The topological analysis of the electron density in the perovskite KNiF3, potassium nickel trifluoride, based on the accurate X-ray diffraction data, has been performed. The topological picture of the atomic interactions differs from that resulting from the classic crystal chemistry consideration. The shapes of atoms in KNiF3 defined by zero-flux surfaces in the electron density are, in general, far from spherical. At the same time, their asphericity in the close-packed layer is very small. The topological coordination numbers of K and Ni are the same as the geometrical ones, whereas topological coordination for the F atom (6) differs from the geometrical value. The latter results from a specific shape of the Ni-atom basin preventing the bond-path formation between F atoms in the same atomic close-packed layer, in spite of the fact that the closest F—F distance is the same as K—F. Judging by the electron density value and curvature at the bond critical points, the K—F interaction in KNiF3 can be considered ionic, while the Ni—F bond belongs to the polar covalent type. No correlation of the topological ionic radii with crystal or ionic radii was found in KNiF3. Critical points in the electrostatic potential have also been studied.
An Attempt to Image Chemical Ordering in Close-Packed Layer of Mg–Zn–Y 18R Long-Period Stacking-Ordered Structure by Scanning Tunneling Microscopy
