The dynamics of strong and fragile glass formers: vibrational and relaxation contributions

Abstract

There are two contributions to the low-frequency excitation spectra (Raman and neutron scattering, specific heat, etc.) of glass formers: relaxations and vibrations. It is shown from the analysis of low-frequency Raman spectra that the relative weight of vibrational over relaxational excitations is larger for less fragile (in Angell's classification) glass formers. The spectra are compared with predictions of mode coupling theory (MCT) for relaxation processes. Qualitatively the predicted behaviour is observed in all analyzed systems. However, some quantitative disagreement due to significant vibrational contribution to the spectra is found for intermediate glass formers. The spectra are also analyzed using a model of vibrations coupled with relaxations. It is found that the temperature at which overdamping of the low-frequency vibrations happens is essentially the critical temperature of MCT. New details of a scenario for the liquid-glass transition are suggested.