Soft Phonon Mode Dynamics and Phase Transitions in Aurivillius Type Structures

We report the dynamics of soft phonon modes and their role in the structural phase transitions of the Aurivillius materials studied by employing terahertz frequency-domain spectroscopy, atomic pair distribution function analysis, and first-principles calculations. We have chosen Bi4Ti3O12 as a model system and discovered soft phonon modes associated with the paraelectric tetragonal to the ferroelectric monoclinic structural transformations. The displacement of Bi results in polarization variations in Bi layered materials, and the anharmonicity in Bi-O bonds plays a significant role in phonon softening and the structural phase transitions.