Mixed-Alkali Effect in the Dilute Foreign-Alkali Region. Failure of the Strong Electrolyte/Cationic Interaction Model

Electrical conductivity and relaxation data were measured for glasses of composition 0.242[XK2O+(1‐X)Na2O]‐0.758SiO2 from 25° to 205°C and in the dilute Na+ ion composition range X= 0.9 to 1. The decrease in electrical conductivity σ 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 σ 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.