EPR and ENDOR studies on CO2− radicals in γ-irradiated B-type carbonated apatites

Abstract

A gamma-irradiated synthetic B-type carbonated apatite calcined at 300°C was studied by EPR and ENDOR. The room temperature EPR spectrum is composed of an axially symmetric species (g_⊥=2.0025 and g_‖=1.9971) attributed to rotating CO₂⁻ radicals. At low temperatures (T<10 K), the CO₂⁻¹H and ³¹P ENDOR spectra were obtained for different magnetic field settings. Computer simulations of these ENDOR spectra, based on the ‘‘molecular orientation–selection’’ principle, enabled one to allocate the CO₂⁻ radicals in the carbonated apatite lattice. It was established that the CO₂⁻ radicals are located on PO₄³⁻ sites with a vacancy on the nearest OH⁻ site. In addition, discussions related to the substitution mechanisms and the nature of the precursors of the CO₂⁻ radicals are presented.