First‐Principles Study of Phonon Instabilities in Aurivillius Bi ₂ TiO ₄ X ₂ Oxyhalides (X = F, Cl, Br, and I)

In this study, density functional theory (DFT) is employed to investigate the impact of halogenation on the m = 1 Aurivillius phase. Four phases are investigated: Bi 2 TiO 4 F 2 , Bi 2 TiO 4 Cl 2 , Bi 2 TiO 4 Br 2 , and Bi 2 TiO 4 I 2 . The structures in which halogens occupy the apical site in the tetragonal I 4 / mmm phase are focused here. Given the increasing atomic radii from fluorine to iodine, the structure extends along the c ‐axis. Several unstable modes are identified through phonon calculations. In contrast to conventional rotation and tilt instabilities typically observed in the oxide Aurivillius phases, these instabilities, along with the polar instability of the c ‐axis present in Bi 2 TiO 4 F 2 , disappear in Bi 2 TiO 4 Cl 2 , Bi 2 TiO 4 Br 2 , and Bi 2 TiO 4 I 2 . Electronic studies reveal that Bi 2 TiO 4 F 2 is a semiconductor with a direct energy gap and this character is maintained for Bi 2 TiO 4 Cl 2 , Bi 2 TiO 4 Br 2 , and Bi 2 TiO 4 I 2 , with however a significant narrowing of the bandgap, primarily attributed to the upward shift of the halogen 2 p ‐states in the valence band.