Site selectivity of dopant cations in Ca3(SiO4)Cl2

A series of static lattice calculations were performed to determine the site selectivity of cations of differing size and valence when substituted onto the Ca sites of the calcium chlorosilicate (Ca3(SiO4)Cl2) lattice, a potential host phase for the immobilisation of halide-rich wastes arising from the pyrochemical reprocessing of plutonium. Atomic-scale simulations indicate that divalent cations are preferentially substituted onto the Ca1 site, whilst tri- and tetravalent cations are preferentially hosted on the Ca2 site, with the Ca1 site favoured for forming the vacancies necessary to charge-balance the lattice as a whole. Multi-defect calculations reveal that the site selectivity of the dopant cations is dependent on their ionic radii; as the ionic radii of the divalent cations increase, substitution onto the preferred site becomes more and more strongly favoured, whereas the inverse is true of the trivalent cations.