High-resolution structural study of zinc ion incorporation at the calcite cleavage surface

The atomic-scale structure of Zn²⁺ incorporated at the CaCO₃ (1014) surface by adsorption from solution was determined by X-ray standing wave triangulation and surface extended X-ray absorption fine structure spectroscopy. At low coverage (approximately 0.1 ML), Zn²⁺ substitutes for Ca²⁺ in the surface layer. Structural relaxation of the adjacent in-plane CO^2-₃ ions in the host surface is shown by the reduced nearest-neighbor distance of Zn-O relative to Ca-O. Relaxation of the Zn²⁺ ion in the out-of-plane direction is shown by the displacement of its lattice position from the ideal Ca²⁺ position. These relaxations, resulting in a local lattice buckling feature at the Zn²⁺ adsorption site, can be fully explained as the combined effect of the electrostatic relaxation of the nearest-neighbor anions in response to the smaller size of Zn²⁺, and the bonding asymmetry due to surface truncation.