Thermal stability and heat capacity changes at the glass transition in K₂O-WO₃-TeO₂ glasses

The thermal stability and heat capacity changes in the glass transition region of K₂O-WO₃-TeO₂ glasses (glass formation range 20-90 mol% TeO₂) have been studied to examine the structural relaxation behavior. The glasses with 60-70 mol% TeO₂ and with both K₂O and WO₃ are thermally stable against crystallization. It is proposed from Raman spectral analyses that TeO₄ trigonal bipyramids change to TeO₃ trigonal pyramids with the addition of K₂O and that Te-O-W bonds are formed in the substitution of WO₃ for TeO₂. Heat capacity changes of ΔC_p = 48-58 J mol^-1 K^-1 (ΔC_p = C_p1 - C_pg, C_pg and C_p1 are the heat capacities of the glasses and supercooled liquids, respectively), and ratios C_p1//C_pg = 1.6-1.8 are obtained for xK₂O·xWO₃·(100 - 2x)TeO₂ glasses. The ΔC_p and C_p1/C_pg increase with decreasing TeO₂ content, indicating an increase in thermodynamic fragility with decreasing TeO₂ content. But, the kinetic fragility estimated from the activation energy for viscous flow is almost constant irrespective of TeO₂ content. These behaviors have been analyzed using the configurational entropy model proposed by Adam and Gibbs. The results indicate that in K₂O-WO₃-TeO₂ glasses, Te-O-Te bonds are weak and bond breakings occur easily in the glass transition region, leading to large configurational entropy changes and thus large ΔC_p.