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<p>The temperature-induced orthorhombic to cubic phase transition in <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\hbox {Li}_2\hbox {C}_2\)</EquationSource> </InlineEquation> 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 <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\hbox {Li}_2\hbox {C}_2\)</EquationSource> </InlineEquation> happens from a phase with directionally ordered <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\hbox {C}_2\)</EquationSource> </InlineEquation> 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.</p>

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

  • 期刊论文

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  • 发表日期：

    2026年3月12日

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  • Johan Klarbring,
  • Stanislav Filippov,
  • Ulrich Häussermann,
  • Sergei I. Simak

摘要

The temperature-induced orthorhombic to cubic phase transition in \(\hbox {Li}_2\hbox {C}_2\) 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 \(\hbox {Li}_2\hbox {C}_2\) happens from a phase with directionally ordered \(\hbox {C}_2\) 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.