Based on a hopping model we show how the mixed alkali effect in glasses canbe understood if only a small fraction c_V ofthe available sites for the mobileions is vacant. In particular, we reproduce the peculiar behavior of theinternal friction and the steep fall (''vulnerability'') of the mobility of themajority ion upon small replacements by the minority ion. The single and mixedalkali internal friction peaks are caused by ion-vacancy and ion-ion exchangeprocesses. If c_V is small, they can become comparable in height even at smallmixing ratios. The large vulnerability is explained by a trapping of vacanciesinduced by the minority ions. Reasonable choices of model parameters yieldtypical behaviors found in experiments.
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