Crystal Chemistry and Structures of (Ca,U) Titanate Pyrochlores

Analysis of multiphase (Ca,U) titanate pyrochlore samples produced under argon or in air, using laboratory X‐ray powder diffraction (XRD) and electron microscopy revealed discrete pyrochlore phase compositions, together with either perovskite or rutile impurities. Investigations of these samples using diffuse reflectance and X‐ray photoelectron spectroscopies revealed mixed U 4+ /U 5+ oxidation states for argon‐annealed samples and U 5+ /U 6+ oxidation states for air‐annealed samples. Single‐phase (Ca 1.25 U 4+ 0.25 U 5+ 0.50 )Ti 2 O 7 (argon) and (Ca 1.40 U 5+ 0.60 )(Ti 1.90 U 6+ 0.10 )O 7 (air) compositions were synthesized and characterized using a combination of synchrotron X‐ray and neutron powder diffraction. Severe intensity reduction of hkl odd XRD peaks from the sample produced in air was found to be the result of U 6+ sharing the Ti site. The refined crystal structures for both single‐phase samples were found to be consistent with the compositions and mixed oxidation states observed by the above spectroscopic measurements. Although the normalized uranium leach rate of the argon‐annealed sample was found to be approximately twice that of the air‐annealed sample, in 7‐day Product Consistency Test the durability of both compounds with respect to leaching by water was found to be excellent. Ca releases were in the range of 0.01–0.03 g/L and U releases were <3 × 10 −5 g/L.