MODE AND SITES OF INCORPORATION OF DIVALENT-CATIONS IN VATERITE

Electron paramagnetic resonance (EPR) spectroscopy, along with other a nalytical methods and techniques (X-ray diffraction, atomic absorption spectroscopy, scanning electron microscopy, etc.) have been used to s tudy the mode of incorporation of divalent cations (Mg2+, Sr2+, Ba2+, Cu2+, Cd2+ and Pb2+) in vaterite. Vaterite, the least stable of the ca lcium carbonate polymorphs, was prepared by spontaneous precipitation from aqueous solutions and the doping of vaterite was performed by add ing a particular divalent cation into a calcium chloride solution befo re mixing it with a sodium (or potasium) carbonate solution. All vater ite samples, pure or doped with divalent cations, were subjected to ga mma-irradiation. By studying EPR spectra of the pure vaterite powder t he following free radicals were determined: (CO3-)-C-. holes freely ro tating at the surface of the spherullites (g = 2.0113); (CO2-)-C-. fro zen in the matrix (g(parallel to) = 2.0031 and g(perpendicular to) = 1 .9998), and freely rotating (CO2-)-C-. (g = 2.0006) probably located b etween the carbonate layers. It was found that the alkaline-earth meta l cations (Mg2+, Sr2+, Ba2+) did not affect the formation and stabilit y of the above free radicals. Cu2+ and Cd2+ doped to the vaterite phas e trapped electrons by forming Cu+ and Cd+, respectively, which led, a s a consequence, to increased formation of freely rotating (CO3-)-C-. holes in the near neighbourhood. Pb2+ released electrons readily by fo rming Pb3+ thus causing a decrease in formation of (CO3-)-C-. holes. I n vaterite doped with Cd2+ two additional paramagnetic centers (g(a) = 1.9886 and g(b) = 1.9939), imbedded in two different sites in the vat erite structure were detected.