We present a detailed analysis of correlations between fragility and other parameters of glass-forming systems. The analysis shows the importance of the ratio between the instantaneous bulk and shear modulus of glass-forming systems, or their Poisson ratio, for structural $\alpha$ relaxation and fast dynamics. In particular, for simple glass formers, the bulk to shear modulus ratio in the glassy state correlates with fragility in the liquid state and is inversely proportional to the intensity of the boson peak. A simple relationship between the temperature dependence of the viscosity of liquids at high temperatures and near the glass transition is used to rationalize these correlations. We argue that the ratio of the moduli controls the high-temperature activation energy of the structural relaxation and in this way affects the fragility. The ratio also defines the amplitude of the structural relaxation (i.e., the nonergodicity parameter) and the latter influences the strength of the boson peak. These observations might explain the puzzling correlation observed between the fragility and fast dynamics in glass-forming systems.

• Received 30 December 2004

DOI:https://doi.org/10.1103/PhysRevE.71.061501