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.