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

Abstract

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 U4+/U5+ oxidation states for argon‐annealed samples and U5+/U6+ oxidation states for air‐annealed samples. Single‐phase (Ca1.25U4+0.25U5+0.50)Ti2O7 (argon) and (Ca1.40U5+0.60)(Ti1.90U6+0.10)O7 (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 U6+ 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.

Pyrochlore based glass-ceramics for the immobilization of actinide-rich nuclear wastes: From concept to reality

“…Previous works on U pyrochlore [18][19][20] revealed that the Ca content of greater than unity was generally found in U pyrochlores sintered in argon or air with valence state of U > 4+. It has also been established that the presence of U 5+ /U 6+ in pyrochlores has no detrimental effect on their overall chemical durabilities [20].…”

“…It has also been established that the presence of U 5+ /U 6+ in pyrochlores has no detrimental effect on their overall chemical durabilities [20]. Therefore the U sample was designed targeting 0.7 f.u.…”

“…The EDS analysis confirmed: (1) there is <4 wt.% of UO 2 in glass and (2) the composition of the pyrochlore phase is close to Ca 1.25 U 0.75-Ti 2 O 7 , suggesting an average U valence state about 4.7+. Note a pre-vious neutron diffraction study [20] confirmed that both U 4+ and U 5+ ions are incorporated on the Ca site and the exact formula should be written as (Ca 1.25 U 4þ 0:25 U 5þ 0:5 )Ti 2 O 7 . The U L 3 -edge XANES spectra result from the dipole transition of the U 2p 3/2 electrons into the empty 6d states mixed in the conduction band; its edge position is known to be sensitive to the formal oxidation state of the U cations [23].…”

HIPed Tailored Pyrochlore-Rich Glass-Ceramic Waste Forms for the Immobilization of Nuclear Waste

“…XRD (Figure 2) showed the sample to contain pyrochlore, rutile and glass (diffuse signal in pattern centered around ~ 20 o 2θ). The [111] peak in the pyrochlore pattern however appears to be missing (~ 14 o 2θ) indicating that the pyrochlore formed in this glass-ceramic was not Ca 1.25 U 0.75 Ti 2 O 7 but Ca 1.4 U 0.6 Ti 1.9 O 7 [18]. SEM analysis (Figure 3) showed the sample to contain pyrochlore, rutile and glass and the EDS analysis confirmed that the pyrochlore present in the sample had a composition close to Ca 1.4 U 0.6 Ti 1.9 O 7 , although the small grain size of the pyrochlore made it difficult to obtain an accurate composition.…”

“…James et al [18] showed that it was possible to produce pyrochlore with the composition Ca 1.25 U 0.75 Ti 2 O 7 by solid state sintering methods in an Ar atmosphere and an air-sintered pyrochlore with the composition Ca 1.4 U 0.6 Ti 1.9 O 7 . Both pyrochlores had mixed U oxidation states (U 4+ , U 5+ and U 6+ ), as expected from charge balance considerations and experimental diffuse reflectance studies.…”