Vibrational Entropic Stabilization of Layered Chalcogenides: From Ordered Vacancy Compounds to Two-Dimensional Layers

Despite the rapid pace of computationally and experimentally discovering new two-dimensional layered materials, a general criterion for a given compound to prefer a layered structure over a nonlayered one remains unclear. Articulating such criteria would allow one to identify materials at the verge of an interdimensional structural phase transition between a 2D layered phase and a 3D bulk one, with potential applications in phase change memory devices. Here, we identify a general stabilization effect driven by vibrational entropy that can favor 2D layered structures over 3D bulk structures at higher temperatures, which can manifest in ordered vacancy compounds where phase competition is tight. We demonstrate this vibrational-entropy stabilization effect for three prototypical ordered vacancy chalcogenides, ZnIn₂S₄, In₂S₃, and Cu₃VSe₄, either by vacancy rearrangement or by cleaving through existing vacancies. The relative vibrational entropy advantage of the 2D layered phase originates mainly from softened out-of-plane dilation phonon modes.