Simulation of Calcium Phosphate Materials for Radioactive Waste Applications

Abstract

Calcium phosphate materials are being considered for use in the immobilization of radiotoxic elements, particularly, those incorporated in legacy and orphan nuclear waste. Many of these elements exist as cations. This thesis describes atomic scale computer simulations that are used to predict cation distributions in calcium fluorapatite and β-tricalcium phosphate. Both quantum mechanical and classical potential approaches are used.

For the apatite structure, a range of isovalent cations are substituted at Ca²⁺ sites and energies of solution are calculated. Furthermore, configurational averaging has been used to predict the energies and lattice parameters associated with mixed calcium/strontium fluorapatites, Ca_10-xSr_x(PO₄)₆F₂. In particular, the preference of a Sr²⁺ ion to occupy a 6h rather than 4f cation site has been determined. This is used to establish the occupancy of 6h and 4f sites across the entire compositional range. The differences between the results of quantum mechanical simulations and classical simulations are also discussed.

For β-tricalcium phosphate, one of the 6a cation sites has previously been assigned half occupancy from fitting to Rietveld powder diffraction data. The different ways of arranging Ca²⁺ ions on the 6a sites, gives rise to a series of unique ordered configurations. The distributions of Ca²⁺ on the 6a site across both a 1x1x1 and 2x1x1 cell size are modelled. A comparison of different models is reported in terms of energy and structure. The mean field average occupancy is also simulated. In order to identify unequivocally the lowest energy structure of this material, it has been demonstrated that a larger cell size must be considered.