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 (10 (1) over bar 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 Angstrom. The Co2+ ions remain octahedrally coordinated, with a first-neighbor Co-O bonding distance of 2.11 Angstrom. 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.