohedral and orthorhombic variants to host oxygen defects. The defect inventory in AUO4 rhombohedral structures is shown to be modulated by the size of the A site cation. When isostructural rhombohedral CaUO4, α-Sr0.4Ca0.6UO4 and SrUO4 obtain a critical amount of oxygen defects they can access novel reversible symmetry lowering and defect ordering transformations forming phases denoted δ. The transformations are purely thermodynamic where the origin is proposed to be related to decreasing entropy from defect ordering balanced by increasing electronic entropy with heating. AUO4 oxides that had been previously poorly described were examined at high resolution. This includes elucidation of NiUO4 polymorphs which provide the transformative “missing link" between the Pbcn and Ibmm orthorhombic variants. Consequently a structural hierarchy is developed for the family of AUO4 oxides that can be used for structure prediction for specific A and U cations. High pressure studies of SrUO4 found anomalous U-O bond lengthening to occur with increasing pressure related to electron delocalisation. This demonstrates the inapplicability of Badger’s rule to all uranyl bearing compounds. With the structural topology of rhombohedral SrUO4, this lengthening process produces bulk moduli comparable to diamond