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; P6_3/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.