Structural phase transitions in malononitrile

Previous studies using nuclear quadrupole resonance and calorimetric methods have shown that there are two second-order phase transitions in malononitrile at 141 and 294.7 K and that the low- and high-temperature phases have the same symmetry. In this paper the crystal structure of the intermediate phase is reported and compared with a previous determination of the high-temperature phase. The unit cell dimensions of the two structures are almost identical and the space groups are Ptext-decoration:overline1 and P21/n, respectively. The structures are related by relative translations and rotations of the molecules and this motion is identified with the eigenvector of the soft mode associated with the transition. The basis of a theory for such a system is developed using the quasi-harmonic approximation.