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 δ phase of crystalline malononitrile, CH2(CN)2 (stable phase below 260 K), has been determined using Rietveld refinement on neutron powder diffraction data. The δ phase has a slightly lower density than the other three low-pressure phases, and unlike those phases it has a polar structure. The reconstructive transition from the β to δ 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 α and δ phases give free energy curves consistent with the phase transitions. It is noted that the transition from the δ to β phase at 260 K is facilitated by the entropy arising from the low-frequency phonons associated with the soft mode for the sequence of α-β-γ phase transitions.