Characterization of vacancies in as-grown and electron irradiated alpha-quartz by means of positron annihilation

Synthetic alpha-quartz is shown to contain a significant concentration (sev eral ppm) of vacancies. The major concentration of vacancies is suggested t o be in the form of divacancies, giving rise to a positron lifetime of 285/-5 ps, but in addition, there is a much smaller concentration of large vac ancy clusters that are observable only after electron irradiation, whereupo n they give rise to a positron lifetime close to 425 ps. Annealing between 900 and 1000 degrees C causes disappearance of divacancies and formation of vacancy clusters giving rise to a positron lifetime close to 300 ps. Above similar to 950 degrees C positronium is formed with an exceptionally long lifetime (3-5 ns) ascribable to the formation of an amorphous phase connect ed with the thermal instability of beta-quartz. Electron irradiation (2.3 M eV at 8 degrees C) gave rise to a 250+/-5 ps lifetime component interpreted to signify formation of neutral monovacancies, V-0 and/or V-Si. Their intr oduction rate is nonlinear, decreasing abruptly by a factor of similar to 5 above a dose of 1x10(17) e/cm(2), an effect which is ascribed to the remov al of irradiation-produced vacancy-interstitial complexes by means of elect ron-hole recombination during the irradiation. (C) 1999 American Institute of Physics. [S0021-8979(99)06313-6].