Glissile Interphase Boundaries Enable Collective Phase Switching in Epitaxial Polar Oxides

ABSTRACT A polar oxide on the brink displays enhanced electromechanical response, classically from a combination of phase competition and domain wall motion. In epitaxial thin films, however, such enhancement is widely suppressed by substrate clamping, which immobilizes structural interfaces. Here, we show that by using a substrate that imposes directionally asymmetric strain, one can fundamentally engineer this constraint to stabilize a special class of glissile interfaces that amplify electromechanical response through their mobility. In BiFeO 3 films grown on LaAlO 3 (103), strain asymmetry stabilizes elastically compatible low‐symmetry variants separated by dislocation‐free, glissile interphase boundaries. These interfaces translate reversibly under weak electrical or mechanical perturbations, mediating collective nanoscale phase transformations that are forbidden under conventional biaxial strain. The resulting dynamics exhibit scale‐free avalanche behavior characteristic of systems poised near an elastic instability. Our findings establish glissile interphase boundaries as an emergent interfacial state in anisotropically‐strained oxides and provide a general framework for engineering phase competition and collective switching phenomena in epitaxial materials.