  We report the dynamics of soft phonon modes and their role towards the
various structural transformations in Aurivillius materials by employing
terahertz frequency-domain spectroscopy, atomic pair distribution function
analysis, and first-principles calculations. We have chosen Bi4Ti3O12 as a
model system and identified soft phonon modes associated with the paraelectric
tetragonal to the ferroelectric monoclinic transition. Three soft phonon modes
have been discovered which exhibit a strong temperature dependence. We have
determined that the anharmonicity in Bi-O bonds plays a significant role in
phonon softening and that Bi cations play an important role in the emergence of
ferroelectricity.
