X- and Q-band EPR of carbonate derived radicals in calcium apatites synthesized at high temperatures and heated tooth enamel

Abstract

The EPR spectra near g = 2 of a few carbonated calcium apatites, synthesized at high temperatures are studied in X- and Q-band. Solid state reactions at high temperatures were used to prepare samples with carbonate substituting for OH⁻ (A-type) and PO₄³⁻ (B-type). In the present study the experience from previous X-band work on the same samples is combined with the higher resolution of Q-band experiments, to gain further insight into the composite nature of the spectra. X-Irradiation of carbonated calcium apatites gives rise to highly composite EPR spectra, due to the presence of different paramagnetic centres. Except for A-type carbonated apatite, the contribution of several types of CO₃³⁻ radicals dominate the spectra. The comparison between X- and Q-band results allows us to identify the different types of CO₃³⁻ (hydroxyl site, phosphate site, surface), together with a smaller CO₂⁻ component. Simulations of the experimental spectra support these assignments. A-type carbonated apatite spectra are dominated by one or more CO₂⁻ contributions. In order to elucidate the composite nature of the spectra, recent hydroxyapatite single crystal results are used to simulate the spectra, again in both frequency bands. Similar simulations of tooth enamel spectra and an evaluation of A-type carbonated apatite as a model system for tooth enamel are presented.