Structural phase transitions in malononitrile, CH(CN): 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 [Formula: see text] phase of crystalline malononitrile, CH(CN) (stable phase below 260 K), has been determined using Rietveld refinement on neutron powder diffraction data. The [Formula: see text] 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 [Formula: see text] to [Formula: see text] 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 [Formula: see text] and [Formula: see text] phases give free energy curves consistent with the phase transitions. It is noted that the transition from the [Formula: see text] to [Formula: see text] phase at 260 K is facilitated by the entropy arising from the low-frequency phonons associated with the soft mode for the sequence of [Formula: see text]-[Formula: see text]-[Formula: see text] phase transitions.