Abstract

Electron paramagnetic resonance (EPR) spectroscopy, along with other analytical methods and techniques (X-ray diffraction, atomic absorption spectroscopy, scanning electron microscopy, etc.) have been used to study the mode of incorporation of divalent cations (Mg2+, Sr2+, Ba2+, Cu2+, Cd2+ and Pb2+) in vaterite. Vaterite, the least stable of the calcium carbonate polymorphs, was prepared by spontaneous precipitation from aqueous solutions and the doping of vaterite was performed by adding a particular divalent cation into a calcium chloride solution before mixing it with a sodium (or potasium) carbonate solution. All vaterite samples, pure or doped with divalent cations, were subjected to γ-irradiation. By studying EPR spectra of the pure vaterite powder the following free radicals were determined: CO3- holes freely rotating at the surface of the spherullites (g=2.0113); CO2- frozen in the matrix (g‖=2.0031 and g⊥=1.9998), and freely rotating CO2- (g=2.0006) probably located between the carbonate layers. It was found that the alkaline-earth metal cations (Mg2+, Sr2+, Ba2+) did not affect the formation and stability of the above free radicals. Cu2+ and Cd2+ doped to the vaterite phase trapped electrons by forming Cu+ and Cd+, respectively, which led, as a consequence, to increased formation of freely rotating CO3- holes in the near neighbourhood. Pb2+ released electrons readily by forming Pb3+ thus causing a decrease in formation of CO3- holes. In vaterite doped with Cd2+ two additional paramagnetic centers (ga=1.9886 and gb=1.9939), imbedded in two different sites in the vaterite structure were detected.

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