The ionic conductivity of synthetic quartz (untreated or H-swept), due to the migration of alkali metal ions which were dissociated from [AlO4-M]0 centers, was investigated at temperatures ranging from 550 to 1700K by means of impedance spectroscopy. Two time-dependent effects were observed during high-temperature annealing. During heat treatment at 900 to 1300K, a monotonic enhancement of the conductivity was detected which was more evident in H-swept samples. Heat treatment at 1400 to 1700K led to a monotonic decrease in the conductivity. The explanation of the former effect was based upon the indirect role which was played by H impurities in ionic transport, and was also supported by infra-red spectroscopic measurements of [AlO4-H]0 centers. The latter effect was attributed to alkali metal-ion desorption. The experimental results were explained in terms of a model which involved 2 interacting dissociation reactions of the [AlO4-M]0 and [AlO4-H]0 defects. By using this model, a satisfactory fit to experimental data was obtained, and the dissociation and migration energies of the alkali-metal ions were deduced to be 1.19 and 0.25eV, respectively.

Ionic Transport in Crystalline SiO2: the Role of Alkali-Metal Ions and Hydrogen Impurities. P.Campone, M.Magliocco, G.Spinolo, A.Vedda: Physical Review B, 1995, 52[22], 15903-8