F+ centre in (Ca-Sr) carbonated hydroxyapatites

Powdered samples of synthetic carbonated hydroxyapatites with variable amounts of strontium substituting for calcium, obtained by precipitation, were studied by ESR at the X-band frequencies after heating at 400 deg C for 24 h, X-irradiation and annealing at 150 deg C for 30 min. The F+ centre spectrum exhibiting orthorhombic symmetry was recorded between RT and 120 K, with = approx. 2. When the amount of carbonate inside the samples increases, the spectral intensity has the same behaviour as that of the quantity of O2- vacancies calculated from results of chemical analysis. Our results are a confirmation of the existence of two different mechanisms of charge compensation when CO32- substitutes for PO43-, and they also show that Sr ions are not located in cation sites adjacent to carbonate ions. (author)