Structural phase transitions in malononitrile, CH₂(CN)₂: crystal structure of the δ phase by neutron powder diffraction, and ab initio calculations of the structures and phonons of the α and δ phases

The crystal structure of the low-temperature phase of crystalline malononitrile, CH₂(CN)₂ (stable phase below 260 K), has been determined using Rietveld refinement on neutron powder diffraction data. The phase has a slightly lower density than the other three low-pressure phases, and unlike those phases it has a polar structure. The transition from the to phase involves a major reconstruction of the structure, including establishing a network of hydrogen bonds. DFT simulations of the structure and phonon dispersion curves of both and phases give free energy curves consistent with the phase transitions. It is noted that the transition from the to phase at 260 K is facilitated by the entropy arising from the low-frequency phonons associated with the soft mode for the sequence of –– phase transitions. (paper)