ENDOR STUDY OF AN O- ION OBSERVED IN X-RAY-IRRADIATED CARBONATED HYDROXYAPATITE POWDERS

A carbonated hydroxyapatite powder synthesized at high temperatures ha s been examined with electron paramagnetic resonance (EPR) and electro n-nuclear double resonance (ENDOR). In the EPR spectra, an intense sig nal assigned to an O- radical was observed. The interactions of the O- radical with three inequivalent sets of P-31 nuclei as well as with o ne set of protons were resolved in the ENDOR powder spectra. By a care ful analysis of the ENDOR powder spectra using computer simulations ba sed on the ''orientation-selection principle,'' a detailed model for t he O- radical and its surroundings could be derived. In this way, it w as established unambiguously that the O- ion is located on a hydroxyl site. In addition, experimental evidence was found that the precursor of the O- radical is a carbonate group, and not a hydroxyl group. Next to the O- radical, EPR and ENDOR resonances of another paramagnetic c enter were observed. After analysis of the spectra and by comparing th e results with previous work, this radical was assigned to a CO33- ion located at a phosphate site.