An EPR study of Cd2+ incorporation in vaterite

Abstract

¹³ C-vaterite, the unstable calcium carbonate polymorph, doped with Cd²⁺ was γ-irradiated and the stable paramagnetic centres were studied using electron paramagnetic resonance (EPR) spectroscopy. The ¹³C isotope has nuclear spin (I=1/2) and as a consequence hyperfine interaction with the unpaired electron occurs. This revealed all paramagnetic centres of carbonate origin, i.e. CO₃^- and CO₂^- free radicals. Two paramagnetic centres induced by cadmium doped to vaterite, one characterized by g_a=1.9939 and the other by g_b=1.9886, were not split by the ¹³C-hyperfine splitting. Therefore, these paramagnetic centres are most probably due to Cdâ†�O^-. The locations of these paramagnetic centres in vaterite are determined and the role of cadmium in the formation of calcite crystals in the vaterite-precipitating system is discussed.