Order-disorder phase transition in dichlorodurene

The structure of dichlorodurene in the ordered phase was determined from neutron diffraction data at 70 K with the help of intra- and intermolecular potential calculations already published [Messager & Sanquer (1974). Mol. Cryst. Liq. Cryst. 26, 373-380]. The space group is the same as at room temperature, P2_1/a, but the cell parameters are integer multiples of those at room temperature, and there are three molecules in the asymmetric unit, instead of half a molecule. The phase transition is of order-disorder type and the first-order nature of this transition is established by the observation of a weak hysteresis.