Preparation of magnesium-substituted hydroxyapatite powders by the mechanochemical–hydrothermal method

Magnesium-substituted hydroxyapatite (Mg-HAp) powders with different crystallinity levels were prepared at room temperature via a heterogeneous reaction between Mg(OH)2/Ca(OH)2 powders and an (NH4)2HPO4 solution using the mechanochemical–hydrothermal route. The as-prepared products contained unreacted Mg(OH)2 and therefore had to undergo purification in ammonium citrate aqueous solutions at room temperature. X-ray diffraction, infrared spectroscopy, thermogravimetric and chemical analyses were performed and it was determined that the purified powders were phase-pure Mg-HAp containing 0.24–28.4 wt% of Mg. The concentration of Mg was slightly lower near the surface than in the bulk of the HAp crystals as indicated by X-ray photoelectron spectroscopy. Dynamic light scattering revealed that the median particle size of the room temperature Mg-HAp powders was in the range of 102 nm–1.2 μm with a specific surface area between 91 and 269 m2/g. Scanning electron microscopy confirmed that the Mg-HAp powders consisted of submicron agglomerates of nanosized crystals, less than ≈20 nm.