Synthesis and structural characterization of Fe3+-doped calcium hydroxyapatites: role of precursors and synthesis method

This study evaluates the role of Ca and P precursors and of two different preparation methods, ceramic and hydrothermal, in the synthesis of undoped- and Fe3+-doped hydroxyapatite (HA), and tests the Fe3+ solubility limits in HA. Previous studies have noted that different synthesis conditions lead to different products in variable amounts and recent literature quotes the highest amount of Fe3+ substituting for Ca in the HA structure in the range 0.2-0.5 as a function of temperature. Samples were characterized by X-ray powder diffraction, Mossbauer and Infrared spectroscopies. The samples obtained by the ceramic method at 800 degrees C contain mixtures of B-type carbonate-apatite and other phosphates whose type and number depend on Ca and P precursors. Samples synthesized at higher temperatures contain mixtures of hydroxyapatite, tricalcium phosphate and of magnetic phases, as haematite and ferrite, whose relative amounts depend on nominal composition and thermal treatment. Samples obtained by a hydrothermal route are richer in HA phase with respect to the solid state one, even if the obtained HA percentage depends on the pH. Mossbauer results suggest that Fe3+ is randomly distributed between Ca1 and Ca2 sites. At the highest doping levels a reduction takes place with the formation of iron(II) and magnetite.