Ultrahigh-strain ferroelasticity in two-dimensional honeycomb monolayers: from covalent to metallic bonding.

We propose a possible ferroelastic switching pathway of two-dimensional (2D) honeycomb lattice (including graphene, BN, stanene, etc.) that may swap its armchair and zigzag direction, reversing an unprecedented strain of 73.2%. Our ab initio calculations reveal that such pathway cannot work in covalent systems like graphene and BN; for monolayer with metallic bonds like stanene, stanane and InBi that have all been synthesized, however, such pathway can be feasible with a low switching barrier (<0.15 eV) and stress (<graphene upon 1% tensile strain), also with the highest energy/stress point in the elastic region. Their distinct behaviors are attributed to the different feature of covalent bonds and metallic bonds: the former is rigid with directionality, while the latter is malleable with ductility. A general trend of linear decrease in switching barrier with uprising metallicity for the same group compounds is revealed. Similar behaviors can be extended to bulk zinc-blended or wurtzite structure that can be deemed as multilayer stacking of buckled monolayer. Binary compounds like InBi monolayer are even multiferroics with both in-plane and vertical ferroelectricity as well as nontrivial topological properties.