Radiation effects in crystalline SiO2: The role of aluminum

Electron spin resonance (ESR) and infrared absorption (IR) experiments have provided information about the role of aluminum in the radiation response of commercially available high-quality synthetic quartz. Samples obtained from two separate sources were investigated, and identical radiation responses were found for the two materials. Interstitial ions such as H+, Li+, and Na+ as well as radiation-induced holes trapped at oxygen ions act as charge compensators for the ever-present substitutional aluminum ions. Usually the charge compensator is located adjacent to the aluminum, and this gives rise to Al-OH−, Al-Li+, Al-Na+, and [Ale+]0 centers. Absolute concentrations of these compensated aluminum centers have been determined as a function of irradiation and annealing temperature for a variety of samples, both swept and unswept. The various treatments simply exchange one type of compensator for another at the aluminum sites, and within experimental error, the sum of the aluminum centers remains constant for a given sample. This direct accountability of all the aluminum ions in hydrogen-swept samples strongly suggests that the 3306- and 3367-cm−1 infrared bands are associated with the Al-OH− center. Also, the ESR and IR results show that the aluminum content of randomly selected bars of high-quality quartz can vary by an order of magnitude.