The safe immobilisation of high-level nuclear waste is a critical challenge for the nuclear industry. Pyrochlore oxides with the general formula A2B2O7 are promising nuclear waste forms due to their structural and chemical stability, radiation tolerance, and compositional flexibility. This project investigates the structural evolution of charge-compensated uranium-doped lanthanum zirconate pyrochlores, La2(UxBxZr1-2x)2O7 (B = Mg2+, Ca2+), designed to enhance uranium solubility by maintaining charge-neutrality.
A coprecipitation synthesis route was used to prepare the two series with 0 ≤ x ≤ 0.3. These materials were characterised using X-ray and neutron diffraction, X-ray absorption spectroscopy, and Raman spectroscopy to probe long- and short-range structural order. Rietveld refinement was performed using synchrotron X-ray and neutron diffraction data, confirming a composition-driven phase transition from ordered pyrochlore (Fd-3m) to defect fluorite (Fm-3m) between 0.2 < x < 0.3.
Zr L-edge XAS revealed gradual disorder of the Zr4+ cation, consistent with partial substitution of Zr onto the 8-coordinate pyrochlore A-sites. Raman spectra also indicated the emergence of uranyl-like (O = U = O) bonding, consistent with such disorder and resulting in non-linear changes in the A-O bond lengths upon doping.
These results demonstrate that charge-compensated doping enhances the solubility limit of uranium while retaining the structural integrity of the pyrochlore structure. These findings provide new insight into the structural evolution of these materials and will inform the future design of stable host materials for long-term nuclear waste storage.