Atomistic computer modeling of the local structure and mixing properties of Sr-bearing fluorapatite

Abstract

The mixing properties and the local structure of the Ca-Sr fluorapatite isomorphic system were studied using the original atomistic computer modeling approach for solid solutions. Calculations were carried out for a 4 × 4 × 4 supercell (2688 atoms) in conjunction with the high-performance computer SKIF MSU Chebyshev. In order to find the most probable (energetically preferable) distribution of Sr over the M1 and M2 cation sites in the fluorapatite solid solution, eight different distributions were tested. Structures with the most preferable x(Sr1): x(Sr2) ratios were used for further investigation of the effect of mixing on the lattice parameters and elasticity of the Ca-Sr fluorapatite solid solution and for the detailed analysis of the local structure. The local structure analysis included investigation of the individual and average M1-O, M2-O, M2-F bond lengths distortions, changes in the M1 and M2 polyhedra volumes as function of Sr content as well as the estimation of the structural relaxation parameters for all compositions of the solid solutions considered in this work.