The re-entrant phase transitions in crystalline malononitrile, CH2(CN)2: a neutron powder diffraction study
The temperature dependence of the crystal structures of the low-temperature α-phase and the intermediate-temperature β-phase of crystalline malononitrile, CH2(CN)2, have been determined using Rietveld refinement on neutron powder diffraction data. The results confirm that the α-phase has the same structure as the high-temperature γ-phase, and the refined structure of the β-phase is in agreement with previous structure determination using single-crystal x-ray diffraction. The variations of the structure with temperature due to the phase transitions are identified and analysed in terms of molecular rotations and spontaneous strains.
Investigation of the β-γ phase transition in single crystal malononitrile by polarized Raman spectroscopy
Temperature dependent, polarized Raman spectra in the lattice mode region for sublimed, single crystal malononitrile through the β–γ phase transition (Tc=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 β–γ phase transition is analogous to the smectic A↔nematic phase transition reported for cyano‐based, liquid crystal systems.
Phase Transitions of Pyrazine
Abstract Phase transitions of pyrazine crystal have been studied by calorimetry, X-ray diffraction and low frequency vibrational spectroscopy. Three phases have been shown to exist: phase I stable up to 28.5°C. phase I1 between 28.5 and 37°C and phase III between 37 and 51°C. Phase III is characterized by the space group Cmmm and its unit cell (Z = 8) is the quadruple of that of phase I. Phase I1 is similar to phase III and the most significant spectroscopic feature of the former is the presence of two Raman bands at 23 and 10 cm-1. The lattice modes of phase 1 and the librational amplitude have been calculated. The amplitudes are very large and amount up to 10° for the motions about the x-axis. They may play a major role in the mechanism of phase transitions and are likely to be responsible for some disagreements between the calculated and observed lattice frequencies.