Stable halogen 2D materials: the case of iodine and astatine

Two-dimensional (2D) materials have applications towards electronic devices, energy storages, and catalysis, et al. So far, most of elemental 2D materials are composed based on groups IIIA, IVA or VA. To expand the 2D material family, the orbital hybridization becomes a key factor to determine stability. Here we predict that sp2d3 hybridization of the outmost electrons in iodine and astatine can build up 2D triangle lattices, delta-iodiene and delta-astatiene, using first-principles calculations. Each atom is connected by σ bonds with nearest 6 atoms and the π bonds are thus introduced. The band gaps can approach zero because of interaction of unpaired single electron between each atom, if the identical bond length is reduced. By inducing compression strain, the Dirac points or topological nontrivial points can be created in the delta-iodiene and delta-astatiene. Our discovery paves a new way to construction of 2D materials.