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

The ground-state electronic structure in various magnetic phases of th e Jahn-Teller-aistorted perovskite KCuF3 has been investigated using t he ab initio periodic unrestricted Hartree-Fock approach. The material is correctly predicted to be an orbitally ordered wide band-gap insul ator 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) p erpendicular to this axis. The adiabatic potential-energy surface corr esponding 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-Tel ler distortion with the superexchange interaction, which is responsibl e for the unusual magnetic behavior, is examined both through an analy sis of the kinetic, Coulomb, and exchange contributions to the total e nergy, and from the dependence of magnetic properties on various geome trical parameters. Two structural polytypes with and without fluorine stacking disorder are found to be virtually isoenergetic, which is con sistent with the experimental difficulty of preparing single-phase cry stals. Charge- and spin-density maps, and densities of states are also reported.