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

Abstract

The crystal structure of the low-temperature δ phase of crystalline malononitrile, CH 2 (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 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.

Unique features of the structural phase transition in acetylene showing simultaneous characteristics of reconstructive, displacive and order–disorder

“…2 Furthermore, new vibrancy has been given to the field through the introduction of new techniques, such as lattice dynamics based on methods using density functional theory, 3,4 which have been the basis for more recent studies in this field. 5–9…”

“…2 Furthermore, new vibrancy has been given to the field through the introduction of new techniques, such as lattice dynamics based on methods using density functional theory, 3,4 which have been the basis for more recent studies in this field. [5][6][7][8][9] Whilst some phase transitions in organic materials involve relatively complex processes, such as charge transfer or conformational changes, there are many phase transitions that primarily involve changes in the arrangements of molecules within the periodic structure of the crystal. In this paper we discuss one of the simplest examples of this, namely of the molecular crystal of acetylene, C 2 H 2 .…”