Effect of synthesis method and reaction conditions on the long-range and local structure of RE₂Ti₂O₇ pyrochlore-type oxides

Current plans for managing high-level nuclear waste may involve sequestration in borosilicate glass and potential disposal in geological repositories. However, there is considerable interest in using ceramic oxides, such as pyrochlores, as alternative nuclear waste forms due to the high chemical durability and waste loading capabilities of these materials. The ceramic method is often used to synthesize RE 2 Ti 2 O 7 pyrochlore-type oxides and requires the use of high temperatures and long reactions times, making this process less economically feasible for large-scale production. Solution-based synthetic routes, such as co-precipitation and sol–gel methods, have been shown to require much lower reaction temperatures by reducing diffusion distances between reactants. This work studied the effect of using these synthetic methods to synthesize RE 2 Ti 2 O 7 (RE = Gd, Yb). The resistance to radiation induced damage of these materials was also investigated. Phase transitions in RE 2 Ti 2 O 7 were investigated using powder X-ray diffraction, X-ray absorption near-edge spectroscopy (XANES), and extended X-ray absorption fine structure. Changes to the structure of ion-implanted materials were investigated using glancing-angle (GA-)XANES. This study demonstrated that solution-based methods used to mix the reactants can allow for the formation of Yb 2 Ti 2 O 7 adopting primarily the pyrochlore-type structure when annealed at temperatures as low as 1000 °C while Gd 2 Ti 2 O 7 adopting the pyrochlore-type structure could be formed using annealing temperatures as low as 800 °C. It was also observed that changing the synthetic method, annealing temperature, annealing time, and having only the pyrochlore structure present or a mixture the pyrochlore- and defect-fluorite type structures did not affect the resistance to radiation induced structural damage of Yb 2 Ti 2 O 7 or Gd 2 Ti 2 O 7 .