Mixed system $Cs_3Cu_3Cl_{8-x}Br_xOH$ with weakly connected Cu-triangles

To study the relationship between the properties of low-dimensional spin systems with weakly coupled Cu-triangles and their crystal structure, single crystals of $Cs_3Cu_3Cl_8OH$ (1) and the new $Cs_3Cu_3Cl_{7.6}Br_{0.4}OH$ (2) were grown. Both compounds are isostructural and crystallize in a monoclinic structure with space group $P2_1/c$. The magnetic susceptibility of (1) shows a maximum at $2.23 K$ and of (2) at $2.70 K$, which are attributed to antiferromagnetic phase transitions. Furthermore, the magnetization along the $b$-axis at $1.9 K$ for both compounds shows a spin-flop transition into a new antiferromagnetic phase. This transition occurs at $0.61 T$ for (1) and at $2.0 T$ for (2). The antiferromagnetic order can be suppressed by a magnetic field $B_{C1}= 1.1 T$ for (1) and $B_{C2}= 1.2 T$ for (2). First single crystal neutron diffraction measured on (1) at different temperatures reveals the magnetic signal on the top of the nuclear reflection at (-1 0 0). Its magnetic ordering temperature was found to be at $T_{N1}= 2.12(3) K$.