Effect of carbonate content on the ESR spectrum near g=2 of carbonated calciumapatites synthesized from aqueous media

Abstract

The ESR spectrum of X-irradiated carbonated apatites synthesized at low temperature was studied as a function of their carbonate content. Using¹³C-enriched samples, four different carbonate-derived radicals and a surface O⁻ ion could be identified. Isotropic CO ₃ ⁻ and CO ₂ ⁻ ions are present at a B site in the apatite lattice, and anisotropic CO ₃ ⁻ and CO ₂ ⁻ radicals are located at the surface of the crystallites. Only the isotropic ESR signals increase with increasing carbonate content. The anisotropic signal ascribed to a surface CO ₂ ⁻ radical is mainly responsible for the so-called asymmetric ESR signal near g=2. It is argued that this surface signal may still be composite and caused by several very similar CO ₂ ⁻ ions. The consequences for phenomenological ESR studies of calcified tissues are discussed.