NEW HIGH-TEMPERATURE RESULTS ON THE IONIC-CONDUCTIVITY OF QUARTZ AND IMPLICATIONS ON THE TRANSPORT MECHANISMS

AC (102-3*105 Hz) measurements have been performed on Sawyer premium-quality synthetic quartz oriented along the z axis, in the temperature range between 240 and 1500 K. The samples were both 'as-received' and X-rayed, in the case of the lower-temperature measurements. The results confirm that the conductivity, in the temperature range considered, is essentially due to extrinsic phenomena, i.e. to the dissociation of Al-Na substitutional centres and to the migration process of alkali ions. An interpretative scheme is proposed, which involves the detailed structure of the attractive potential of Na+ ion in the vicinity of an aluminium impurity and the increase in the radius of the z-axis channels in the beta -quartz-to-tridymite transition.