Abstract
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.
Highlights
Of recent recognition is the general principle that van der Waals forces can compete against covalent and ionic forces to control chemical bonding.[1]
What is revealed for both ABP2X6 laminar materials considered is that antiferroelectric structures intrinsically involve two different types of inter-layer interactions with signi cantly different van der Waals forces, one type being much more attractive than the van der Waals force acting between ferroelectric structures while the other is much less attractive
The difference between these two van der Waals bonding effects compared to the weak dipole–dipole stabilization energy operative in ferroelectric structures controls the properties of crystals and nano akes
Summary
Of recent recognition is the general principle that van der Waals forces can compete against covalent and ionic forces to control chemical bonding.[1]. We show that strong speci c van der Waals interactions associated with so 8,9 copper ions go beyond this to control internal ferroelectric ordering in polar laminar materials containing sheets of so sulfur or selenium atoms. These forces prove to be sufficiently strong to, in certain cases, overturn the natural electrostatic forces driving ferroelectricity to make antiferroelectric structures – a somewhat rare effect that is only recently becoming understood.[10]
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