Recent advancements in two-dimensional van der Waals (vdW) ferroelectric materials have highlighted the significance of both interlayer and intralayer degrees of freedom in governing their functional properties. Among these, ${\mathrm{CuInP}}_2{\mathrm{S}}_6$ (CIPS) stands out due to the unique displacement behavior of ${\mathrm{Cu}}^{+}$ ions within the layers and across the vdW gap in the layered CIPS structure. In this work, we present a comprehensive first-principles study of the structural, vibrational, ferroelectric, and electronic properties of CIPS under hydrostatic pressure (0–18 GPa). Our calculations predict a pressure-induced structural transition from the monoclinic $Cc$ phase to a trigonal $P31c$ phase above 6.8 GPa. At higher pressures, a metastable centrosymmetric $P\overline{3}1c$ phase can be stabilized, which serves as the paraelectric reference for the ferroelectric $P31c$ phase. Our lattice dynamics analysis reveals two zone-center instabilities, polar ${\mathrm{\Gamma }}_2^{-}$ and antipolar ${\mathrm{\Gamma }}_2^{+}$, in the centrosymmetric reference $P\overline{3}1c$ structure, leading to the identification of distinct ferroelectric ($P31c$) and antipolar ($P\overline{3}$) phases. By mapping the two-dimensional potential energy surface as a function of these two lattice instabilities, we further identify a low-energy-barrier polarization switching pathway for the ferroelectric $P31c$ phase, with the antipolar $P\overline{3}$ phase serving as an intermediate state, supporting the theory of asynchronous switching mechanism in CIPS. Using the modern theory of polarization, we distinguish between low- and high-polarization variants of the $P31c$ phase. Electronic structure analysis shows pressure-induced band-gap narrowing and a transition from direct to indirect gap, while retaining semiconducting character. These results provide key insights for designing pressure-tunable ferroelectric and electronic functionalities in layered materials, and highlight their potential for applications in nonvolatile memory, pressure-sensitive sensors, and flexible electronics.

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• Antiferroelectricity
• Electric polarization
• Electronic structure
• Ferroelectricity
• Phase transitions
• Phonons
• Pressure effects