Positron lifetime and angular correlation measurements have been made on the plastic crystal succinonitrile. They confirm the phase transition at 234 ± 1 K. In the plastic phase the average orthopositronium lifetime increases with temperature and saturates at the highest temperatures. This is interpreted as orthopositronium trapping in thermally created defects, probably vacancies. A defect formation energy of 0.36 ± 0.1 eV is deduced. This is lower than predicted values of the vacancy formation energy.

• Received 23 July 1979

DOI:https://doi.org/10.1103/PhysRevLett.43.1407