Pyrochlore is a complex oxide with general formula VIIIA2
X (X – additional anion in specific position, usually O2-). It has a fluorite-derived structure (space group Fd3m) where one-eighth of oxygens are missing. Two different structural sites for cations exist, resulting in the cell parameter of pyrochlore is twice that of the fluorite lattice. Some pyrochlore-based actinide waste forms have been produced in the systems: CaO-CeO2-TiO2, CaO-UO2(ThO2)-ZrO2, CaO-UO2(ThO2)-Gd2O3-TiO2-ZrO2, and CaO-ThO2-SnO2. The oxide precursors were cold pressed at 200 – 400 MPa and sintered at 1500 – 1550 °C for 6-10 hours in air or at 1300 – 1350 °C for 5-50 hours in pure oxygen (Ce-doped batches only). The use of pure oxygen was used to try to put Ce in the tetravalent state required to form a pyrochlore.
The products were examined with XRD, SEM/EDS, and TEM. In the ceramics with bulk compositions: CaCeTi2O7, CaThZr2O7, (Ca0.5GdTh0.5)(TiZr)O7, (Ca0.5GdU0.5)(TiZr)O7, and CaThSn2O7, pyrochlore was the major phase, but fluorite-structured oxide was also present. In the samples with target compositions CaUZr2O7 and (Ca0.5GdU0.5)Zr2O7 pyrochlore was not identified, rather a fluorite-structured oxide appeared. Different occurrences of pyrochlore and defect fluorite in the ceramics produced are due to crystal chemical constraints.