R3c to P4bm phase transition assisted small to large polaron crossover in sodium bismuth titanate-based ferroelectrics

Sodium bismuth titanate (NBT) reveals a rhombohedral (R3c) phase at room temperature. Ferroelectricity reduces with the advent of a tetragonal (P4bm) phase at the depolarization temperature, Td ∼ 456 K. AC conductivity (σac) studies exposed a small-to-large polaron transition at Td. Barrier energy (WH) was ∼1.60 eV at T < Td for the small polarons in the R3c phase, which drastically reduced to ∼0.043 eV with the advent of the P4bm phase for the large polarons for T > Td. This is associated with the sharp rise in conductivity for T > Td. Ab initio calculations consider the electronic distortion due to oxygen vacancies, which creates trap states in the band structure. The energy gap (ΔE) between the trap states and the conduction band was ∼1.4 eV (R3c) and ∼0.2 eV (P4bm). These values are comparable to the experimental WH. The P4bm phase is more distorted than the R3c phase from charge density and structural distortion calculations. This indicates the formation of large polarons in the P4bm phase, compared to that of small polarons in R3c. The formation energy of the polaron (Epolaron) was calculated from the structural distortion and electron localization energies. The P4bm phase shows lower Epolaron (−0.26 eV) than R3c (−0.36 eV), indicating higher conductivity for the P4bm phase. NBT was chemically modified by adding BCZT to validate the small to large polaronic crossover at Td. This is discussed in light of σac measurements. WH decreased with BCZT incorporation, thereby increasing the conductivity. This is a consequence of the increased lattice distortion due to BCZT incorporation.