Decoupling between enthalpy relaxation and viscous flow and its structural origin in fragile oxide glass-forming liquids

The structural relaxation kinetics at the glass transition in tellurium oxide (TeO₂)-based glasses has been examined from viscosity and heat-capacity measurements to clarify the features of the structural relaxation in fragile oxide glass-forming liquids. A large decoupling between enthalpy relaxation and viscous flow, i.e., a large discrepancy between the activation energies for the enthalpy relaxation (recovery), ΔH, and viscous flow, E_η, has been demonstrated in TeO₂-based glasses. The values in xK₂O·xMgO·(100 - 2x)TeO₂ glasses, for example, are ∼919-1051 kJ/mol for ΔH and ∼ 577-701 kJ/mol for E_η, given the ratio of ΔH/_Eη ≈ 1.44-1.59. Some viscosity and heat-capacity data (all data have been reported previously) obtained from similar experiments in Sb₂O₃-B₂O₃ glasses belonging to the category of strong glass-forming liquids have been reanalyzed in this paper for comparison; a strong coupling was found to exist between ΔH and E_η, i.e., ΔH/E_η ≈ 0.98-1.18. An origin of decoupling between ΔH and E_η in fragile glass-forming systems such as TeO₂-based glasses has been discussed by considering the glass structure model for fragile glasses; strongly bonded correlated (highly constrained) regions are surrounded or connected by weakly bonded noncorrelated (unconstrained) parts.