Effects of density and temperature on correlations between fragility and glassy properties

A fundamental question in condensed matter science is to understand what governs the increase of relaxation time, and ultimately the glass formation, in liquids upon cooling. This phenomenon, which can be observed in chemically very different systems ranging from metals to polymers, has had practical importance for thousands of years but remains poorly understood from a fundamental point of view. The glass transition is general but the relaxation time has qualitatively different temperature dependencies in different systems. This qualitative difference can be quantified via the fragility. Within the past decades a lot of experimental effort has been put into correlating isobaric fragility at atmospheric pressure with other properties of the liquid and the glass. Here we show how such correlations to the isobaric fragility in some cases reflect the effect of density on the relaxation time while they in other cases are related to the intrinsic effect of temperature on the relaxation time. We specifically demonstrate that different properties related to the vibrational dynamics in the glass might contain information about different aspects of the mechanisms that govern the viscous slowing down. This insight is central if the correlations are to be used in the development of theories and models of the glass formation.