Studies of Phase Transformation in Molecular Crystals Using the Positron Annihilation Technique

An examination has been made of the brittle/plastic phase transformation in the molecular crystals cyclohexane, DL-camphene and succinonitrile using the positron annihilation technique. In each material, the transition is characterized by a distinct increase in ortho-positronium lifetime. The influence of impurities on the transition was examined for DL-camphene. Addition of the impurity tricylene in concentrations in the range 0.14-4.0 mol % resulted in a lowering of the transition temperature from 176 to 167 K and a broadening of the transition region.