Distinct magnetic regimes through site-selective atom substitution in the frustrated quantum antiferromagnet Cs ₂CuCl _4-xBr _x

We report on a systematic study of the magnetic properties on single crystals of the solid solution Cs ₂CuCl _4-xBr _x (0 ≤ x ≤ 4), which include the two known end-member compounds Cs ₂CuCl ₄ and Cs ₂CuBr ₄, classified as quasi-two-dimensional quantum antiferromagnets with different degrees of magnetic frustration. By comparative measurements of the magnetic susceptibility χ(T) on as many as 18 different Br concentrations, we found that the in-plane and out-of-plane magnetic correlations, probed by the position and height of a maximum in the magnetic susceptibility, respectively, do not show a smooth variation with x. Instead, three distinct concentration regimes can be identified, which are separated by critical concentrations x _c1 =; 1 and x _c2 =2. This unusual magnetic behavior can be explained by considering the structural peculiarities of the materials, especially the distorted Cu-halide tetrahedra, which support a site-selective replacement of Cl ^- by Br ^- ions. Consequently, the critical concentrations x _c1 (x _c2) mark particularly interesting systems, where one (two) halide-sublattice positions are fully occupied.