Underpinning the use of indium as a neutron absorbing additive in zirconolite by X-ray absorption spectroscopy

Indium (In) is a neutron absorbing additive that could feasibly be used to mitigate criticality in ceramic wasteforms containing Pu in the immobilised form, for which zirconolite (nominally CaZrTi₂O₇) is a candidate host phase. Herein, the solid solutions Ca_1-xZr_1-xIn_2xTi₂O₇ (0.10 ≤ x ≤ 1.00; air synthesis) and Ca_1-xU_xZrTi_2-2xIn_2xO₇ (x = 0.05, 0.10; air and argon synthesis) were investigated by conventional solid state sintering at a temperature of 1350 °C maintained for 20 h, with a view to characterise In³⁺ substitution behaviour in the zirconolite phase across the Ca²⁺, Zr⁴⁺ and Ti⁴⁺ sites. When targeting Ca_1-xZr_1-xIn_2xTi₂O₇, single phase zirconolite-2M was formed at In concentrations of 0.10 ≤ x ≤ 0.20; beyond x ≥ 0.20, a number of secondary In-containing phases were stabilised. Zirconolite-2M remained a constituent of the phase assemblage up to a concentration of x = 0.80, albeit at relatively low concentration beyond x ≥ 0.40. It was not possible to synthesise the In₂Ti₂O₇ end member compound using a solid state route. Analysis of the In K-edge XANES spectra in the single phase zirconolite-2M compounds confirmed that the In inventory was speciated as trivalent In³⁺, consistent with targeted oxidation state. However, fitting of the EXAFS region using the zirconolite-2M structural model was consistent with In³⁺ cations accommodated within the Ti⁴⁺ site, contrary to the targeted substitution scheme. When deploying U as a surrogate for immobilised Pu in the Ca_1-xU_xZrTi_2-2xIn_2xO₇ solid solution, it was demonstrated that, for both x = 0.05 and 0.10, In³⁺ was successfully able to stabilise zirconolite-2M when U was distributed predominantly as both U⁴⁺ and average U⁵⁺, when synthesised under argon and air, respectively, determined by U L₃-edge XANES analysis.