# Ab initio determination of phase stabilities of dynamically disordered solids: rotational C2 disorder in Li2C2

## TL;DR

This paper explains how a material's structure changes with temperature using advanced simulations, focusing on a specific compound's phase transition.

## Contribution

A novel ab initio method is introduced to determine phase stabilities in dynamically disordered solids.

## Key findings

- The free energy difference between ordered and disordered phases of Li2C2 is calculated using AIMD thermodynamic integration.
- The entropy stabilizing the dynamically disordered cubic phase is captured through stress behavior analysis.

## Abstract

The temperature-induced orthorhombic to cubic phase transition in \documentclass[12pt]{minimal}
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\begin{document}$$\hbox {Li}_2\hbox {C}_2$$\end{document} 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 \documentclass[12pt]{minimal}
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\begin{document}$$\hbox {Li}_2\hbox {C}_2$$\end{document} happens from a phase with directionally ordered \documentclass[12pt]{minimal}
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\begin{document}$$\hbox {C}_2$$\end{document} dimers to a structure, where they are dynamically disordered. 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.