Synthesis and characterisation of the uranium pyrochlore betafite [(Ca,U)2(Ti,Nb,Ta)2O7]

Betafite of composition [(Ca, U)2(Ti, Nb, Ta)2O7] was prepared via a solid state synthesis route. The synthesis was shown to be sensitive to initial reactant ratios, the atmosphere used (oxidising, neutral, reducing) and time. The optimum conditions for the synthesis of betafite were found to be heating the reactants required at 1150°C for 48h under an inert gas atmosphere. XRD characterisation revealed that the synthesised betafite contained minor impurities. EPMA analysis of a sectioned surface showed very small regions of Ca-free betafite on grain boundaries as well as minor rutile impurities. Some heterogeneity between the Nb:Ta ratio was observed by quantitative EPMA but was generally within the nomenclature requirements stated for betafite. SEM analysis revealed the synthesised betafite was comprised mostly of hexaoctohedral crystals of ~3μm in diameter. XPS analysis of the sample showed that the uranium in the synthesised betafite was predominately present in the U5+ oxidation state. A minor amount of U6+ was also detected which was possibly due to surface oxidation.