Magneto-Structural Properties of the Layered Quasi-2D Triangular-Lattice Antiferromagnets Cs2CuCl4-xBrx for x=0, 1, 2, and 4

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 similar or equal to 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 J(c) = (J(2) + 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.