EPR study of radiation-induced defects in carbonate-containing hydroxyapatite annealed at high temperature

Abstract

Radiation-induced (γ or UV) paramagnetic defects in carbonate-containing hydroxyapatite (HAP) annealed at high (600–950 °C) temperature were studied by EPR. The complex spectra reveal the presence of different paramagnetic species. Their contributions were found to be strongly dependent on the annealing temperature as well as microwave power, thus, by the adjustment of experimental conditions some of the components can be eliminated that allowed to record EPR spectra caused by no more than two types of paramagnetic defects. All experimental spectra were analyzed using computer simulation. The parameters of the paramagnetic defects detected were determined, and the centers models were discussed. It was found that high-temperature annealing influences essentially the formation of radiation-induced defects in HAP. The СО33−, О− centers and oxygen vacancy VO− were shown to be the main stable γ-induced defects in the HAP annealed at high temperatures. New paramagnetic defect with the parameters g|| = 2.002, g⊥ = 2.0135 was detected and tentatively identified as an O-related radical. The γ-induced EPR response from СО33− radicals was found to be more intense than response from CO2− in non-annealed HAP. UV-irradiation was found to create smaller amounts of paramagnetic defects in comparison with γ-rays. Besides, oxygen vacancy VO− was not observed, while two other centers (СО3− and the center of unknown nature) appear in the UV-induced EPR spectra.