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X-Ray Diffractometry and X-Ray Photoelectron Spectroscopy Investigations of Nanocrytalline Hydroxyapatite Synthesized by a Hydroxide Gel Technique

Nanocrystalline hydroxyapatite (HA), Ca10(PO4)6(OH)2, has been synthesized by a precipitate-conversion technique using hydroxide gel. Hydroxyapatite crystallizes in a hexagonal structure (space group; P63/m) having lattice parameters; a=9.407 Å and c=6.883 Å, and around 37 nm in size for the 800°C-annealed sample. Substantial crystalline characteristics are observed for the material heat-treated at 80°C. With the increase of air-annealing temperature, from 80°C to 800°C, lattice expansion along all crystallographic axes occurs indicating a structural change in the HA lattice. These results have been attributed to (1) the formation of a relative Ca2+ deficiency in HA that incorporates various chemical species in the anionic sites compared to their lower temperature air-annealed counterparts, and (2) the precipitation of CaO with increasing air-annealing temperature. The X-ray photoelectron spectroscopy studies indicate a distorted anionic structure and two electronic states for the O1 s, corroborating the observed X-ray diffraction results.