Electron density of KNiF₃: analysis of the atomic interactions

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.