Ionic transport in crystalline SiO2: The role of alkali-metal ions and hydrogen impurities

The ionic conductivity of synthetic quartz (both untreated and ''hydrogen swept''), due to the migration of alkali-metal ions (M(+)) dissociated from [AlO4-M](0) centers, has been investigated in an extended temperature range (550-1700 K) by impedance spectroscopy measurements. Two time-dependent effects, important for the overall interpretative framework, have been observed during high-temperature annealings: (a) during thermal treatments performed in the 900-1300 K temperature range, a monotonic enhancement of the conductivity is detected, more evident in ''hydrogen swept'' samples; (b) thermal treatments in the 1400-1700 K range induce a monotonic decrease of the conductivity. The interpretation of the former effect is based on the indirect role of hydrogen impurity in ionic transport, also supported by infrared spectroscopy measurements on the [AlO4-H](0) centers; the latter effect is attributed to alkali-metal ion desorption. The experimental results are interpreted in the framework of a model featuring two interacting dissociation reactions, of the [AlO4-M](0) and of the [AlO4-H](0) defects, respectively. Following this model, a satisfactory fit of the experimental data is performed, leading to the determination of the dissociation and migration energies of the alkali-metal ions as 1.19 eV and 0.25 eV, respectively.