X-ray diffraction and structural study of the phase behavior of (Et2Me2N)2CuBr4

The synthesis and characterization of a new member of the β-K2SO4 family, (Me2Et2N)2CuBr4, are reported. DSC studies indicate the presence of a phase transition at Tc=29°C with ΔS/R=13.7J/mol. The room temperature structure is monoclinic, space group, P1121/n with a=9.657(4)Å, b=15.954(6)Å, c=13.950(6)Å, γ=92.94(2)° and V=2146.3(15)Å3, with Z=4. Structural analysis gave R=0.060 and ωR=0.084 for 2098 reflections with |F|≥3σ(F). The high temperature phase is orthorhombic, space group Pmcn, with a=9.628(2), b=16.713(3), c=13.787(3)Å and V=2218.5(9)Å3 with Z=4. The structural analysis gave R=0.206 and ωR=0.227 with substantial disorder present. The room temperature structure corresponds to a B3g distortion of the parent psuedo-hexagonal Pmcn structure. The strains associated with the phase transition are particularly large. The primary structural features associated with the phase transition include rotation and disorder of the cations, displacements of the Cu and N atoms off of the mirror planar of the Pmcn phase, and rotation of the CuBr42− anions.