Thermal expansion of mixed−alkali germanate glasses

Abstract

Thermal expansion measurements were made for several series of Na₂O⋅K₂O⋅GeO₂ glasses, for one series of Na₂O⋅Rb₂O⋅GeO₂ glasses, and for one series of Na₂O⋅Cs₂O⋅GeO₂ glasses. Negative deviations from additivity were observed in the critical and deformation temperatures in every case, with the deviations increasing with total alkali concentration. Positive deviations from additivity were noted in the thermal expansion coefficients of the Na₂O+K₂O and Na₂O+Rb₂O glasses, whereas the Na₂O+Cs₂O glasses exhibited negative deviations from additivity. Changes in the coordination of the network forming elements had no effect on the nature of the deviations from additivity. Finally, since these deviations seem so similar to those observed in other alkali−free glass systems, the common usage of the term ’’mixed−alkali effect’’ to describe their occurrence is questioned.