Heat Capacity Changes at the Glass Transition in Mixed‐Alkali Tellurite Glasses

The heat capacity changes (ΔCp) at the glass transition in mixed-alkali tellurite glasses of (20-x)Li2O-xNa2O-80TeO2 that show different fragilities, depending on the Na2O/Li2O ratio, have been measured to obtain more-detailed information on the structure of tellurite glasses or supercooled liquids with a fragile character. A large increase in Cp is clearly observed at the glass transition in all the samples. The values of the heat capacity of glasses and supercooled liquids (Cpg and Cpe, respectively) and ΔCp (equal to Cpe-Cpg) at the glass transition temperature, Tg, are almost the same, irrespective of the Na2O/(Na2O+Li2O) ratio, indicating the absence of a clear, mixed-alkali effect on Cp at Tg. The values of Cpg at 200°C, Cpe at 300°C, and ΔCp at Tg are } 75, 120, and 45 J·(mol·K)−1, respectively. The ratio of the heat capacity of the glassy state to the heat capacity of the supercooled liquid state, i.e., Cpe/Cpg, is 1.6; this large value indicates that the (20-x)Li2OxNa2O80TeO2 system should be included in the category of fragile liquids. It has been suggested that the structural units of TeO4 trigonal bipyramid and TeO3 trigonal pyramid are weakly connected to each other and, thus, the intermediate structure varies largely as the temperature increases, which leads to a fragile character. The change in the fragility that is caused by the mixing of dissimilar alkali ions is not detected through ΔCp (i.e., there is no mixed-alkali effect on thermodynamic fragility); however, it is observed from the viscosity behavior at } }Tg (i.e., the presence of the mixed-alkali effect on kinetic fragility).