On the Ordering Mechanism of Cu + in 2D van der Waals Multiferroic CuCrP 2 S 6

ABSTRACT CuCrP 2 S 6 is a van der Waals multiferroic where the tunable Cu + sublattice underpins its exceptional ferroelectric and electronic switching properties. Yet, the microscopic mechanism governing Cu + ordering has remained elusive. Here, we combine single‐crystal X‐ray and neutron diffraction with pair distribution function analysis to uncover a temperature‐driven evolution of Cu + ordering, giving rise to an incommensurate quasi‐antipolar phase between the paraelectric and antiferroelectric states. The modulation originates from correlated Cu + occupancy redistribution coupled to breathing distortion of surrounding S 3 triangles, establishing a symmetry‐adapted lattice distortion mode. Diffuse scattering persisting over 35 K above the transition confirms that the structural instability follows an order‐disorder mechanism. The spontaneous off‐centering of Cu + positions CuCrP 2 S 6 as a model platform for correlated order‐disorder phenomena in 2D layered ferroics, and provides design principles for next‐generation memory and logic devices.