Bulk heterogeneity in barium titanate above the Curie temperature

We use dark-field x-ray microscopy to reveal evidence of subtle structural heterogeneity in BaTiO3 single crystals at temperatures of 150 °C—well above the Curie temperature of 125 °C. The heterogeneity exhibits domain-like ordering on the scale of several micrometers, pronounced curvature, and a preference for ⟨110⟩ lattice directions. Complementary high-resolution x-ray reciprocal space measurements suggest that the features originate from point defects (most likely oxygen vacancies) that coalesce along pre-existing domain walls during aging. A simple thermodynamic model suggests that the weak elastic strains associated with the heterogeneity are likely to locally raise the Curie temperature in their vicinity, creating nucleation sites for the ferroelectric phase upon cooling through the ferroelectric phase transition.