Ab initiostudy of magnetic interactions inKCuF3andK2

Abstract

The ab initio cluster model approach has been used to study the electronic structure and magnetic coupling of KCuF 3 and K 2 CuF 4 in their various ordered polytype crystal forms. Due to a cooperative Jahn-Teller distortion these systems exhibit strong anisotropies. In particular, the magnetic properties strongly differ from those of isomorphic compounds. Hence, KCuF 3 is a quasi-one-dimensional ͑1D͒ nearest neighbor Heisenberg antiferromagnet whereas K 2 CuF 4 is the only ferromagnet among the K 2 M F 4 series of compounds ͑M ϭMn, Fe, Co, Ni, and Cu͒ behaving all as quasi-2D nearest neighbor Heisenberg systems. Different ab initio techniques are used to explore the magnetic coupling in these systems. All methods, including unrestricted Hartree-Fock, are able to explain the magnetic ordering. However, quantitative agreement with experiment is reached only when using a state-of-the-art configuration interaction approach. Finally, an analysis of the dependence of the magnetic coupling constant with respect to distortion parameters is presented. ͓S0163-1829͑99͒09831-8͔

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

“…B3LYP overestimates the J values by a factor of ∼ 2; the Fock‐35 approach performs better because this fraction of exact exchange has been chosen to optimize the value of the magnetic coupling constants in a set of related compounds 62, 116. This general trend in describing the magnetic coupling constants has been previously analyzed in embedded cluster models of these and other closely related TM systems 71, 73, 82, 85.…”

“…In this approach, second‐order perturbation theory leads to J ∼ t 2 / U , in a two‐electron, two‐center problem, which permits one to relate J and the parameters t and U defining the Hubbard Hamiltonian. Previous calculations with the same model (see references in Section 1) and recent work on cluster models at various levels of theory 61, 71, 73, 82, 120 have shown that the UHF approach can, in principle, account for the superexchange mechanism but provide too small antiferromagnetic coupling constants—usually 20–30% of the experimental value 61, 71, 82, whereas LDA and GGA largely overestimate these magnitudes by a factor of 5–10. This different behavior can be rationalized by the changes in J arising from changes in the effective parameters of the Hubbard Hamiltonian (or, more precisely, in the t / U ratio, i.e., the delocalization/repulsion ratio) in a variable extent, which may be dramatically exaggerated, as in LDA or GGA, or underestimated, as in UHF.…”

“…7), at variance with respect to LDA and PW‐GGA, where no stabilization appears when the distortion is included. The magnetic coupling in the copper compounds considered here have been extensively studied by Moreira et al 81–85 using the embedded cluster model approach. These authors have shown that the magnetic coupling constant is a local property that can be accurately calculated with sophisticated configuration interaction calculations on relatively small embedded cluster models of the solids.…”

“…The CRYSTAL approach used in this work has been previously applied to Sr 2 CuO 2 Cl 2 by Moreira and Dovesi 79 and Mackrodt et al 80, Su et al 51 (UHF), and Perry et al 70 (UB3LYP) on closely related cuprate compounds, whereas KCuF 3 has been investigated by Towler et al 56 at the UHF level. The magnetic coupling in the copper compounds here considered have been extensively studied by Moreira et al 81–85 using the embedded cluster model approach. Model Hamiltonian calculations 86, 87 have also been performed to study the magnetic interactions in K 2 CuF 4 .…”

The electronic states of the neutral vacancy in diamond: a quantum mechanical approach

“…In particular, their performance for open-shell systems is by far superior to pure DFT functionals. This is the case, for example, of the calculation of the superexchange coupling constants in transition metal compounds [23][24][25]. For open-shell compounds with strongly localized electrons, as is the case here, the "correct" picture appears to correspond to some percentage of exact exchange in a hybrid functional.…”