van der Waals forces control ferroelectric-antiferroelectric ordering in CuInP₂S₆ and CuBiP₂Se₆ laminar materials

We show how van der Waals (vdW) forces outcompete covalent and ionic forces to control ferroelectric ordering in CuInP₂S₆ nanoflakes as well as in CuInP₂S₆ and CuBiP₂Se₆ crystals. While the self-assembly of these 2D layered materials is clearly controlled by vdW effects, this result indicates that the internal layer structure is also similarly controlled. Using up to 14 first-principles computational methods, we predict that the bilayers of both materials should be antiferroelectric. However, antiferroelectric nanoflakes and bulk materials are shown to embody two fundamentally different types of inter-layer interactions, with vdW forces strongly favouring one and strongly disfavouring the other compared to ferroelectric ordering. Strong specific vdW interactions involving the Cu atoms control this effect. Thickness-dependent significant cancellation of these two large opposing vdW contributions results in a small net effect that interacts with weak ionic contributions to control ferroelectric ordering.