IMF2017

First-principles modeling of materials is key to fundamental understanding of materials properties. Bi₄Ti₃O₁₂ (BiT) is a complex layered oxide belonging to the Aurivillius class with alternating layers of perovskite-like and anti-fluorite-like ionic arrangements in the crystal structure. We have modeled the phonon dynamics of BiT using density functional theory (DFT) in order to understand the origin of lattice instabilities and structural phase transitions. The low temperature ferroelectric phase is monoclinic with space group P_c and the high temperature paraelectric phase is tetragonal. Phonopy and the density functional perturbation theory have been used to study the phonon modes whereby terahertz frequency-domain spectroscopy for highly textured BiT films and theoretical results are matched. Three low-frequency optical phonon modes were observed at room temperature form the THz spectroscopy: 0.61 THz, 0.83 THz and 0.95 THz. The DFT calculations predict the optical phonon modes at 1.13 THz, 1.14 THz, and 1.37 THz. Further analysis showed that the three low frequency phonons shifted by 7.1%, 2.6% and 1.3%, respectively, as temperature is increased from −273 ^oC to 527 ^oC. The corresponding displacement of phonon frequencies for hydrostatic expansion of the lattice by 1% is found to be 12.9%, 5.3% and 2.6%, providing the clue that the phonon softening is associated with the volume expansion of the lattice. In addition to that, phonons in presence of point defects such as the oxygen vacancies are explored which show site dependent trends. The effect of impurities on bismuth and titanium sites within the perovskite layers on the polarization is also explored.