Secondary relaxations due to fast-ion diffusion in AgI-rich borate glasses observed by Brillouin scattering

Abstract

Brillouin spectra from a series of fast-ion conducting (AgI${)}_{\mathrm{x}}$-(${\mathrm{Ag}}_2$O-n${\mathrm{B}}_2$ ${\mathrm{O}}_3$ ${)}_{1\mathrm{-}\mathrm{x}}$ glasses have been recorded in the temperature range 295–620 K. Anomalous hypersonic attenuation was observed which increases with increasing AgI content. It is attributed to secondary relaxation processes caused by ${\mathrm{Ag}}^{+}$ ions jumping between available sites in the glass network under the perturbation of thermally induced mechanical stress. By combining the present results for the (x=0.6, n=2) glass with reported ultrasonic and sub-audio-frequency mechanical relaxation data, the relaxation time has been found to adhere to an Arrhenius law for the ten decades of frequency of observation. The very broad distribution of relaxation times observed at lower frequencies and temperatures is dramatically narrowed down with increasing frequency and temperature and approaches Debye relaxation at Brillouin frequencies. Contrary to the general predictions of the weak electrolyte model the number of mobile ${\mathrm{Ag}}^{+}$ ions is, from comparison of the Brillouin, ultrasonic, and sub-audio-frequency relaxation strengths, found to be almost constant in the temperature range 120–500 K. Room-temperature velocity data extrapolate to those of pure α-AgI with increasing AgI concentration, thereby supporting the model in which the AgI is introduced in the glass network in the form of α-AgI-like microdomains.