Low-temperature synthesis of CaZrTi2O7 zirconolite-type materials using ceramic, coprecipitation, and sol–gel methods
Materials adopting the zirconolite-type structure have been investigated previously for use in many applications, including for the sequestration of nuclear waste. However, these materials are often formed using high-temperature methods (e.g., 1400 °C), which are not easy to scale up to produce large quantities. The effects of solution-based synthesis methods (coprecipitation and sol–gel) on the long- and short-range structures of zirconolite-type oxides annealed at various temperatures was investigated in the current study. Powder X-ray diffraction (XRD) was used to study the formation of the zirconolite phase at all temperatures studied (700 to 1400 °C). While the zirconolite structure was observed to be formed over the temperature range of 900 to 1400 °C, a mixture of zirconolite and defect fluorite phases were observed when the annealing temperature was 800 °C and only the defect fluorite structure was observed when the annealing temperature was 700 °C. Changes in the long-range structure of the zirconolite-type materials were evaluated by Rietveld refinement of the diffraction patterns. X-ray absorption near edge spectroscopy (XANES) was used to probe the local structure of Ti and Zr within the materials. Examination of the Ti K- and Zr K-edge XANES spectra showed no change in the local structure of the zirconolite-type materials annealed at temperatures as low as 900 °C while changes were observed when lower annealing temperatures were used owing to the presence of both the zirconolite and defect fluorite phases. It has been shown in this study that zirconolite-type materials can be synthesized at temperatures as low as 900 °C using coprecipitation or sol–gel methods due to the atomic scale mixing of reactants which occurs in solution. The radiation resistance of the materials synthesized using the ceramic, coprecipitation or sol–gel methods was investigated by implanting the materials with high energy Au ions and studying the changes in the local structure of these materials using glancing angle XANES. The resistance of these materials to radiation induced damage was found to be similar regardless of the synthesis method used to form them, which is important if these materials are to be used as nuclear waste forms.