Heat capacities of a series of mixed-alkali glasses of composition(in mol%) 24.4(Na20 + K,O) -75.6Si0, were measured in the transition region by differential scanning calorimetry. The glass heat capacities at 700 K and the equilibrium liquid heat capacities are the same for all glasses on a per-gatom basis and equal, respectively, to 5.6&0.1 and 6.8k 0.1 cal/g.atom K. The glass transition temperatures exhibited negative deviations from additivity, but the heat capacity curves in the transition region of all the glasses for identical heating rates and thermal histories could be superimposed on the same reduced plot. This behavior indicates that the shapes of the structural relaxation functions are the same for all the glasses. These results support Shelby's conclusion that there is no unique "mixed-alkali effect" on thermodynamic or structural relaxation properties and that the term should be reserved for propcrties relating to ionic transport.

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Electrical conductivity and relaxation data were measured for glasses of composition 0.242[XKzO+(l -X)Na,O] -0.758Si0, from 25" to 205°C and in the dilute Na+ ion composition range X =0.9 to 1. The decrease in electrical conductivity u when the majority alkali K+ is replaced with the foreign alkali Na+ is most rapid in the very dilute range X =0.98 to 1 and becomes less rapid at larger Na+ contents. The electric field relaxation times agree well with the single-alkali mechanical relaxation times. The electric field relaxation function becomes more exponential with increasing foreign alkali content, indicating a corresponding decrease in the concentration of mobile ions. Attempts to account for the decrease in u with increasing foreign alkali content by assuming that the single-alkali glass is a strong electrolyte and that the foreign alkali complexes with and immobilizes the majority alkali were unsuccessful. Calculated values of the number of majority alkali ions complexed by a foreign-alkali ion were unreasonably large.