Isothermal mechanical relaxations in single and mixed alkali silicate glasses

Very low frequency damping measurements, carried out in various mixed alkali silicate glasses (1 − x)Na2O · xLi2O · 3SiO2 with x = 0, 0.25, 0.5 and 0.75, indicate new experimental features. A new relaxation peak at very low temperature is evidenced in the single alkali glass (x = 0) in addition to the two classical relaxation peaks previously described and assigned, respectively, to the motion of alkali ions (ionic peak) and a local rearrangement of the glassy structure involving non-bridging oxygen (NBO peak). For the three mixed alkali glasses, the mixed alkali relaxation peak is shown to be superimposed to two satellite peaks corresponding to the ionic and NBO peaks. Moreover, an 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 recent models proposed for the alkali effect and a model developed previously for mechanical relaxation in ionic glasses below T g.

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