V. Noethiglaslo et L. Brecevic, MODE AND SITES OF INCORPORATION OF DIVALENT-CATIONS IN VATERITE, Journal of the Chemical Society. Faraday transactions (Print), 94(14), 1998, pp. 2005-2009
Citations number
23
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
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.