The Structural Change Associated with Amorphous Solidification: A General Order Parameter Description

A central problem posed by the transformation of liquid into glass on cooling is to develop a general atomic level description of amorphous solidification. The answer proposed in this paper is to measure a configuration’s capacity to restrain the motion of the constituent atoms. Here we show that the instantaneous normal modes can be used to define a measure of atomic restraint that accounts for the difference between fragile and strong liquids and the collective length scale of the supercooled liquid. This measure provides a microscopic account of solidification that is independent of the rate of cooling. We show that a related measure can be obtained experimentally from the analysis of the temperature dependence of speckle scattering. These results represent a significant simplification of the description of amorphous solidification and a potentially powerful systematic treatment of the impact of thermal and mechanical history, atomic interactions, molecular shape and flexibility and surface effects on the structural transformation from supercooled liquid to amorphous solid.