Phase Transitions of Pyrazine

Abstract

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.