Reversible shuffle twinning yields anisotropic tensile superelasticity in ceramic GeSe.

Superelasticity is a reversible, nonlinear strain response to stress stimuli beyond the linear elastic regime. It is commonly associated with a martensitic transformation in its host material, usually a metal or polymer. Except for the ceramic crystals ZrO(2) and BaTiO(3), which show superelasticity under compressive stress, inorganic materials with covalent or ionic bonding usually do not exhibit superelastic behaviour because of large energy barriers for structural transitions. Here we show anisotropic tensile superelasticity in the ceramic crystal GeSe, which originates from reversible shuffle twinning. Through in situ transmission electron microscopy mechanical testing, we trace the evolution from a linear elastic behaviour to a nonlinear superelastic plateau in stress-strain curves and concurrently observe the generation of stripy-shaped twin domains along the <110> direction. Density functional theory calculations paired with molecular dynamics simulations reveal a release of elastic potential energy upon the shuffle twinning process from a Z-shaped to an anti-Z-shaped bond configuration, which is responsible for the observed tensile superelasticity. This mechanism makes the observed superelasticity highly directional. In line with the anisotropic Young's modulus and Poisson's ratio in GeSe, experiments confirm that superelastic response emerges only when we apply strain along or close to the zigzag direction. We expect to find similar anisotropic superelasticity in ceramic semiconductors with similar crystal structure such as SnSe, SnS or GeS.