Energetics of Ce and Pu incorporation into zirconolite waste-forms

The General Utility Lattice Program (GULP) has been used to model the zirconolite lattice, calculate the energies of substituting Ce(3+), Ce(4+), Pu(3+), Pu(4+) and Fe(3+) into the lattice both as single and multi-defect systems and model the formation of Ce(3+), Ce(4+), Pu(3+) and Pu(4+) doped zirconolite lattices. These results have been compared against experimental observations, with particular emphasis on those Ce containing solid solutions that exhibit Ce(3+)/Ce(4+) mixed valence characteristics. It is found that the Ce(3+)/Ce(4+) mixed valence is as a result of reduction within the lattice, with the Ce(3+) being stabilised on the Ca site, and that this behaviour would not be expected for the corresponding Pu solid solutions.