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

The temperature-induced orthorhombic to cubic phase transition in Li2C2is a prototypical ex-ample of a solid to solid phase transformation between an ordered phase, which is well describedwithin 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 disordered. We provide a description of this transition within the recently developed method (Klarbring et al.,Phys.Rev. Lett. 121, 225702 (2018)) 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.