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.