XAFS study on the trace amounts of ytterbium ions incorporated in calcium carbonate crystal

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 L _III-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) Yb ³⁺, major Yb species in the calcite, is located at the Ca ²⁺ 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⁺.