Various polymorphs of group III–VI (GaSe, InSe, GaTe) monolayers with quasi-degenerate energies: facile phase transformations, high-strain plastic deformation, and ferroelastic switching

Abstract

The rare existence of large plastic deformation in brittle covalent systems with little plastic regime can be attributed to the features of covalent bonds, which are rigid with directionality compared with ductile metallic bonds with delocalized electrons. In this work, we show ab initio calculation evidence that group III–VI monolayers such as InSe and GaTe possess a series of metastable polymorphs with similar energies. The energy differences between the ground state and those polymorphs are almost negligible (<kBT at room temperature) and can even be further reduced to zero on critical alloying, which is hitherto unreported. The ground state will be tunable owing to its sensitivity to strain or charge injection, which may be used as shape memories. Those low-energy phases with similar lattice constants in one direction and gradually varying lattice constants in another direction may enable a large plastic region from 0% to 12.6% via the ‘multi-intermediate strategy,’ also rendering large plastic deformation or large-strain ferroelasticity that most ductile metals cannot sustain before a fracture, wherein they can be a series of intermediate states with similar energies that may make the pathway much smoother.