α- and β-tricalcium phosphate:: A density functional study -: art. no. 205205

Biologically important alpha- and beta-tricalcium phosphates (TCPs) have been investigated using ab initio density functional calculations. alpha- and beta-TCP have particularly large unit cells amounting to 312 and 273 atoms, respectively. The relationship between alpha-TCP and its three subcells, as well as the influence of the distribution of the Ca vacancies apparently existing in beta-TCP have been studied. The calculated structural parameters for all the TCP phases, are in substantial agreement with experiment. The Ca-O distance varies continuously, while the P-O bonds distribute over a very narrow range. Oxygens hold the majority of the bonding electrons which reflects the ionic nature of the alpha and beta phases. The results suggest that beta-TCP is more stable than the alpha phase, and that beta-TCP with uniformly distributed Ca vacancies is the most stable structure. The three 1/3 alpha-TCP subcells relax to similar structures, and we found that the full alpha-TCP cell and its subcells have very similar stability, electronic and structural properties, suggesting that the subcell can be a good approximation for studying alpha-TCP.