Impurity structure in a molecular ionic crystal: Atomic-scale x-ray study of CaCO₃:Mn²⁺

The lattice sites and spatial disorder of isolated Mn²⁺ ions in calcite were examined with x-ray standing waves, and the structure of the surrounding ions was examined with extended x-ray absorption fine-structure spectroscopy. The Mn²⁺ ion is found to be on-center substitutional at the Ca²⁺ site, with spatial disorder comparable to that of Ca²⁺. The first-neighbor Mn-O distance is found to be the same as that in the isostructural MnCO3. The radial distance of the closest Mn-Ca shell is reduced by ∼2%from the undistorted Ca-Ca distance. Based on these measurements, an atomic-scale structural model of the Mn²⁺ site suggests that the intramolecular distortion in the first-neighbor CO^2-₃ anions plays a key role in establishing the conserved first-neighbor Mn-O distance while maintaining ordering with respect to the lattice. The CaCO₃:Mn²⁺ structure is shown to be characteristically distinct from those of analogous impurities in monatomic ionic crystals.