IONIC TRANSPORT IN CRYSTALLINE SIO2 - THE ROLE OF ALKALI-METAL IONS AND HYDROGEN IMPURITIES

The ionic conductivity of synthetic quartz (both untreated and ''hydro gen swept''), due to the migration of alkali-metal ions (M(+)) dissoci ated from [AlO4-M](0) centers, has been investigated in an extended te mperature range (550-1700 K) by impedance spectroscopy measurements. T wo time-dependent effects, important for the overall interpretative fr amework, have been observed during high-temperature annealings: (a) du ring 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 1400-170 0 K range induce a monotonic decrease of the conductivity. The interpr etation 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 attribut ed to alkali-metal ion desorption. The experimental results are interp reted in the framework of a model featuring two interacting dissociati on reactions, of the [AlO4-M](0) and of the [AlO4-H](0) defects, respe ctively. Following this model, a satisfactory fit of the experimental data is performed, leading to the determination of the dissociation an d migration energies of the alkali-metal ions as 1.19 eV and 0.25 eV, respectively.