A comparison of crystalline and molten structures of zirconolite (CaZrTi₂O₇), a potential plutonium wasteform medium, by molecular dynamics simulation and topological analysis

Abstract

Molecular dynamics simulations of the ceramic compound zirconolite (CaZrTi₂O₇), a potential crystalline wasteform host for plutonium, were carried out for ideal and experimental crystalline forms and a simulated molten state, and the connectivities of the resulting structures were compared. Local primitive-ring topological clusters were determined for individual atoms, and averages of ring counts were calculated for atom types within each form of zirconolite. The ideal crystalline structure and the best experimental structure, deduced by Rossell from neutron diffraction data, proved very similar, though the Rossell local clusters contained small variations from the ideal. Molten zirconolite appeared very different; it exhibited much larger ring counts and local clusters, together with a tendency for Ca and Ti (but not Zr) cation clustering. The technique of looking at ring counts for individual atoms was found to be very sensitive to small changes in the structure, though more suited to comparison of the two crystalline structures because of their uniformity. Significant connectivity differences and heterogeneity in the molten structure were best compared by considering the average local cluster. The structure of metamict zirconolite, amorphized by [alpha]-recoil of incorporated waste actinides, is conjectured to exhibit some characteristics of both crystalline and molten forms, likely stabilized by polymerization of cation coordination units signaled by the observed clustering of like Ca and Ti cations observed in the molten state.