Fragility and compressibility at the glass transition

TL;DR

This paper investigates the glass transition by analyzing compressibility and sound velocities, revealing that vibrational and relaxational contributions relate differently to fragility, with implications for understanding glassy dynamics.

Contribution

It distinguishes vibrational and relaxational parts of compressibility at the glass transition and links their behavior to fragility using the concept of constrained degrees of freedom.

Findings

Vibrational part follows the fragility-independent Lindemann criterion.

The fragility dependence mainly arises from the relaxational part.

Data shows a linear fragility relation with deviations at higher fragilities.

Abstract

Isothermal compressibilities and Brillouin sound velocities from the literature allow to separate the compressibility at the glass transition into a high-frequency vibrational and a low-frequency relaxational part. Their ratio shows the linear fragility relation discovered by x-ray Brillouin scattering [1], though the data bend away from the line at higher fragilities. Using the concept of constrained degrees of freedom, one can show that the vibrational part follows the fragility-independent Lindemann criterion; the fragility dependence seems to stem from the relaxational part. The physical meaning of this finding is discussed. [1] T. Scopigno, G. Ruocco, F. Sette and G. Monaco, Science 302, 849 (2003)