Structural, electronic and magnetic properties of KMF3 (M = Mn, Fe, Co, Ni)

The structural, electronic and magnetic properties of the perovskite systems KMF3 (M = Mn, Fe, Co, Ni) have been investigated with CRYSTAL95, a periodic ab initio Hartree-Fock program. An all-electron Gaussian basis set has been used. The equation of state has been determined first for the cubic structure; then deviations from cubic symmetry have been explored, with the result that the Mn, Fe and Co systems are found to be slightly more stable in a tetragonal geometry. The systems are almost fully ionic, with net charges for K and M of + 1, ca. -0.9 and cn. + 1.8 \e\, respectively. The antiferromagnetic (AFM) is always more stable than the ferromagnetic (FM) phase; the energy difference Delta E = E(FM) - E(AFM) is shown: (a) to be additive with respect to the number of M-M first neighbours; (b) to increase with decreasing lattice parameter according to an inverse power law; and (c) to become zero when the M-(F) over cap-M angle approaches 90 degrees. The super-exchange coupling constants, evaluated from AE by using an Ising model hamiltonian, are in qualitative agreement with the experimental data (from 30% to 45% of the latter). Mulliken population data, charge and spin density maps and density of states are used to illustrate the electronic structure.