Revealing Intrinsic Strain and Domain Evolution Across the Pristine Phase Transition Using PbTiO3 Epitaxial Films with Negligible Residual Strain

Displacive ferroelectrics exhibit a strong coupling between lattice anisotropy and intrinsic structural distortions, and epitaxial constraints in thin films make it essential to directly monitor strain-driven structural responses under controlled conditions. Here, we report in situ high-temperature X-ray diffraction of hydrothermally grown PbTiO3 epitaxial films with negligible residual strain on (100) Nb-doped SrTiO3 substrates, where minimized substrate-induced lattice and thermal strains during deposition enable ferroelectric-phase growth without a growth-induced phase transition. Upon heating, the films undergo a discontinuous ferroelectric-to-paraelectric transition, during which partial relaxation is enabled by thermally active defect migration, likely associated with misfit dislocations. Upon cooling, the structure evolves relatively continuously, with a domains nucleating at the transition temperature to relieve anisotropic misfit strain, followed by gradual relaxation through domain wall evolution. These findings highlight the importance of determining the strain relaxation mediated by domain formation and, together with theoretical predictions, offer a framework for distinguishing strain-structure relationships in epitaxial thin films from those in bulk counterparts.