First-principles study of substitutional magnesium and zinc in hydroxyapatite and octacalcium phosphate.

First-principles calculations are performed for Mg(2+) and Zn(2+) substitution in hydroxyapatite (HAp) and octacalcium phosphate (OCP), because the foreign ions are known to play an important role for bone formation. In order to study their possible location in the system of HAp in contact with the aqueous solution, OCP is considered as a structural model of the transition region between HAp and the solution. It is found that, when the foreign ions substitute for Ca sites, the surrounding oxygen ions undergo considerable inward relaxation, due to their smaller ionic sizes than Ca(2+), which results in the smaller coordination numbers with oxygen as compared with those of Ca in bulk HAp and OCP. From the calculated defect formation energies, it is likely that the substitutional foreign ions are quite difficult to dissolve into HAp whereas can be more easily incorporated in OCP. In particular, Zn(2+) can more favorably substitute for the specific Ca site of OCP, as compared to Mg(2+), which is attributed with covalent bond formation between Zn and the surrounding oxygen ions. It is thus considered that zinc may play its role to promote bone formation by being incorporated into the transition region between HAp and the surrounding solution.