Critical points (CPs) in ferroelectrics are known for their association with giant dielectric and electromechanical responses, yet their structural origin, especially in nonrelaxor systems, remains unclear. We combine in situ biasing-heating transmission electron microscopy with first-principles-based simulations to map polarization disorder across the electric-field-temperature ($E$, $T$) phase diagram of single-crystal ${\mathrm{BaTiO}}_3$ (BTO). Near-CP conditions, Fourier-space analysis of high-resolution images shows diffuse broadening at the (001) point, suggesting randomized Ti off centering and local lattice distortion. Simulation reproduces and extends these trends: selected piezoelectric and dielectric coefficients peak along the ferroelectric phase boundary and its Widom-line continuation, while probability-density functions for unit cell dipoles broaden (higher disorder) near the CP and then narrow with increasing field (field-aligned ordering). This unified evidence identifies a disorder-driven enhancement of piezoelectricity in BTO near the CP and explains its decline in the supercritical regime. The results clarify the microscopic origin of the response maximum and suggest a practical route: tuning the disorder-order balance to maximize piezoelectric and other functional properties.

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• Critical phenomena
• Microstructure
• Ferroelectrics
• Piezoelectrics
• First-principles calculations
• Molecular dynamics
• Transmission electron microscopy