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

Shi-Kuan Sun; Lucy M Mottram; Thomas Gouder; Martin C Stennett; Neil C Hyatt; Claire L Corkhill

doi:10.1038/s41598-023-36571-w

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

Sci Rep. 2023 Jun 26;13(1):10328. doi: 10.1038/s41598-023-36571-w.

Authors

Shi-Kuan Sun^{1

2}, Lucy M Mottram¹, Thomas Gouder³, Martin C Stennett¹, Neil C Hyatt^{1

4

5}, Claire L Corkhill^{6

7}

Affiliations

¹ Immobilisation Science Laboratory, Department of Materials Science and Engineering, University of Sheffield, Sheffield, S1 3JD, UK.
² School of Material Science and Energy Engineering, Foshan University, Foshan, 528000, Guangdong, China.
³ European Commission, Joint Research Centre (JRC), Postfach 2340, 76125, Karlsruhe, Germany.
⁴ School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, 99164, USA.
⁵ School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK.
⁶ Immobilisation Science Laboratory, Department of Materials Science and Engineering, University of Sheffield, Sheffield, S1 3JD, UK. c.corkhill@sheffield.ac.uk.
⁷ School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK. c.corkhill@sheffield.ac.uk.

Abstract

Crystal chemical design principles were applied to synthesise novel U⁴⁺ dominant and titanium excess betafite phases Ca_1.15(5)U_0.56(4)Zr_0.17(2)Ti_2.19(2)O₇ and Ca_1.10(4)U_0.68(4)Zr_0.15(3)Ti_2.12(2)O₇, 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 CaUTi₂O₇ (r_A/r_B = 1.75). U L₃-edge XANES and U 4f_7/2 and U 4f_5/2 XPS data evidenced U⁴⁺ 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.

Abstract

Grants and funding