Direct Observation of Structural Disorder Near Critical Point in BaTiO_{3} Crystal

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid-State Chemistry

Background:

• Critical points (CPs) in ferroelectrics correlate with significant dielectric and electromechanical properties.
• The precise structural origins of these responses, particularly in nonrelaxor ferroelectrics, remain incompletely understood.

Purpose of the Study:

• To investigate the structural origin of critical points in ferroelectrics.
• To map polarization disorder across the electric-field-temperature (E, T) phase diagram of barium titanate (BTO).
• To elucidate the relationship between disorder and enhanced piezoelectricity.

Main Methods:

• In situ biasing-heating transmission electron microscopy (TEM) was employed to observe structural changes.
• First-principles-based simulations were used to model polarization behavior and material properties.
• Fourier-space analysis of TEM images and probability-density functions of unit cell dipoles were utilized.

Main Results:

• Diffuse broadening at the (001) point in TEM images indicated randomized Ti off-centering and local lattice distortion near CPs.
• Simulations showed peaks in piezoelectric and dielectric coefficients along the ferroelectric phase boundary and its Widom-line continuation.
• Increased dipole disorder near CPs was observed, followed by field-aligned ordering with increasing electric field.

Conclusions:

• Disorder is identified as the primary driver for enhanced piezoelectricity in BTO near critical points.
• The study clarifies the microscopic origins of maximum response and explains the decline in the supercritical regime.
• Tuning the disorder-order balance presents a practical strategy for optimizing piezoelectric and other functional properties.