Structural transitions and polyamorphism in glass-forming oxides

Brillouin light scattering was used to monitor the structural evolution of glass-forming oxide melts as a function of temperature, including the glass transition range. The storage and loss components of the high-frequency complex mechanical modulus are interpreted in terms of the rigidity resulting from network connectivity and the energy dissipated in aperiodic motions of small structural moieties, respectively. A series of glass-forming oxides with variable fragility was investigated. Most revealing is the fact that thermo-rheological simplicity does not apply to high-frequency processes in these supercooled oxide melts. In order to describe the temperature dependence of the complex moduli consistently for all systems we investigated, we enhanced Maxwell's theory of visco-elasticity to explicitly account for the transition between thermodynamically distinct structural states corresponding to liquid and glass respectively. Using complementary information pertaining to short-range structural characteristics, based on atomic-scale computer simulations and experiments, we have identified mechanisms that are invoked upon transition between structural states. Thereby, we uncovered some attributes of the beginning and ending configurations associated with, what we believe to be, the polyamorphic transformations that underlie the glass transition.