POSITRON-ANNIHILATION STUDY OF DEFECTS IN SUCCINONITRILE

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. © 1979 The American Physical Society.