van der Waals forces control ferroelectric–antiferroelectric ordering in CuInP(2)S(6) and CuBiP(2)Se(6) laminar materials

We show how van der Waals (vdW) forces outcompete covalent and ionic forces to control ferroelectric ordering in CuInP(2)S(6) nanoflakes as well as in CuInP(2)S(6) and CuBiP(2)Se(6) 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.