Stacking-insensitive ferroelectricity in layered ScPS 3 : Lone-pair-driven polarization and strain-stabilized ferroelectricity against the antiferroelectric ground state

Interlayer stacking dependence of polarization remains a critical bottleneck for 2D vdW ferroelectrics in nanoelectronics. Herein, first-principles calculations reveal stacking-insensitive ferroelectricity in layered ScPS3, driven by out-of-plane perturbation of the P–P dimers. Unlike conventional ion-displacement ferroelectrics such as CuInP2S6, where polarization comes from metal-ion off-centering, the polarization of ScPS3 arises from local ionic displacement within the P–P dimers together with the the P lone pairs. This polarization originates from the intralayer asymmetry of the monolayer and remains nearly unchanged for different stacking orders (AA, AB), circumventing the stringent fabrication tolerances of sliding ferroelectrics. Although the antiferroelectric (AFE) phase is identified as the thermodynamic ground state and the ferroelectric (FE) phase is metastable at zero strain, biaxial strain beyond ±6% stabilizes the FE phase and enables a reversible AFE-to-FE phase transition. Together with the low switching barriers and the stacking-independent polarization (~0.73–0.74 μC/cm2), these results establish ScPS3 as a versatile platform for strain-engineered memories and flexible optoelectronics.