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 abinitio 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 ca. +1.8 ∣e∣, respectively. The antiferromagnetic (AFM) is always more stable than the ferromagnetic (FM) phase; the energy difference ΔE=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–M angle approaches 90°. The super-exchange coupling constants, evaluated from ΔE 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.