  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 and 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
