Strain-Reshaped Mexican-Hat-Like Energy Landscape Enables Ferroelastic Variant Selection in 2D β′-In ₂ Se ₃

Two-dimensional β′-In 2 Se 3 hosts coexisting ferroelectric, antiferroelectric, and ferroelastic orders, yet the microscopic origin of its strain-driven ferroelastic switching remains unclear. Here, we demonstrate that strain can effectively modulate the in-plane polarization orientation variants of β′-In 2 Se 3 by reshaping a Mexican-hat-like potential energy landscape. Using first-principles calculations, we show that β-In 2 Se 3 is dynamically unstable and transforms into β′ through an in-plane displacement of the central-layer Se atoms (Se-CL), which follows a continuous Mexican-hat-like potential. Mechanical strain reshapes this potential: biaxial strain preserves its rotational symmetry, whereas uniaxial strain breaks it, converting the Mexican-hat-like profile into an anisotropic canyon-like landscape that lifts valley degeneracy. This symmetry breaking pins the Se-CL displacement direction and thereby controls the orientation of in-plane ferroelastic variants. This study reveals the microscopic coupling between lattice distortion and domain switching in 2D ferroelastics, highlighting their potential for strain-controlled, low-power 2D ferroic devices.