title: Combination of EPR measurements and DFT calculations to study nitrate impurities in the carbonated nanohydroxyapatite.
abstract: We demonstrate the application of the combined experimental-computational approach for studying the anionic impurities in hydroxyapatite (HAp). Influence of the carbonation level (x) on the concentration of the NO3(2-) radicals in the HAp nanocrystals of Ca10-xNax(PO4)6-x(CO3)x(OH)2 with x in the range 0 < x < 2 and average sizes of 30 nm is investigated by different analytical methods including electron paramagnetic resonance (EPR). Stable NO3(2-) radicals are formed under X-ray irradiation of nano-HAp samples from NO3(-) ions incorporated in trace amounts during the wet synthesis process. Density functional theory (DFT) based calculations show energetic preference for the PO4 group substitution by NO3(-) ions. Comparison of the calculated and experimental spectroscopic parameters (g and hyperfine tensors) reveals that EPR detects the NO3(2-) radicals located in the positions of the PO4 group only. It is shown that with the increase in x, the carbonate ions substitute the NO3(2-)/NO3(-) ions. DFT calculations confirm that carbonate incorporation in HAp structure is energetically more favorable than the formation of the nitrate defect.
authors: Institute of Physics, Kazan Federal University , Kremlevskaya Street 18, 420008 Kazan, Russian Federation.
authors: Timur
authors: T
authors: Biktagirov
authors: Marat
authors: M
authors: Gafurov
authors: Georgy
authors: G
authors: Mamin
authors: Elena
authors: E
authors: Klimashina
authors: Valery
authors: V
authors: Putlayev
authors: Sergei
authors: S
authors: Orlinskii
mesh_terms: D002254:Carbonates / Q000737:chemistry*; D017886:Durapatite / Q000737:chemistry* / Q000528:radiation effects; D005609:Free Radicals / Q000737:chemistry; D053758:Nanoparticles / Q000737:chemistry* / Q000528:radiation effects; D009566:Nitrates / Q000737:chemistry*; D009589:Nitrogen Oxides / Q000737:chemistry*; D011789:Quantum Theory; D013816:Thermodynamics; D014965:X-Rays
chemical_list: D002254:Carbonates; D005609:Free Radicals; D009566:Nitrates; D009589:Nitrogen Oxides; D017886:Durapatite