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The effect of prolonged annealing treatments at elevated temperatures (900-1700 K) on the a.c. ionic conductivity of Sawyer PQ 'hydrogen swept' quartz has been investigated by impedance spectroscopy measurements. Transient and irreversible effects have been observed during the annealings: specifically, a monotonic enhancement of the conductivity is detected in the 900-1300 K temperature range, while at higher annealing temperatures an opposite effect is evidenced. The experimental results can be interpreted in the frame of a phenomenological model featuring two equilibrium reactions; an indirect role of hydrogen impurity in the ionic transport dynamics is evidenced, also by means of infrared spectroscopy measurements.

A wavelength resolved thermally stimulated luminescence (TSL) study has been carried out for the first time on X-irradiated chemical vapor deposition SiO2 films deposited on a silicon substrate, from room temperature, to 400 degrees C. Upon irradiation, the TSL glow curve features a prominent structure at a maximum temperature, T-max, of approximately 62 degrees C (heating rate = 1 degrees C/s); the analysis of the emission wavelength shows a peak at 457 nm (2.71 eV). The shape of the TSL peak is complex, and cannot be described in the frame of classical first- or second-order kinetics. Moreover, T-max has a strong and monotonic shift to higher temperatures after partial pre-heating treatments while the spectral emission is not modified: a similar phenomenon has already been observed in bulk fused silica of various types, different from crystalline quartz which does not present any shift of T-max as a function of pre-heating. These results suggest that the TSL peak is characterized by a distribution of trap parameters: specifically, a continuous distribution of trap energy or of the frequency factor can be taken into account. Considerations on the characteristics of the trap parameter distribution are made.

Spectrally resolved thermostimulated-luminescence (TSL) measurements have been carried out on samples of synthetic quartz, submitted to different treatments in order to change the type of charge compensation at the substitutional aluminum sites. Near-infrared (NIR) optical absorption, electron paramagnetic resonance (EPR), and ac ionic conductivity measurements have been performed to monitor the kinetics of defects involved in the treatments. A specific treatment, consisting in a prolonged high-temperature annealing of the samples in vacuum (10 h at 1400 C°, 1 Pa), is shown to reduce the presence of ionic charge compensators at the Al sites and, at the same time, to induce an intense 380-nm TSL, undetected before the treatment. The assignment of the 380-nm emisson to [AlO4]0 centers is proposed and is seen to be in good agreement with a number of experimental results presented so far. © 1995 The American Physical Society.

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.

A comparative and simultaneous study of TSL and TSC above room temperature (20-400 degrees C) has been performed on ''as-grown'' and ''hydrogen-swept'' synthetic quartz crystals. Following X-irradiations, TSL spectra (heating rate = 1 degrees C/s) feature a number of peaks: at 75 degrees C an intense structure is observed (the well-known ''100 degrees C'' peak of quartz); the analysis of this peak obtained by numerical methods has shown that it follows monomolecular kinetics, giving a value of 0.83 eV for the trap depth. Additional peaks are observed at 110 degrees C and 160 degrees C, followed by weaker and less resolved emissions above 200 degrees C. TSC peaks at 80 degrees C, 120 degrees C and 160 degrees C, particularly evident in as-grown samples when measured with the electric field applied along the x-axis, can be associated to the corresponding TSL peaks. However, spectra performed with the electric field applied along the z-axis evidence different features. In as-grown samples a strong and broad peak at approximately 132 degrees C is observed, while hydrogen-swept samples are characterized by two peaks at 180 degrees C and 275 degrees C. Such an anisotropic character, and the fact that no TSL structures are observed in the same temperature range, support the hypothesis of an ionic nature for the latter peaks. TSC ''pre-dose'' measurements of the 75 degrees C peak show that no current enhancement is observed upon irradiation and heating treatment: this result is in accordance with previous radioluminescence and thermally stimulated exoelectron emission experiments and supports the proposed model of the dynamics of this effect.

Thermally Stimulated Luminescence (TSL) and phosphorescence measurements excited by X- and beta rays have been performed above room temperature on fused quartz and on thin SiO2 films deposited on a silicon substrate by the technique of chemical vapour deposition. Although in these types of SiO2 the basic TSL features, noticeably the prominent glow peak at 60 < T < 120°C, are similar to those already studied in quartz, some behaviours are different. Specifically, the maximum temperature of the glow peak shifts linearly to higher temperatures as a consequence of partial pre-heating treatments and the phosphorescence decay shows a t-1 power law. The implications of these results are discussed also in comparison with the corresponding ones of crystalline quartz and with preliminary data on thermally stimulated currents measurements. © 1994.

Optical absorption measurements in the 3-9 eV energy range and electron paramagnetic resonance (EPR) have been performed at 300 K and at 80 K on Sawyer PQ quartz. Both as-received and neutron-irradiated samples (neutron fluences up to 3 x 10(18) neutrons cm-2) have been studied. The absorption spectra have been analysed in terms of a sum of elementary Gaussian components. The effect of the neutron irradiations is to induce the presence of absorption bands al 7.6 eV (E band), at 7.1 eV (D band) and at 6 eV (E' region). The most intense peak of all the spectra is the E band (7.6 eV); its shape is complex and the existence of a double structure can be suggested. The D band is evidenced for the first time in neutron-irradiated synthetic quartz; our measurements show that this structure is correlated with the 6.0 eV band. The 'E'' region is complex; in fact, at the highest neutron fluence the optical absorption spectrum reveals the existence of four structures, at 4.7, 5.2, 5.6 and 6.0 eV, where the 4.7 eV band has the same characteristics as the D0 band, which is present in amorphous silica. The comparison between the results of EPR measurements and the analysis of the complex structure of this absorption indicates that the correlation between optical absorption bands in this region and the EPR E'-centre signals suggested previously must be re-examined.

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.