In Situ Observations of Ferroelastic Domain Evolution in Epitaxial WO ₃ Films

ABSTRACT Ferroelastic materials, characterized by reversible structural phase transitions and mechanical hysteresis, hold significant promise for strain‐engineered applications. Tungsten trioxide (WO ₃ ) is a prototypical ferroelastic system exhibiting rich phase behaviors, yet the mechanisms governing domain evolution—particularly in thin films under thermal activation—remain unresolved. Here, WO ₃ films have been fabricated on LaAlO ₃ substrates via pulsed laser deposition. High‐resolution scanning transmission electron microscopy and X‐ray diffraction confirmed coherent epitaxial growth and the formation of hierarchical ferroelastic twins with labyrinthine domain structures. Through combined in situ transmission electron microscopy, in situ X‐ray reciprocal space mapping, and thermally driven domain evolution from b ‐domain to a ‐ or c‐ domain configurations have been elucidated. Geometric phase analysis revealed giant strain gradients in wide domain walls (∼10 nm), suggesting potential flexoelectric effects. This work uncovers the mechanisms of ferroelastic domain evolution in WO ₃ films and establishes the critical roles of epitaxial strain and thermal activation in mediating functional oxide phases, providing fundamental insights into strain‐mediated manipulation of ferroelastic domains.