Thermodynamics and dynamics of a monoatomic glass former. Constant pressure and constant volume behavior

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 (SW) model. We have found an approximately linear temperature 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 SW 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 to interpret experiments, and configurational thermodynamics used to describe the dynamics in the Adam-Gibbs (AG) equation. However we are limited computationally to work at temperatures above the "crossover" temperature at which the breakdown in the Adam-Gibbs relation has been identified in laboratory studies. We find a new break in the slope of the constant pressure AG plot (in the same sense but at much higher temperature than with laboratory data) when the excess entropy is used in the AG equation. This break, which we associate with anharmonic vibrational effects, is not seen when the configurational entropy is used. The simulation diffusivity data are equally well fitted by the AG equation and by a new equation, derived within the Gaussian excitations model, that emphasizes enthalpy over entropy as the thermodynamic control variable for transport in viscous liquids. We show that the modified SW model has close links to the behavior observed for bulk metallic glasses, both in its diffusional and in its thermodynamic properties.