Thermodynamic Behavior of a Model Covalent Material Described by the Environment-Dependent Interatomic Potential

TL;DR

This study uses molecular dynamics to explore the thermodynamic properties of silicon modeled with EDIP, revealing unusual behaviors like multiple disordered phases, density anomalies, and potential critical points, highlighting the model's utility for studying disordered systems.

Contribution

It demonstrates that the EDIP model for silicon captures complex thermodynamic phenomena, including phase transitions and anomalies, not typically seen in simpler models.

Findings

Existence of two disordered phases with a first-order transition.

Density decreases upon melting of the amorphous phase.

Negative thermal expansion in both crystalline and amorphous phases.

Abstract

Using molecular dynamics simulations we study the thermodynamic behavior of a single-component covalent material described by the recently proposed Environment-Dependent Interatomic Potential (EDIP). The parameterization of EDIP for silicon exhibits a range of unusual properties typically found in more complex materials, such as the existence of two structurally distinct disordered phases, a density decrease upon melting of the low-temperature amorphous phase, and negative thermal expansion coefficients for both the crystal (at high temperatures) and the amorphous phase (at all temperatures). Structural differences between the two disordered phases also lead to a first-order transition between them, which suggests the existence of a second critical point, as is believed to exist for amorphous forms of frozen water. For EDIP-Si, however, the unusual behavior is associated not only with the open nature of tetrahedral bonding but also with a competition between four-fold (covalent) and five-fold (metallic) coordination. The unusual behavior of the model and its unique ability to simulation the liquid/amorphous transition on molecular-dynamics time scales make it a suitable prototype for fundamental studies of anomalous thermodynamics in disordeered systems.