Fragility of oxide melts as a thermodynamic parameter

Oxide melts demonstrate a deviation from the Arrhenius-type viscosity behaviour in which the activation energy of viscosity changes from a high value at low temperature to a low value at high temperature. This change in activation energy with temperature can be used to classify network liquids as strong or fragile depending on the form of the dependence of logarithm of viscosity versus reduced temperature 1/T. Their activation energy changes only slightly with temperature for strong glass-forming liquids which demonstrate nearly Arrhenius temperature dependence of viscosity. In contrast the activation energy of fragile glass forming liquids changes significantly with temperature and their viscosity deviates significantly from Arrhenius behaviour. Doremus suggested a criterion for oxide melt fragility utilising the ratio of the activation energy at high viscosity to that at low viscosity. We demonstrate that the Doremus criterion of fragility can be expressed in terms of thermodynamic parameters of the defects mediating viscous flow in the oxide melts: R D =1+H d /H m , where H d is the enthalpy of formation and H m is the enthalpy of motion of such defects. Hence the fragility of oxide melts is an intrinsic thermodynamic parameter of melts which can be determined unambiguously by experiment.