Ac conductivity and ion transport in K+-for-Na+ ion-exchanged glasses: exchange experiments below and above the glass transition temperature

Samples of commercial soda-lime silicate glass were ion exchanged in KNO3/NaNO3 melts below and slightly above the glass transition temperature (TG), and the ion dynamics and transport were characterized by means of ac conductivity spectroscopy. Our results demonstrate unambiguously that mixed alkali effects are present in ion-exchanged glasses. However, these effects depend strongly on the thermal history. For the glasses exchanged below TG we do not observe a conductivity minimum as a function of the degree of exchange, in agreement with the results of other studies. On the other hand, when the exchange was done slightly above TG, a conductivity minimum was detected. Nevertheless, the spectral shape of the conductivity and the dependence of the dc conductivity on the degree of exchange point to significant differences in the ion dynamics between glasses exchanged slightly above TG and melt-grown glasses of the same compositions.