A multimodal X-ray spectroscopy investigation of uranium speciation in ThTi2O6 compounds with the brannerite structure

Abstract

ThTi2O6 derived compounds with the brannerite structure were designed, synthesised, and characterised with the aim of stabilising incorporation of U5+ or U6+, at dilute concentration. Appropriate charge compensation was targeted by co-substitution of Gd3+, Ca2+, Al3+, or Cr3+, on the Th or Ti site. U L3 edge X-ray Absorption Near Edge Spectroscopy (XANES) and High Energy Resolution Fluorescence Detected U M4 edge XANES evidenced U5+ as the major oxidation state in all compounds, with a minor fraction of U6+ (2–13%). The balance of X-ray and Raman spectroscopy data support uranate, rather than uranyl, as the dominant U6+ speciation in the reported brannerites. It is considered that the U6+ concentration was limited by unfavourable electrostatic repulsion arising from substitution in the octahedral Th or Ti sites, which share two or three edges, respectively, with neighbouring polyhedra in the brannerite structure.

Long-term, sustainable solutions to radioactive waste management

“…In this collection, Corkhill et al 2 investigated titanate ceramics with the pyrochlore structure using photoemission and X-ray absorption spectroscopy (XAS) and showed the possibility of chemical flexibility in terms of crystal-chemical design principles of pyrochlore structure. Additionally, Corkhill et al 3 explored the mineral brannerite UTi 2 O 6 as a possible host phase for the immobilization of long-lived actinides, such as plutonium. Their investigation demonstrated that the ThTi 2 O 6 brannerite structure can incorporate a small fraction of U 6+ alongside a more significant inventory of U 5+ , achieved through charge compensation on the Th or Ti site.…”