Characteristics of synthetic hydroxyapatites and attempts to improve their thermal stability

Synthetic hydroxyapatites (HA) with widely different crystallinity degrees were prepared starting from calcium hydroxide and phosphoric acid. The powders were carefully characterised by various physico-chemical techniques. Additions of Ca(OH)2 were performed in order to improve HA thermal stability to firing treatments up to temperatures significantly higher than those commonly employed for safe HA sintering. A positive effect in this sense was found, yielding HA almost pure samples up to 1400–1450°C and maintaining considerable HA concentrations up to 1600°C. An explanation in terms of stoichiometric defectiveness, linked to the quantitatively estimated contemporary presence of HPO42− and CO32− substituting PO43− and OH−, was attempted.