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.