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 reg ion of K2O-WO3-TeO2 glasses (glass formation range 20-90 mol% TeO2) have be en studied to examine the structural relaxation behavior. The glasses with 60-70 mol% TeO2 and with both K2O and WO3 are thermally stable against crys tallization. It is proposed from Raman spectral analyses that TeO4 trigonal bipyramids change to TeO3 trigonal pyramids with the addition of K2O and t hat Te-O-W bonds are formed in the substitution of WO3 for TeO2. Heat capac ity changes of Delta C-p = 48-58 J mol(-1) K-1 (Delta C-p = C-pl - C-pg, C- pg and C-pl are the heat capacities of the glasses and supercooled liquids, respectively), and ratios C-pl/C-pg = 1.6-1.8 are obtained for xK(2)O . xW O(3). (100 - 2x)TeO2 glasses. The Delta C-p and C-pl/C-pg increase with dec reasing TeO2 content, indicating an increase in thermodynamic fragility wit h decreasing TeO2 content. But, the kinetic fragility estimated from the ac tivation energy for viscous flow is almost constant irrespective of TeO2 co ntent. These behaviors have been analyzed using the configurational entropy model proposed by Adam and Gibbs. The results indicate that in K2O-WO3-TeO 2 glasses, Te-O-Te bonds are weak and bond breakings occur easily in the gl ass transition region, leading to large configurational entropy changes and thus large Delta C-p. (C) 1998 Elsevier Science B.V. All rights reserved.