Switchable Out‐of‐Plane Polarization in 2D LiAlTe 2

Covalent‐polar semiconductors that show intrinsic 2D vertical polarization present new device opportunities. These materials differ from ordinary ferroelectrics in that they are able to maintain polarization normal to a surface even with an unscreened depolarization field. Identifying phases that exhibit intrinsic 2D vertical polarization is an ongoing challenge. Here the discovery of a new promising phase via computational material design, specifically 2D LiAlTe 2 , is reported. The design concept is developed from a physical understanding of 3D hyperferroelectric covalent polar semiconductors. A structure determination method combining a swarm intelligence algorithm and first‐principles calculations is used to identify energetically stable structures. In addition to the expected layered version of bulk LiAlTe 2 , β‐LiAlTe 2 , a novel 2D structure, γ‐LiAlTe 2 , is found. In this phase, the vertical dipole can be switched between 0.07 and −0.11 e Å. This switching is triggered by the movement of the Li atom between two local energy minima. The associated asymmetric double‐well energy profile can be continuously tuned by the applied electric field as well as strain. There is, therefore, a reversible transition between two polar states. This discovered off‐plane switchability provides an opportunity for a 2D γ‐LiAlTe 2 ‐ based interfacial phase change memory device; for example, by growing γ‐LiAlTe 2 /GeTe heterostructures.