Structure of Plastic Crystalline Succinonitrile: High-Resolution in situ Powder Diffraction

The temperature dependent (150-290 K) crystal structure of the low-temperature cc-phase. and high temperature beta-phase, of succinonitrile hits been determined by high resolution it) situ powder diffraction. The alpha-phase has a monoclinic unit cell that contains four gauche molecules and belongs to the P2(1)/a space group. The crystal undergoes a reversible first-order phase transition at 233 K into the high temperature beta-phase. The lattice parameters increase with temperature and the phase transition leads to an abrupt 6.7% increase in volume. The beta-phase crystallizes into it bcc-structure that belongs to the Im (3) over barm space group. The high temperature phase however, is a highly disordered plastic crystal at room temperature that contains both gauche and trans molecules. The non-linearity in the overall isotropic temperature-factor indicates other possible phase transitions in the temperature range of 233-250 K.