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

Synthetic α-quartz is shown to contain a significant concentration (several ppm) of vacancies. The major concentration of vacancies is suggested to 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 vacancy clusters that are observable only after electron irradiation, whereupon they give rise to a positron lifetime close to 425 ps. Annealing between 900 and 1000 °C causes disappearance of divacancies and formation of vacancy clusters giving rise to a positron lifetime close to 300 ps. Above ∼950 °C positronium is formed with an exceptionally long lifetime (3–5 ns) ascribable to the formation of an amorphous phase connected with the thermal instability of β-quartz. Electron irradiation (2.3 MeV at 8 °C) gave rise to a 250±5 ps lifetime component interpreted to signify formation of neutral monovacancies, V0 and/or VSi. Their introduction rate is nonlinear, decreasing abruptly by a factor of ∼5 above a dose of 1×1017 ...