Local structure of 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 (Formula presented) ions incorporated at the calcite (Formula presented) 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 (Formula presented) ions selectively occupy the (Formula presented) lattice sites with an inward relaxation of 0.34 Å. The (Formula presented) 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 (Formula presented) ion above the surface. The structure of (Formula presented) incorporated at the calcite surface is successfully described by a model in which the (Formula presented) 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.