Abstract
Transformation‐range viscosities are reported for Na2O‐, K2O‐, Rb2O‐, and Cs2O‐GeO2 glasses. A minimum in viscosity occurs between 1 and 5 mol% alkali oxide and a maximum between 17 and 22 mol% alkali oxide. The thermal‐expansion coefficients of the Na2O‐GeO2 glasses also exhibit a minimum at < 5 mol% Na2O. The accepted structural model for alkali germanate glasses does not predict the minimum at low R2O concentrations. A modification of this model is suggested to explain these data.

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