Polymorphs Of Group ?-? Monolayers:Phase Transformations And Ferroelastic/Ferroelectric Switching

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-? monolayers such as In Se and Ga Te possess a series of metastable polymorphs with similar energies.Those polymorphs were named as H,T,and n M phases,respectively.The energy differences between the ground state and those polymorphs are almost negligible(<k _BT 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 that most ductile metals cannot sustain before a fracture,wherein they can be a series of intermediate states with similar energies.This thesis also predicts some of polymorphs can be a series of intermediate states,which can greatly reduce the energy costs for ferroelectric/ferroelastic switching.There are three symmetry-equivalent directions of n M phase,and three orientation states are denoted as O1,O2,and O3.3M(O1)can be transformed to 3M(O2)or 3M(O3)via strain following the pathway 3M(O1)-6M(O1)-T-6M(O2/O3)-3M(O2/O3),hopping over a barrier of around 0.09eV/f.u.