Structural developments in fragile glass forming oxides

Brillouin light has been used to measure the complex mechanical modulus of the molecular scale structure in oxide melts. The storage modulus reflects the degree of networking and structural integrity, while the loss modulus, which can be attributed to aperiodic motions of structural components, provides a measure of the mobility of these entities. From the loss modulus it is possible to derive the zero-shear rate viscosity, which is characteristic of a structure in thermodynamic equilibrium, and thereby provide a less encumbered insight into the molecular scale mechanisms that are underlying to the viscous momentum transport. In this paper we compare the visco-elastic behavior of alkali borates and alkali tellurites, particularly in the temperature range between T_g and the equilibrium melting point. While in borates several mechanisms can be identified as contributing to structural relaxation, in tellurites a single mechanism dominates. The assumption of thermo-rheological simplicity, however, is invalid, and a new visco-elastic model has been developed for the description of our results.