Van der Waals forces control ferroelectric-antiferroelectric ordering in CuInP <inf>2</inf> S <inf>6</inf> and CuBiP <inf>2</inf> Se <inf>6</inf> laminar materials

Publication Type:
Journal Article
Chemical Science, 2018, 9 (39), pp. 7620 - 7627
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© The Royal Society of Chemistry 2018. 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.
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