Relaxation processes in ionic glasses studied by ultrasonic and dielectric spectroscopy

Ultrasonic measurements on several ionic glasses having different composition (silver phosphates, borates, molybdates, doped or modified by different salts) have shown the existence of a relaxation process, related to the long-range ionic diffusion in the glassy matrix [1]. In some cases, evidence has been provided that the temperature dependent behaviour of the acoustic attenuation at ultrasonic frequencies, and the corresponding changes of the ultrasonic sound velocity, are due to the superposition of at least two different relaxational contributions [1,2]. However, at temperatures well below the temperature range were the main mechanical loss peak occurs, ultrasonic measurements have revealed the existence of an additional feature in the acoustic attenuation, probably arising from ionic localized motion [3]. In order to understand the origin of all these different relaxational contributions occurring below the glass transition temperature, and their relation with the composition of the material, the dielectric response of different ionic glasses has been investigated as a function of frequency (1 mHz – 40 GHz) and of temperature (450 K – 3 K). Dielectric measurements have been performed using a broadband measurement setup [4], different impedance analyzers and network analyzers, a high precision capacitance bridge, and also waveguides in different frequency bands. The results have been analyzed in terms of frequency dependent conductivity, complex permittivity, complex modulus, and discussed in comparison with the results of mechanical measurements at ultrasonic frequencies. Some aspects of the experimental dielectric data have been considered in particular: (i) the shape of the electrical response (frequency dependent conductivity or electric modulus) and the factors (composition, temperature) that could have an influence on it; (ii) the appearance of a nearly constant loss behaviour in the low temperature / high frequency part of dielectric spectra. The results have been compared with available models concerning the dynamical response of ionic materials [5,6,7,8] and the widely debated origin of the nearly constant loss regime [5,9,10].