Ab initio simulation of Si-doped hydroxyapatite

Si-doped hydroxyapatite is a bioceramic useful as a bone repair material retaining the hexagonal structure of hydroxyapatite up to about 2 wt % Si. Different mechanisms for charge compensation for the SiO44- ion substituting for the PO43- have been proposed. Also, variations are reported in the dependence of the lattice parameters on the Si doping. These may be a result of different charge compensation mechanisms which may in turn depend on the method of preparation of the material. Calculation using ab initio total energy methods have been performed to investigate different mechanisms for charge compensation in Si-doped hydroxyapatite. Mechanisms involving an OH vacancy, an oxygen vacancy, and an additional hydrogen are studied. These mechanisms correspond to different degrees of dehydration and, consequently, depend on water partial pressure and chemical potential. Full relaxation of the atomic positions and the unit cell parameters was performed, and ground-state energies of the equilibrium structures, equilibrium lattice parameters, and atomic arrangements were obtained. The results indicate that which charge compensation mechanism is stable depends on the chemical potential of water. For small values of the water chemical potential the mechanism involving an OH vacancy is stable, but for larger values the mechanism leading to the formation of HSiO4 is Stable. The other mechanisms considered are unstable.