High-temperature positron lifetime measurements were used to investigate vacancy-like defects. A detailed 3-state trapping model which allowed for de-trapping was used to show that monovacancy-like defects trapped positrons with a binding energy of almost 0.3eV, while vacancy clusters represented traps which were so deep that no detectable de-trapping occurred at temperatures of up to 800K.

S.Dannefaer, D.Kerr: Physical Review B, 1994, 50[19], 14096-103