Vibrational study of phase transition of solid malononitrile

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Abstract
Solid phases and phase transitions of crystals of malononitrile, CH2(CN)2, have been studied by Raman and infrared spectroscopy. Among twelve Raman active lattice modes of T′ phase a soft mode has been identified and assigned to a R′b libration. The T′–F–T transitions can be considered as second order and of displacive type since the soft mode frequency decreases continuously to zero at Tc when the transition temperature is approached from either above or below. Critical exponent β∼1/2 has been determined from the soft mode frequency versus Tc-T plot. Half-width of the soft mode band has also been studied as a function of temperature and relaxation times have been extracted from the corresponding curves. The stable low temperature phase L has been characterized spectroscopically: unlike T′, F, T phases it contains CH...N hydrogen bonds. The behavior of its lattice frequencies with temperature indicates that L–F phase change is a first order transition.