Effects of a quantum-mechanical lattice on the electronic structure and d–d spectrum of the (MnF6)4− cluster in Mn2+ :KZnF3

Abstract

The electronic structure of the Mn2+ :KZnF3 impurity system has been computed by means of a Hartree–Fock–Roothaan cluster model. First, the Mn2+ center has been simulated by the (MnF6 )4− unit in vacuo. Then, the effects of the KZnF3 lattice have been included in the cluster calculation using three different lattice models. The well-known point–charge approximation has been compared with two rigorous quantum–lattice models derived from the ideas of the theory of electronic separability. In these two models the lattice ions are represented by an effective lattice potential and a lattice projection operator that enforces the cluster–lattice orthogonality. In the Coulomb or Hartree model the cluster–lattice exchange interactions are neglected. The ab initio model potential (MP) lattice model makes use of model potentials for representing the lattice ions and includes an accurate nonlocal exchange operator. According to the present results, the point–charge lattice model destroys the acceptable picture of the electronic ground state of the (MnF6 )4− unit obtained at the cluster-in vacuo stage, giving a continuously repulsive nuclear potential. The quantum–lattice models restore the bound ground state and give an equilibrium geometry qualitatively correct and quantitatively reasonable. The seven d–d electronic transitions observed in this system are computed within a quarter of eV after correcting the Hartree–Fock description with a configuration interaction limited to the octahedral configurations of the d5 problem and a semiempirical correlation-energy correction. Lattice effects on vertical transition energies turn out to be small. However, important indirect lattice effects are encountered in the theoretical spectrum as a consequence of the lattice-induced modification of the equilibrium geometry of the ground state.