Magneto‐Structural Properties of the Layered Quasi‐2D Triangular‐Lattice Antiferromagnets Cs₂CuCl_4−xBr_x for x = 0, 1, 2, and 4

Abstract

A study of the magnetic susceptibility under variable hydrostatic (He gas) pressure on single crystals of Cs2CuCl4−xBrx is presented. This work includes the border compounds x = 0 and 4, known as good realizations of the distorted triangular‐lattice spin‐1/2 Heisenberg antiferromagnet, as well as the recently discovered well‐ordered isostructural systems Cs2CuCl3Br1 and Cs2CuCl2Br2. For the determination of the exchange coupling constants J and J′ of their anisotropic triangular lattice, the susceptibility data are fitted by the recently proposed J–J′ model [Schmidt and Thalmeier, New J. Phys. 2015, 17, 073025]. Its application on magnetic susceptibility data, validated for the border compounds, yields a degree of frustration J′/J = 0.47 for Cs2CuCl3Br1 and J′/J ≃ 0.63–0.78 for Cs2CuCl2Br2, making these systems particular interesting representatives of this family. From the evolution of the magnetic susceptibility under pressure up to about 0.4 GPa, the maximum pressure applied, two observations were made for all the compounds investigated here. First, it has been found that the overall energy scale, given by Jc = (J2 + J′2)1/2, increases under pressure, whereas the ratio J′/J remains unchanged in this pressure range. These experimental observations are in accordance with the results of DFT calculations performed for these materials. Secondly, for the magnetoelastic coupling constants, extraordinarily small values are obtained, about two orders of magnitude smaller compared to other Cu‐based quantum magnets. These observations have been assigned to a structural peculiarity of this class of materials, consisting of well‐isolated magnetic units not sharing any common coordination element.

Microscopic Modeling of Correlated Systems Under Pressure: Representative Examples

“…In order to explain the experimental observations, ab initio density functional theory calculations for Cs 2 CuCl 4 were performed at various values of pressure . We find a systematic shift of the pressure scale which is needed to match the simulated and measured structures, similarly to other systems considered in this review .…”

“…In this approach, the total energy calculated from first principles for a set of different lattice volumes is fitted to the aforementioned equation of state which gives the equilibrium lattice parameters and total energy. It should be noted here that often a uniform shift of the pressure scale is required to improve the agreement between the simulated and the experimentally measured structural parameters, as we have found for quantum magnets and iron‐based ThCr 2 Si 2 systems . This is related to the fact that the volume is usually under‐ or overestimated for most common approximations to the exchange‐correlation energy in density functional theory.…”

“…Application of pressure is shown to modify the magnetic interactions in both of these materials . The exchange coupling constants J and correspond to different exchange paths and exhibit a strong dependence on the subtle details of the crystal structure (Figure c), such as the local geometries of the CuCl 4 tetrahedra and the bonding angles in the superexchange bridge CuClClCu .…”

“…The exchange coupling constants J and correspond to different exchange paths and exhibit a strong dependence on the subtle details of the crystal structure (Figure c), such as the local geometries of the CuCl 4 tetrahedra and the bonding angles in the superexchange bridge CuClClCu . From the fit of the susceptibility measured on single crystals to a model, the ratio is shown to remain unchanged under small hydrostatic pressure up to 0.5 GPa.…”

“…This parameter choice is justified by the fact that the bond angles in the optimized structure are in a reasonable agreement with the X‐ray diffraction data at zero pressure . In general, magnetic degrees of freedom need to be included for the structure optimization of these frustrated systems, similarly to the iron‐based superconductors, and for Cs 2 CuCl 4 we assume the simplest case of a ferromagnetic order of the Cu atoms . Although this is not the magnetic order shown by the system at low temperatures, the assumption of ferromagnetism is sufficient to account for the existence of Cu moments in the ab initio simulations of pressure‐induced structural changes.…”