Sliding van der Waals polytypes

Compared with electronic phase transitions, structural phase transitions of crystals are challenging to control owing to the energy cost of breaking dense solid bonds. Recently, electric field switching of stacking configuration between honeycomb layers, held together by relatively weak van der Waals attractions, has been demonstrated. Different structural configurations — or polytypes — of 2D van der Waals materials host diverse electronic orders such as intrinsic polarizations and magnetism. In this Perspective, we discuss stacking energies, symmetries and orbital overlaps that underlie the band structures and internal charge distributions of these polytypes and their effect on properties such as interfacial ferroelectricity, ladder-like cumulative polarization, superconductivity and orbital magnetic orders. We also identify the challenges of harnessing these switching mechanisms for rapid, local and practical multiferroic devices. van der Waals polytypes are a class of periodic crystals that differ in their stacking configurations and can transform from one to another by discrete interlayer shifts. This Perspective discusses recent reports of the properties, structural stabilities and switching responses of polytypes and highlights challenges towards multiferroic opportunities.