Crystal chemical design, synthesis and characterisation of U(IV)-dominant betafite phases for actinide immobilisation

Crystal chemical design principles were applied to synthesise novel U(4+) dominant and titanium excess betafite phases Ca(1.15(5))U(0.56(4))Zr(0.17(2))Ti(2.19(2))O(7) and Ca(1.10(4))U(0.68(4))Zr(0.15(3))Ti(2.12(2))O(7), in high yield (85–95 wt%), and ceramic density reaching 99% of theoretical. Substitution of Ti on the A-site of the pyrochlore structure, in excess of full B-site occupancy, enabled the radius ratio (r(A)/r(B) = 1.69) to be tuned into the pyrochlore stability field, approximately 1.48 ≲ r(A)/r(B) ≲ 1.78, in contrast to the archetype composition CaUTi(2)O(7) (r(A)/r(B) = 1.75). U L(3)-edge XANES and U 4f(7/2) and U 4f(5/2) XPS data evidenced U(4+) as the dominant speciation, consistent with the determined chemical compositions. The new betafite phases, and further analysis reported herein, point to a wider family of actinide betafite pyrochlores that could be stabilised by application of the underlying crystal chemical principle applied here.