MAGNETIC-INTERACTIONS AND THE COOPERATIVE JAHN-TELLER EFFECT IN KCUF3

The ground-state electronic structure in various magnetic phases of the Jahn-Teller-aistorted perovskite KCuF3 has been investigated using the ab initio periodic unrestricted Hartree-Fock approach. The material is correctly predicted to be an orbitally ordered wide band-gap insulator with effectively one-dimensional magnetic properties, in that the estimated antiferromagnetic exchange coupling constant J(c) along the c axis is much larger than the ferromagnetic exchange constant J(a) perpendicular to this axis. The adiabatic potential-energy surface corresponding to cooperative distortions of CuF6 octahedra has the form of a classical Jahn-Teller double well, with the equilibrium distortion close to that observed experimentally. The interaction of the Jahn-Teller distortion with the superexchange interaction, which is responsible for the unusual magnetic behavior, is examined both through an analysis of the kinetic, Coulomb, and exchange contributions to the total energy, and from the dependence of magnetic properties on various geometrical parameters. Two structural polytypes with and without fluorine stacking disorder are found to be virtually isoenergetic, which is consistent with the experimental difficulty of preparing single-phase crystals. Charge- and spin-density maps, and densities of states are also reported.