Synthesized by the wet chemical precipitation technique hydroxyapatite powders (HAp) with the sizes of the crystallites of (20-10 50) nm and 1 m were analyzed by multi-frequency (9 and 95 GHz) electron paramagnetic resonance (EPR) methods. 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 axially symmetric NO 3 2paramagnetic centers detected after X-ray irradiation of the product at room temperature are measured with high accuracy. Assignments of the observed EPR spectra as compared with that given in literature are discussed. Analyses of electron-nuclear double resonance (ENDOR) data from 1 H and 31 P nuclei and ab-15 initio density functional theory (DFT) calculations allow suggesting that the paramagnetic centers and nitrate anions as the precursors of NO 3 2radicals preferably occupy PO 4 3site in the HAp structure. 65 110 Troullier-Martins pseudopotentials [27] with the plane-wave cutoff energy of 70 Ry.

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Incorporation of carbonate ions to the crystal structure of carbonated hydroxyapatite (CHAp) leads to the formation of point defects (vacancies) in Ca-and OH-sublattices as well as to microstrains revealed in CHAp nanocrystals. Various techniques, such as XRD, FTIR, TEM, FESEM/EDX, TG/DTA, AES (ICP), wet chemical analysis, Ca-ionometry, microvolumetric analysis of evolved CO 2 , BET adsorption, were applied to determine an efficiency of carbonate substitution, and to quantify the elemental composition, as well as to characterize the structure of the carbonated hydroxyapatite and the site(s) of carbonate substitution. It was shown that there is insignificant incorporation of Na into the crystal structure of HAp. Over the range of 0 -4 % wt. (x<0.25), the substitution of OH-by CO 3 2-takes place leading to A-Type of CHAp, further increase of CO 3 2--content enhances PO 4 3--substitution giving AB-type of CHAp. According to in vitro test, the bioactivity of the samples is increasing with the growth of carbonate content due to accumulation of the defects in CHAp nanocrystals.

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