Light-scattering study of slow and fast dynamics in a strong inorganic glass former

Abstract

The dynamic properties of glassy and liquid ${\mathrm{As}}_{2}{\mathrm{O}}_{3}$ are investigated over a wide temperature range, in both the microscopic and macroscopic time domains by Brillouin scattering (BS) and photon correlation spectroscopy (PCS). The two characteristic properties of sound propagation, velocity, and attenuation were found to exhibit considerable, although unexpected, changes very close to the glass transition temperature ${T}_{g}.$ The high-frequency density fluctuations were quantitatively treated using a phenomenological formulation for the corresponding memory function, which considers both slow and fast processes. The obtained viscoelastic parameters were found to follow physically acceptable temperature dependencies. Both density and orientation autocorrelation functions show a very narrow distribution of relaxation times with a shape parameter close to 0.8. The peculiarities of the sound-velocity and the sound-absorption coefficient as well as the comparison between the PCS and the BS relaxation times confirmed the existence of two relaxation processes differing by 10 orders of magnitude near ${T}_{g}.$ The difference in activation energies, for the fast process, between strong and fragile glasses is discussed on the basis of the stability of asymmetric double-well potentials over a relaxation period. Evidence is provided conforming to the two fluid model predictions, invoking long-range density fluctuations. Pseudotransformations of chemically and topologically ``acceptable'' structures seem to be the driving force for low-energy excitations in network bonded glasses.