Local structure of Co²⁺ incorporated at the calcite surface: An x-ray standing wave and SEXAFS study

Following adsorption from a dilute water solution, the lattice site and first-neighbor bonding distances of Co²⁺ ions incorporated at the calcite (101¯4) surface were determined with atomic resolution by the combination of x-ray standing wave triangulation and polarization-dependent surface extended x-ray absorption fine-structure spectroscopy. The incorporated Co²⁺ ions selectively occupy the Ca²⁺ lattice sites with an inward relaxation of 0.34 Å. The Co²⁺ ions remain octahedrally coordinated, with a first-neighbor Co-O bonding distance of 2.11 Å. The octahedral coordination suggests that a coadsorbed species from the solution remains bonded to the Co²⁺ ion above the surface. The structure of Co²⁺ incorporated at the calcite surface is successfully described by a model in which the Co²⁺ sites are mainly determined by relaxation due to surface lattice asymmetry, and the first-neighbor Co-O relaxation by reconformation of the adjacent carbonate molecules.