Periodic approach to the electronic structure and magnetic coupling in KCuF3, K2CuF4, and Sr2CuO2Cl2 low‐dimensional magnetic systems

Abstract

AbstractThe ab initio periodic unrestricted Hartree–Fock method and several generalized gradient algorithm (GGA) and hybrid density functional theory (DFT) approaches have been used to investigate the ground‐state electronic and magnetic properties of three low‐dimensional magnetic copper insulators, namely, KCuF3, K2CuF4, and Sr2CuO2Cl2. A Gaussian atomic basis set optimized for the crystal environment has been used to construct the crystalline orbitals in the LCAO approximation as implemented in the CRYSTAL code. The interaction between magnetic moments on Cu2+ ions (d9 local configuration) strongly depends on the crystal structure and hence on the resulting electronic structure: Sr2CuO2Cl2 is representative of 2‐D antiferromagnetic systems, closely related to the high‐Tc parent cuprate superconductors; K2CuF4 is a 2‐D ferromagnetic system and KCuF3 behaves as a quasi‐1‐D antiferromagnetic system. The most stable electronic state, the relevant magnetic coupling constants, and the magnetic form factors are calculated by means of the various Hamiltonians. The systems turn out to be large band‐gap insulators in the UHF and hybrid DFT approximations, whereas they become metallic or narrow band‐gap semiconductors in the local density approximation or GGA approximations. UHF gives a qualitatively correct description of this kind of magnetic insulators and, more important, good relative values for the relevant magnetic coupling constants. Hybrid functionals (like B3LYP) largely improve, with respect to the other Hamiltonians, the magnitude and nature of the band‐gap, the spin densities on the Cu2+ ions, and the most important magnetic coupling constants, providing a reasonable, semiquantitative description of this kind of strongly correlated materials. In addition, those functionals correctly account for the distortions due to the Jahn–Teller effect in KCuF3, in contrast to LDA and PW‐GGA, where no stabilization appears when the distortion is included. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004