Microstructure and composition of synroc samples crystallized from a CaCeTi₂O₇ chemical system: HRTEM/EELS investigation

Ce-pyrochlore, CaCeTi₂O₇ is a chemical analogue for CaPuTi₂O₇, which is a proposed ceramic endmember waste form for the disposition of excess weapon-usable plutonium in geological repositories. Ce-pyrochlore was synthesized by firing and annealing in air a mixture of CeO₂, TiO₂, and CaCO₃ with a stoichiometry of CaCeTi₂O₇. The annealed products contain Ce-pyrochlore, Ce-bearing perovskite, CeO₂, and minor CaO. The mixture annealed at a temperature of 1140 °C contains more pyrochlore phase than that annealed at a higher temperature (1300 °C), indicating that a low temperature condition favors the formation of the Ce-pyrochlore. The Ca/Ce ratio of the pyrochlore is slightly lower than the ideal ratio (one). Electron energy-loss spectroscopy results show that there is a small fraction of Ce³⁺ present in the pyrochlore. Ce present in perovskite is dominated by Ce³⁺. High-resolution TEM images show that the boundary between pyrochlore and perovskite is semi-coherent. No glassy phases were observed at the grain boundary between pyrochlore and perovskite, nor between CeO₂ and pyrochlore. It is postulated, based on the presence of trivalent Ce in the Ce-pyrochlore, that neutron poisons such as trivalent cation Gd would be incorporated into the CaPuTi₂O₇ phase.