Electrical conductivity of synthetic and natural quartz crystals

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Abstract
Electrical conductivity of α quartz is ionic, the current being carried by alkali ions (M+) moving in channels parallel to the c axis. A study was made of conductivity σ of one natural and three synthetic crystals in order to better understand these ionic defects. ac measurements were made over a wide range of frequencies, and complex‐impedance analysis was used to separate out the bulk conductivity from electrode effects. The activation energy E for the synthetic crystals was always close to 1.36 eV and the pre‐exponential A was in the range of 106 K/Ω cm. A decrease in σ resulted from electrodiffusion (sweeping) in hydrogen. For the as‐received natural crystal, the value of E was 0.82 eV, but this E value increased appreciably after hydrogen sweeping or after irradiation plus high‐temperature anneal. The principal observations could not be interpreted in terms of the conventional theory in which the Al3+‐M+ pair and its dissociation products are the only defects involved. A modified theory, in which additional unassociated Al is present as compensation either for H+ or for oxygen vacancies, allows a detailed interpretation of the results. Supporting data are also obtained from infrared measurements.