Tunable Ferroelectricity in Van der Waals Layered Antiferroelectric CuCrP₂S₆

Recent success in experimental and theoretical works on metal thiophosphates (MTPs) paved the way to add multiple functionalities of complex oxides, such as ferroelectricity, in 2D materials. To realize multiferroicity and magnetoelectric coupling on layered van der Waals materials, incorporating magnetic ions in the ferroelectric framework is desirable. Unfortunately, replacing the metal ion with a magnetic one in MTPs results in antiferroelectricity in which spontaneous macroscopic polarization is absent. Herein, the emergence of a tunable local ferroelectric state in antiferroelectric CuCrP₂S₆ possessing magnetic Cr³⁺ ion is reported. The spontaneous macroscopic polarization is observed, which is switchable by an external poling field through controlling a defect-dipole polarization in the quasi-antipolar state. The observations suggest that the formation of defect dipoles, which is facilitated by an order-disorder-type structural transition, is likely related to a metastable Cu site within the van der Waals gap and therefore is a smoking gun of the existence of a uniaxial quadruple potential well. The interaction between the defect-dipole polarization and dipoles in the antipolar matrix may lead to the emerging local ferroelectricity in antiferroelectric CuCrP₂S₆. The findings suggest a possibility of utilizing the local ferroelectricity of multiferroic MTPs for novel 2D applications.