Cu⁺-Driven Ferroionic Structure and Pressure-Tunable Magnetism in Layered Thiophosphate CuVP₂S₆.

Abstract

Two-dimensional (2D) van der Waals (vdW) magnets offer a versatile platform to explore fundamental physics and low-dimensional functionalities. Metal thiophosphates (MTPs) with mobile Cu⁺ ions exhibit a ferroionic state, where polarization arises from ionic redistribution among multiple nearly degenerate sites. CuVP₂S₆ uniquely combines intrinsic ferromagnetism from the V sublattice with Cu⁺-driven ferroionic configurational freedom, enabling direct exploration of how ionic dynamics influence magnetic interactions. Herein, high-quality CuVP₂S₆ single crystals are synthesized, and their structural and physical properties are systematically investigated. Temperature-dependent neutron diffraction elucidates a ferroionic structure with dynamic distributions of copper ions across multiple crystallographic sites. The versatile occupations are driven by local symmetry-controlled orbital interactions between copper ions and surrounding ligands through a second-order Jahn-Teller mechanism. Magnetic measurements identify a ferromagnetic (FM) transition below 3.3 K. The pressure-controlled magnetocrystalline anisotropy and interlayer exchange interactions mediated by Cu⁺ migration are demonstrated, boosting the Curie temperature remarkably by over 60% and inducing a soft-to-hard FM transition unparalleled within the MTP family. These results demonstrate that ionic configurational freedom provides an efficient route to control magnetism, opening new possibilities for spintronic applications.