Crystal structures of methylene bromide and methylene iodide

The crystal structures of the three solid phases of methylene iodide and the stable phase of deuterated methylene bromide have been determined using the neutron powder profile structure refinement technique. For CH2I2, both phase I at 50K and phase III at 16K, and for CD2Br2, the stable phase at 15K, have eight molecules per unit cell on general sites of the monoclinic space group C2/c. The structure of CH2I2, phase II, has been determined at 16K, 30K, 50K and 255K. This phase is face centred orthorhombic, space group Fmm2, with four molecules per unit cell on sites with C 2v symmetry. Molecular bond lengths and angles are given for all structures. The lattice energy of each structure has been calculated using an empirical potential. Phase II is found to be the most stable phase for CH2I2, but the energy differences between all three phases are small.