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

Ferroelastic materials, characterized by reversible structural phase transitions and mechanical hysteresis, hold significant promise for strain-engineered applications. Tungsten trioxide (WO₃) 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, WO₃ films have been fabricated on LaAlO₃ 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 WO₃ 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.