H. Tsuno, H. Kagi, Y. Takahashi
Dec 1, 2005
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Journal
Physica Scripta
Abstract
We measured XAFS spectra of trace amount of ytterbium (Yb) incorporated in calcite, a stable phase of calcium carbonate. Calcium carbonate was precipitated from a mixed solution of CaCl2 aq and NaHCO3 aq with a given amount of YbCl3. Concentrations of Yb were 5 μmol kg−1 in the starting solutions and Yb/Ca molar ratio in the precipitated calcium carbonate was 1.2 × 10−3. Ytterbium LIII-edge XAFS spectra were collected in the fluorescence mode at the beamline BL-12C of the KEK-PF. Analysis of the XAFS results indicated, (1) Approximately 15% Yb ion existed as divalent ions in the calcite. (2) Yb3+, major Yb species in the calcite, is located at the Ca2+ site in the calcite. The local structure around Yb3+ is significantly different from that of Ca2+. The coordination number of Ca2+ in the crystal structure of calcite is 6 and Ca–O distance is 2.36 Å. In contrast, the nearest Yb–O coordination for Yb-doped calcite is split into two shells, the nearest 4 oxygen atoms and the second nearest 2 oxygen atoms, and their Yb–O distances are 2.24 Å and 2.77 Å, respectively. It was shown that localized structural relaxation occurred around Yb3+ which has smaller ionic radius than Ca2+.