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.

中文翻译：

---

范德华力控制 CuInP2S6 和 CuBiP2Se6 层状材料中的铁电-反铁电排序†

我们展示了范德华 (vdW) 如何克服共价力和离子力来控制 CuInP ₂ S ₆纳米片以及 CuInP ₂ S ₆和 CuBiP ₂ Se ₆晶体中的铁电有序性。虽然这些二维层状材料的自组装明显受到 vdW 效应的控制，但这一结果表明内部层结构也受到类似的控制。使用多达 14 种第一原理计算方法，我们预测这两种材料的双层应该是反铁电的。然而，反铁电纳米薄片和块体材料被证明体现了两种根本不同类型的层间相互作用，与铁电有序相比，vdW 力强烈支持其中一种，强烈反对另一种。涉及铜原子的强特定 vdW 相互作用控制了这种效应。这两个大的相反 vdW 贡献的依赖于厚度的显着抵消导致小的净效应，该净效应与弱离子贡献相互作用以控制铁电排序。