Ab initio Determination of Phase Stabilities of Dynamically Disordered Solids: rotational C2 disorder in Li2C2

TL;DR

This paper uses ab initio molecular dynamics and stress-strain thermodynamic integration to study the phase transition in Li2C2, revealing how entropy stabilizes the dynamically disordered phase and proposing a new approach for such materials.

Contribution

It introduces a novel ab initio method combining thermodynamic integration and machine learning to analyze phase stability in dynamically disordered solids.

Findings

Quantified free energy difference between phases

Identified entropy as key stabilizer of the disordered phase

Proposed combined AIMD and machine learning approach

Abstract

The temperature-induced orthorhombic to cubic phase transition in Li2C2 is a prototypical example of a solid to solid phase transformation between an ordered phase, which is well described within the phonon theory, and a dynamically disordered phase with rotating molecules, for which the standard phonon theory is not applicable. The transformation in Li2C2 happens from a phase with directionally ordered C2 dimers to a structure, where they are dynamically disorderd. We provide a description of this transition by employing ab initio molecular dynamics (AIMD) based stress-strain thermodynamic integration on a deformation path that connects the ordered and dynamically disordered phases. The free energy difference between the two phases is obtained. The entropy that stabilizes the dynamically disordered cubic phase is captured by the behavior of the stress on the deformation path. We also show that a combination of the stress-strain thermodynamic integration and machine learning force field methodologies appears as a promising path in studies of dynamically disordered materials.