Heat capacity changes and structural relaxation at glass transition in mixed-alkali tellurite glasses

The heat capacity changes and enthalpy relaxation at the glass transition in mixed-alkali tellurite glass-forming liquids of (20 - x)Li₂O-xNa₂O-80TeO₂ with different fragilities have been measured. An increase in heat capacity, ΔC_p = 45 J mol^-1 K^-1, is observed and the value is almost the same irrespective of the Na₂O/(Na₂O + Li₂O) ratio, i.e. no mixed-alkali effect (MAE) on thermodynamic fragility. The ratio of the heat capacities of the glassy state (C_pg) and supercooled liquid state (C_pl), C_pe/C_pg, is 1.6, indicating that the above system should be included in the category of fragile liquids. The MAE is observed on the activation energy for the relaxation process at the glass transition, ΔH, i.e. the presence of MAE on the kinetic fragility. Although the structural units of TeO₄ and TeO₃ are connected weakly with each other and thus the intermediate structure varies with increasing temperature, the rearrangement kinetics of TeO₄ and TeO₃ during heating is affected by the alkali mixing.