A Reversible Non-disruptive Phase Transition Shown by the Zinc Iodide Dimethylformamide Complex ZnI₂(dmf)₂

At 228 K crystals of ZnI₂(dmf)₂ show a reversible phase transition which does not disrupt the lattice. Above the transition temperature the space group is C2/c and the cell contains eight symmetrically equivalent molecules. Cooling to below the transition temperature has little effect on the cell parameters or on the Zn- and I-atom positions, but the space group is now P2₁/n and the asymmetric unit comprises two conformationally different molecules. These arise from cooperative rotations of either ca +25 or −43° about the Zn—O bond of one of the dmf ligands in the high-temperature form. This displacive transition involves large movements of some atoms. The corresponding chloride and bromide are isomorphous with the higher temperature C2/c form, but it is only with the iodide that the weaker intermolecular forces permit the unusual phase change. The transition has been followed by differential scanning calorimetry, which gives an enthalpy change of 1.44 (5) kJ mol⁻¹.

-1