Magnesium doped hydroxyapatite: Synthesis and characterization

Hydroxyapatite (HA) represents a good model system for biological apatites which are characterized by poor crystallinity and non stoichiometry due to the presence of significant amount of foreign ions. Some of these ions are known to enter into the HA crystal lattice, others have been suggested to be only absorbed on the apatite surface. X-ray diffraction, infrared absorption, and chemical investigations have been performed on hydroxyapatite synthesized in the presence of different magnesium concentration. Magnesium inhibits the crystallization of hydroxyapatite through a reduction of Ca / P molar ratio and crystal size of apatite; contemporarily it increases the nucleation rate of HA causing a trapping of CO₃ ^2- ions into the apatitic structure mainly in B position. The samples become completely amorphous between 35 and 50 Mg atom percent, at higher magnesium concentration different crystalline phases form. The results of the X-ray powder pattern fitting indicate that the HA crystal structure at most hosts magnesium amount of about 13% mol. Magnesium substitutes preferentially one of the two different calcium atoms positions: in our case the favourite substitution is in Ca (1), i.e. the Ca atoms lying within the phosphate groups columns (4f). On the basis of these results, the key role of magnesium on the crystallization, crystal growth, and thermal stability of hydroxyapatite has been used to explain the relevant properties of biological apatites.