Computational study of the substitution of early actinides and Ce into zirconolite

Substitutions of tetravalent early actinides (Th – Am) and Ce into the ceramic nuclear waste form zirconolite, CaZrTi2O7, have been analysed using hybrid Density Functional Theory in conjunction with the Periodic Electrostatic Embedded Cluster Method. Substitutions on to both Zr4+ and Ca2+ sites were modelled, with Fe3+ and Al3+ used for charge balancing. The mean displacements of a central region of ions in the cluster indicated redox behaviour which was then confirmed through analysis of spin density data. Strong correlations were found between the substitution energies of these ions and the ratio of the ionic radii of substituents and substituted ions. Similar redox behaviour and substitution energies were seen for substitutions of Ce4+ and Pu4+ into the lattice. Spin density data of molecular M(OH)n (M = Ce, Th - Am, n = 3–5) showed a preference for the +3 oxidation state for later actinides - as seen in the zirconolite clusters.