Vibrational Entropic Stabilization of Layered Chalcogenides: From Ordered Vacancy Compounds to 2D Layers

TL;DR

This paper reveals that vibrational entropy can stabilize 2D layered chalcogenides over 3D structures at higher temperatures, especially in vacancy-ordered compounds, aiding the discovery of new layered materials.

Contribution

It introduces a general vibrational-entropy stabilization criterion for layered structures, demonstrated on specific chalcogenides, linking phonon modes to phase stability.

Findings

Vibrational entropy favors 2D layers at high temperatures.

Softened out-of-plane phonon modes drive stabilization.

Applicable to vacancy-ordered layered compounds.

Abstract

Despite the rapid pace of computationally and experimentally discovering new two-dimensional layered materials, a general criteria for a given compound to prefer a layered structure over a non-layered one remains unclear. Articulating such criteria would allow one to identify materials at the verge of an inter-dimensional structural phase transition between a 2D layered phase and 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 and 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.