van der Waals Forces Control the Internal Chemical Structure of Monolayers within the Lamellar Materials CuInP2S6 and CuBiP2Se6

Following the recent demonstration that van der Waals forces control the ferroelectric ordering of layers within nanoflakes and bulk samples of CuBiP2Se6 and CuInP2S6, it is demonstrated that they also control the internal geometrical structure of isolated monolayers of these materials. This internal structure involves large displacements of copper atoms, either normal to the layer plane or else within the plane, that change its ligation environment. In both cases, the van der Waals dispersion force out-competes traditional bonding effects to control the structure. However, we find that the aspects of the dispersion force giving rise to each effect are uncorrelated: long-range effects control the interlayer ferroelectric ordering, whereas short-range effects control the internal layer structure. These conclusions are drawn considering the predicted properties of monolayers, bilayers, and bulk materials obtained using 14 density-functional theory-based methods. Although the different methods used often predict starkly different quantitative results, they concur as to the basic nature of ABP2X6 materials. Of the methods used, only the PBE-D3 and optPBEvdW methods were found to predict a wide range of observed properties without serious disparity. Finding optimal computational methods remains a significant challenge for which the unusual multiscale nature of the van der Waals interactions in ABP2X6 materials provides demanding criteria.