Mode and sites of incorporation of divalent cations in vaterite

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 (g_a=1.9886 and g_b=1.9939), imbedded in two different sites in the vaterite structure were detected.