Depending on the crystal growth conditions, an orthorhombic (O-type) or a tetragonal (T-type) structure can be found in the solid solution Cs2CuCl4-xBrx (0 < x < 4). Here we present measurements of the temperature-dependent magnetic susceptibility an
d isothermal magnetization on the T-type compounds x = 1.6 and 1.8 and compare these results with the magnetic properties recently derived for the O-type variant by Cong et al., Phys. Rev. B 83, 064425 (2011). The systems were found to exhibit quite dissimilar magnetic properties which can be assigned to differences in the Cu coordination in these two structural variants. Whereas the tetragonal compounds can be classified as quasi-2D ferromagnets, characterized by ferromagnetic layers with a weak antiferromagnetic inter-layer coupling, the orthorhombic materials, notably the border compounds x = 0 and 4, are model systems for frustrated 2D Heisenberg antiferromagnets
Starting from Cs2CuCl4 and Cs2CuBr4 our project focuses on the growth of the Cs2CuCl4-xBrx mixed crystals from aqueous solution and the investigation of the occurring structural variations. The well known orthorhombic structure (space group Pnma) of
the end members of this system is interrupted within the intermediate composition range Cs2CuCl3Br1 - Cs2CuCl2Br2, if the growth takes place at room temperature. Within this range a new tetragonal phase is found (space group I4/mmm). However, in case the growth temperature will be increased to 50 {deg}C, the existence of the orthorhombic structure can be extended over the whole Cs2CuCl4-xBrx mixed system. A detailed analysis of the composition dependence of the lattice parameters is used to draw conclusions on the incorporation of Cl- and Br-ions at different sites which is important for the magnetic interactions between the Cu-ions.
