The iso-structural viscosity, configurational entropy and fragility of oxide liquids

This paper describes how the fragility of a liquid is linked to the ratio between the energy barrier (E_eq) for
the equilibrium viscous behavior and that (E_iso) for the non-equilibrium iso-structural viscous behavior.
Using the concept of iso-structural viscosity, two functions describing the variation of the configurational
entropy (S_c) with temperature (T) are obtained from the Avramov-Milchev (AM) and the Vogel-Fulcher-
Tammann (VFT) viscosity equations, respectively. The two S_c(T) functions exhibit different relations to
the liquid fragility. The AM S_c(T) function is a power function with the exponent of F - 1, where F is
the AM fragility index. In this case, Sc vanishes at T = 0 K. For the VFT function, Sc vanishes as T is lowered
to a finite temperature T₀, whereas it reaches the maximum value S_c,max at infinitively high T. S_c,max is proportional
to the VFT fragility index. Thus, the VFT equation is not only a dynamical, but also a thermodynamic
model. It is proved that for oxide liquids, the VFT equation describes viscosity data better than the
AM equation, provided the pre-exponential factor η₀ is fixed to a generally accepted value, e.g.,
10^-3.5 Pa s.