Thermodynamics and Dynamics of a Monoatomic Glass-Former. Constant Pressure and Constant Volume Behavior

TL;DR

This study uses simulations to explore the thermodynamics and dynamics of a monoatomic glass-former, revealing new insights into entropy contributions, landscape non-Gaussianity, and transport control variables.

Contribution

It introduces a new analysis of the configurational landscape and thermodynamics of a monoatomic glass-former, including a novel entropy-based break in Adam-Gibbs plots and a new equation emphasizing enthalpy for transport.

Findings

Linear increase in the effective Gaussian width of inherent structures.

Identification of a new slope break in the Adam-Gibbs plot with excess entropy.

A new equation for diffusivity emphasizing enthalpy over entropy.

Abstract

We report constant-volume and constant-pressure simulations of the thermodynamic and dynamic properties of the low-temperature liquid and crystalline phases of the modified Stillinger-Weber (mSW) model. We have found an approximately linear increase of the effective Gaussian width of the distribution of inherent structures. This effect comes from non-Gaussianity of the landscape and is consistent with the predictions of the Gaussian excitations model representing the thermodynamics of the configurational manifold as an ensemble of excitations, each carrying an excitation entropy. The mSW model provides us with both the configurational and excess entropies, with the difference mostly attributed to vibrational anharmonicity. We therefore can address the distinction between the excess thermodynamic quantities often used in the Adam-Gibbs (AG) equation. We find a new break in the slope of the constant pressure AG plot when the excess entropy is used in the AG equation. The simulation diffusivity data are equally well fitted by applying a new equation, derived within the Gaussian excitations model, that emphasizes enthalpy over entropy as the thermodynamic control variable for transport in viscous liquids.