Mechanical relaxations in single and mixed alkali silicate glasses

Isothermal mechanical spectroscopy (IMS) have been performed in single and mixed alkali silicate glasses (1 − x)Na2O·xLi2O·3SiO2 with x = 0, 0.25, 0.5, 0.75. alkali silicate glass Na2O·3SiO2 exhibits several relaxation peaks. Classically, a peak located at about 230 K at 1 Hz is assigned to the stress induced motion of alkali ions (ionic peak) and a second one (NBO peak) at about 500 K at 1 Hz is linked with a local rearrangement of the glassy structure involving non-bridging oxygen. Moreover, a small relaxation peak is evidenced for the first time at very low temperature (∼185 K at 1 Hz). This new peak is attributed to the motion of the alkali ion inside its site. g measurements carried out in the mixed alkali silicate glasses evidence the classical mixed alkali peak and point out new experimental features. Firstly, two satellite relaxation peaks are superimposed to the mixed alkali peak and correspond to the ionic and NBO peaks, respectively. Moreover, Arrhenius plot of the mixed alkali peak shows that the apparent activation energy is different below and above 340 K. These experimental results are discussed using the recent models for the alkali effect and the Meyer–Bunde model for mechanical relaxation in ionic glasses below the glass transition temperature Tg.