Microstructure and Composition of a Ce-pyrochlore: A Chemical Analog for Pu-pyrochlore

摘要

Ce-pyrochlore (CaCeTi_2O_7), is a chemical analogue for CaPuTi_2O_7, which is a proposed ceramic waste form for deposition of excess weapon-usable Pu in geological repositories. Ce-pyrochlore was synthesized by firing and annealing a mixture of Ce(NO_3)_4, TiO_2, and Ca(OH)_2 with a stoichiometry of CaCeTi_2O_7 at 1,300 °C for 50 hours. The annealed product contains Ce-pyrochlore, Ce-bearing perovskite, CeO_2 (cerianite), and minor CaO. Electron energy-loss spectroscopy (EELS) was used to determine the valence of Ce in the synthesized materials using the shape of the Ce M_(4,5) edge. Cerium in the perovskite is dominated by Ce~(3+). The Ce~(4+)/∑Ce in the pyrochlore is 0.8, giving (Ca_(0.87)Ce_(0.20)~(3+)Ce_(0.86)~(4+)Ti_(0.05))Ti_2O_7. High-resolution TEM images show that the boundary between pyrochlore and perovskite is semicoherently bonded. The orientational relationship between the neighboring pyrochlore and perovskite is not random. There are no glassy phases observed at the grain boundaries between pyrochlore and perovskite, and between CeO_2 and pyrochlore. It is postulated, based on the presence of trivalent Ce in the Ce-pyrchlore, that the neutron poisons such as Gd can be incorporated into the CaPuTi_2O_7 phase.