Elastic softening and hardening at intersections between twin walls and surfaces in ferroelastic materials

Surfaces play a key role during ferroelastic switching and define the interactions of materials with ionic species and biological systems. Here, we perform molecular dynamics simulations and identify ridges and valleys with rounded singularities around the intersections between twin walls and surfaces. Two dominant length scales stem from the elastic bending of the surface layer (>30 lattice units) and local atomic reshuffles (some five lattice units). For static twin walls, which do not shift laterally under external stress, the intrinsic change in Young’s modulus involves softening near valleys and hardening near ridges. The boundary-induced changes in the surface Young’s modulus are of the order of 0.7%.