Investigation of the β–γ phase transition in single crystal malononitrile by polarized Raman spectroscopy

Abstract

Temperature dependent, polarized Raman spectra in the lattice mode region for sublimed, single crystal malononitrile through the D-r phase transition (T,=294.7 K) are reported. Symmetry assignments and frequencies of vibrational modes are made and compared to previous studies. Nine out of 12 possible lattice modes are observed in each phase with three of the observed modes being affected by the phase transition. The temperature dependence of these three affected modes is attributed to cyano group and molecular dipole interactions between molecules in the crystal. The behavior of malononitrile through the /?-r phase transition is analogous to the smectic A+nematic phase transition reported for cyano-based, liquid crystal systems.

The re-entrant phase transitions in crystalline malononitrile, CH2(CN)2: a neutron powder diffraction study

“…The change in crystal symmetry from P2 1 /n to P 1 space groups occurs by a distortion of the α or γ -phase that can be described by the irreducible representation B g at zero wavevector. Whilst the study of Le Calvé et al [13] did not determine the symmetry of the soft mode, the only symmetries of the Raman-active modes are A g and B g , and a subsequent study by Bonafede and Eckhardt [18] accounted for all A g modes. This implies that the soft mode has symmetry B g , independently confirming the change in space group.…”

“…Most studies on the phase transitions in malononitrile have been based on spectroscopic methods [11][12][13][14][15][16][17][18]. Of particular note are the studies of Le Calvé et al [13] and Krauzman et al [14], who identified the soft mode responsible for the two displacive phase transitions.…”