Structural Phase Transition in Bismuth Oxide due to Cooperative Pseudo Jahn Teller distortion

Bismuth oxide is the base of several promising materials for various applications concerning the microelectronics and energy conversion industry. The present investigation aims to unveil the underlying reason for its phase transitions, e.g., δ→β→α, yet not completely understood, based on a combination of experimental and first principle studies. The calculations confirm that the orientations of the stable and metastable phases are attributed to the positions of lone pairs from the 6s² orbitals of the Bi atoms. However, the structural relationship between the different phases of the crystal raises the more fundamental question of the underlying mechanism governing the phase transitions in this system. The electron localization function calculations predict the location of these lone pairs, and the crystal orbital hamiltonian population calculations confirm the bonding and antibonding between the orbitals. These results indicate that the observed phase transitions are due to the cooperative pseudo-Jahn-Teller distortion in the crystal originating from mixing the ground state Bi_6s and excited Bi_6p states. Therefore, we endorse the role of O_2p states in stabilizing the lone pair activity, which leads to a pseudo-Jahn Teller distortion that cooperatively induces the phase transition.