Nitrogen-containing species in the structure of the synthesized nano-hydroxyapatite

Synthesized by the wet chemical precipitation technique, hydroxyapatite (HAp) powders with the sizes of the crystallites of $20$–$50$ nm and $1$ ${\mu}$m were analyzed by different analytical methods. By means of electron paramagnetic resonance (EPR) it is shown that during the synthesis process nitrate anions from the reagents (by-products) could incorporate into the HAp structure. The relaxation times and EPR parameters of the stable axially symmetric NO$_3^{2-}$ paramagnetic centres detected after $X$-ray irradiation are measured with high accuracy. Analyses of high-frequency ($95$ GHz) electron-nuclear double resonance spectra from $^1$H and $^{31}$P nuclei and $ab$ initio density functional theory calculations allow suggesting that the paramagnetic centres and nitrate anions as the precursors of NO$_3^{2-}$ radicals preferably occupy PO$_4^{3-}$ site in the HAp structure.