Distinct magnetic regimes through site-selective atom substitution in the frustrated quantum antiferromagnet Cs2CuCl4-xBrx

We report on a systematic study of the magnetic properties on single crystals of the solid solution Cs2CuCl4-xBrx (0 <= x <= 4), which include the two known end-member compounds Cs2CuCl4 and Cs2CuBr4, classified as quasi-two-dimensional quantum antiferromagnets with different degrees of magnetic frustration. By comparative measurements of the magnetic susceptibility chi(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.