Optical properties and ferromagnetic order in K2 CuF4

Abstract

A crystal field analysis of the 3d9 multiplet of Cu+ + in K2CuF4 is presented, taking into account optical absorption spectra, linear dichroism, and the anisotropy of the g tensor. The calculations are based on local D'4h symmetry arising from elongated ligand octahedra with their long axes in the c planes, as it has been proposed theoretically and confirmed by X-ray studies. An additional orthorhombic distortion must be assumed to explain the dichroic properties. Magnetic circular dichroism, Faraday rotation, and magnetic field dependence of the linear birefringence have been used to measure in- and out-of-plane magnetization curves below Tc = 6.25 K. The Faraday rotary dispersion in the visible region and domain effects are discussed

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

“…The intermediate nature of the hybrids maintains to a large extent the charge transfer nature of the gap as well as its correct magnitude, which are among the most prominent features one would like theoretical approaches properly describe. The B3LYP value of 2.5 eV for the optical gap in Sr 2 CuO 2 Cl 2 is an excellent approximation to the experimentally observed values in the 1.4 to 1.9 eV range 103–105; for K 2 CuF 4 the calculated 3.3‐eV gap compares well with the ∼ 5‐eV value inferred from the optical absorption spectra of Kleemann and Farge 98 on this transparent material, where the strong charge–transfer absorption edge starts near 40,000 cm −1 . The larger value of the gap in the fluorides with respect to the oxides is in‐line with the larger ionicity.…”

“…In this work we consider the simplest structure isomorphous to that of K 2 NiF 4 . K 2 CuF 4 is a transparent nonmetallic ferromagnet 87, 98 and exhibits a 1.0μ B localized magnetic moment on each Cu atom (at T = 0 K) lying on ab planes forming a quasi‐2‐D Heisenberg ferromagnetic system at low temperature. The Curie temperature of this compound is 6.25 K in zero field 99–101 and the in‐plane magnetic coupling constant, J ab , has been determined from thermal analysis 101 and by neutron diffraction 100 and is in the range 15–23 K. A small interplane magnetic coupling J c ≃ 0.016 K has been also reported by Yamada 101 that explains the phase transition at T c .…”

Changing the Usual Interpretation of the Structure and Ground State of Cu2+-Layered Perovskites

“…For this simple reason, it has systematically been accepted [9][10][11][12][13][14][15][16]30,31 that the JTE is behind the local distortion around the metal cation in RCuCl4 compounds. A similar situation holds for purely inorganic materials displaying a layer structure, such as K2CuF4 [32][33][34] , Rb2CuCl4 35 , Cs2AgF4 36 or Na3MnF6 37,38 . Under that assumption, the principal axis of the distorted octahedron would however lie in the layer plane, a circumstance which is certainly surprising in view of the axial character displayed by layered compounds.…”

A Novel Three‐Dimensional Copper(II) 1,2‐Ethylenediphosphonate Framework with Channel‐like Voids

“…Since the Cu 2+ coordination polyhedron has D 2 h symmetry four d–d transition bands are expected according to selection rules 36. A precise inspection shows that the polyhedron deviates only slightly from D 4 h symmetry (three transition bands), therefore the energy levels of the d xz and d yz orbitals are very close to each other, resulting in almost identical transition bands 37,38. According to Billing and Hathaway 36 the order of the energy levels is d > d > d xy > d xz ≥ d yz .…”