Atomistic structural mechanism for the glass transition: Entropic contribution

Abstract

A popular Adam-Gibbs scenario has suggested that the excess entropy of glass and liquid over crystal dominates the dynamical arrest at the glass transition with exclusive contribution from configurational entropy over vibrational entropy. However, an intuitive structural rationale for the emergence of frozen dynamics in relation to entropy is still lacking. Here we study these issues by atomistically simulating the vibrational, configurational, as well as total entropy of a model glass former over their crystalline counterparts for the entire temperature range spanning from glass to liquid. Besides confirming the Adam-Gibbs entropy scenario, the concept of Shannon information entropy is introduced to characterize the diversity of atomic-level structures, which undergoes a striking variation across the glass transition, and explains the change found in the excess configurational entropy. Hence, the hidden structural mechanism underlying the entropic kink at the transition is revealed in terms of proliferation of certain atomic structures with a higher degree of centrosymmetry, which are more rigid and possess less nonaffine softening modes. In turn, the proliferation of these centrosymmetric (rigid) structures leads to the freezing-in of the dynamics beyond which further structural adjustements become highly unfavourable, thus explaining the kink in the configurational entropy at the transition.

First-principles study on the specific heat of glass at glass transition with a case study on glycerol

“…Essentially the same conclusion was already deduced by experiment 65 and MD simulation 32 for metallic glasses, although the different term entropy is used in place of energy. The description in terms of energy has a merit to explain the issue of fragility, which becomes soon clear.…”

“…The same holds for entropy: only the total entropy S is physically meaningful, while the configurational entropy S c is defined as the remaining part of S after subtracting the vibrational part to entropy S ph . 32 b. Energy dissipation due to atom relaxation.…”

“…On account of the above difficulties in calculating ∆C p , in the first part of this paper, general principles for calculating specific heat of liquids and glasses are given, which is described in Sec II. Recently, essentially the same approach based on the total-energy theory has been published by Han et al 32 Their approach is based on the entropic representation. Here, the energy representation with explicitly specifying state variables is employed.…”

First-principles study on the specific heat jump in the glass transition of silica glass and the Prigogine-Defay ratio

“…By analyzing the three components, we conclude that the contribution from structural energy, E st , is responsible for the specific-heat jump of silica glass. By considering similar conclusions in other studies on different glasses, 27,61,62 although the degree of contribution of ∆C st is different, it is certain that the property that ∆ p is predominated by ∆ st is a general property of glasses. The value of ∆C p of silica glass varies between 0.50R and 0.68R, depending on the cooling rate.…”

Does the Adam-Gibbs relation hold in simulated supercooled liquids?

“…First, using the excess entropy, S exe = S liq −S cry , instead of S conf = S liq −S glass is an approximation, in general. The validity of S glass ≈ S cry has been widely studied [64,[93][94][95][96][97][98][99] and its validity seems to be non-universal [64]. Typically, S glass is determined from the heat capacity of the non-equilibrium glass state, and so it still involves some approximations compared to its theoretical definition [63].…”