Chapter 7: Mixed Monovalent Germanate Glasses Check Access

Deviations from additivity in the properties of glasses containing two different alkali ions are attributed to the mixed alkali effect (MAE), or, if the glass contains any non-alkali monovalent ions, the mixed monovalent ion effect (MME). The densities and refractive indices of germanate and aluminogermanate glasses do not display a MME. The thermal expansion coefficients of Na/K and Na/Rb germanate glasses exhibit a positive MAE, while those of Na/Cs germanate glasses exhibit a negative MAE. The isothermal electrical conductivities of mixed alkali germanate and aluminogermanate glasses exhibit a positive MAE if the alkali oxide concentration is low and a negative MAE at higher alkali oxide concentrations. The isothermal ionic diffusivities of sodium and rubidium ions exhibit a slightly negative MAE. The T_g of mixed alkali germanate glasses always exhibits a negative MAE. Mixed alkali germanate glasses are more stable against crystallization than single alkali germanate glasses. The hydroxyl spectra of the Na/K and Li/Cs glasses indicate that there are no non-bridging oxygens (NBOs) in glasses containing between 90 and 100 mol% GeO₂, with NBOs appearing for glasses containing less GeO₂. Mixed alkali germanate glasses form more hydroxyl during melting than do the corresponding single alkali germanate glasses.