Ab InitioCalculations on KNiF3: Ligand-Field Effects

Restricted Hartree-Fock molecular-orbital calculations have been carried out for various states of the cluster Ni${\mathrm{F}}_6^{4-}$ "in vacuo" and in a surrounding of several sets of point charges representing the perovskite lattice KNi${\mathrm{F}}_3$. All electrons were included. A "double-$\zeta$" basis set of contracted Gaussian orbitals has been used. The calculations were performed with the computer program IBMOL IV. The Hartree-Fock approximation gives a reasonable description of the covalency effects and the spectral properties of KNi${\mathrm{F}}_3$. The calculated hyperfinefield parameters $f_s$ and $f_{\sigma }$ and the $10Dq$ value are about 10-25% smaller than the experimental values. Similar deviations were found for the spectral transition energies. It is argued that magnitude and sign of the crystal field splitting in this compound can be understood in terms of the well-known ionic electrostatic model provided the Born repulsion is properly taken into account.