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

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.