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 CaZrTi 2 O 7 ) is a candidate host phase. Herein, the solid solutions Ca 1-x Zr 1-x In 2x Ti 2 O 7 (0.10 ≤ x ≤ 1.00; air synthesis) and Ca 1-x U x ZrTi 2-2x In 2x O 7 (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 In 3+ substitution behaviour in the zirconolite phase across the Ca 2+ , Zr 4+ and Ti 4+ sites. When targeting Ca 1-x Zr 1-x In 2x Ti 2 O 7 , 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 In 2 Ti 2 O 7 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 In 3+ , consistent with targeted oxidation state. However, fitting of the EXAFS region using the zirconolite-2M structural model was consistent with In 3+ cations accommodated within the Ti 4+ site, contrary to the targeted substitution scheme. When deploying U as a surrogate for immobilised Pu in the Ca 1-x U x ZrTi 2-2x In 2x O 7 solid solution, it was demonstrated that, for both x = 0.05 and 0.10, In 3+ was successfully able to stabilise zirconolite-2M when U was distributed predominantly as both U 4+ and average U 5+ , when synthesised under argon and air, respectively, determined by U L 3 -edge XANES analysis.