The lattice sites and spatial disorder of isolated ${\mathrm{Mn}}^{2+}$ 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 ${\mathrm{Mn}}^{2+}$ ion is found to be on-center substitutional at the ${\mathrm{Ca}}^{2+}$ site, with spatial disorder comparable to that of ${\mathrm{Ca}}^{2+}.$ The first-neighbor Mn-O distance is found to be the same as that in the isostructural ${\mathrm{MnCO}}_3.$ The radial distance of the closest Mn-Ca shell is reduced by $\sim 2\%$ from the undistorted Ca-Ca distance. Based on these measurements, an atomic-scale structural model of the ${\mathrm{Mn}}^{2+}$ site suggests that the intramolecular distortion in the first-neighbor ${\mathrm{CO}}_3^{2-}$ anions plays a key role in establishing the conserved first-neighbor Mn-O distance while maintaining ordering with respect to the lattice. The ${\mathrm{CaCO}}_3:{\mathrm{Mn}}^{2+}$ structure is shown to be characteristically distinct from those of analogous impurities in monatomic ionic crystals.

• Received 6 October 2000

DOI:https://doi.org/10.1103/PhysRevB.63.144104