The effect of chemical substitution on the local structure of garnet-type oxides that have nuclear waste sequestration applications
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The garnet-type structure has been investigated widely for numerous applications (i.e. laser, battery, diode etc.). The structure shows high chemical flexibility and compositional diversity. Material properties can be tuned and influenced by chemical substitution of garnet type materials. Garnet has recently been investigated as a potential host matrix for the immobilization of actinide elements found in high level nuclear waste. The general formula unit of garnet is X3A2B3O12. The structure has three different polyhedral sites with different sizes and coordination numbers: 8 coordinate dodecahedral site (X-site); 6 coordinate octahedral site (A-site); and 4 coordinate tetrahedral site (B-site). To understand the effect of chemical substitution in cation site disorder, four different series of garnet-type materials (Y3AlxGa5-xO12, Y3AlxFe5-xO12, Y3GaxFe5-xO12, and Y3AlxGayFe5-x-yO12) were initially synthesized under the same conditions. The local structures of the A- and B- sites were investigated to understand the site occupancies. Factors that affect the site preference of Al, Ga, and Fe were studied using powder XRD and X-ray absorption near edge spectroscopy (XANES). For further study, a series of garnet materials (Y3-zCezAlFe4O12) was investigated in to understand the factors affecting the oxidation state of Ce, which can be used as a simulator element for actinides. Powder XRD and XANES have been used to study the change in oxidation state of Ce in Y3-zCezAlFe4O12 (0.05 ⩽ z ⩽ 0.20) materials. The Ce L3-edge spectra confirmed that Ce has a mixture of oxidation state (3+ and 4+) in the system. The largest concentration of Ce4+ was observed when the materials were post-annealed at 800 oC following synthesis of the materials at 1400 oC. The observation of this variation in Ce oxidation state is a result of the temperature dependant Ce3+/Ce4+ redox couple with Ce4+ being favoured at lower temperatures. Examination of the Fe K-edge XANES spectra indicated that only Fe3+ was present within the materials and that the average Fe coordination number increased with increasing concentration of Ce4+ to charge balance the system. This study suggests that this system can be described as an oxygen-stuffed garnet-type structure with the formula being Y3-zCezAlFe4O12+δ.
DegreeMaster of Science (M.Sc.)
SupervisorGrosvenor, Andrew P.
CommitteeScott, Robert W.J.; Ansdell, Kevin; Urquhart, Stephen
Copyright DateSeptember 2018
Nuclear waste sequestration