First-principles Study of Intergrowth Bismuth Layer-structured Ferroelectric Bi7Ti4NbO21

Intergrowth bismuth layer-structured ferroelectric Bi7Ti4NbO21 material (iBTN) is a natural super lattice structure with excellent properties. Ferroelectric iBTN and parent compounds Bi3TiNbO9 (BTN) and Bi4Ti3O12 (BiT) were studied with first-principles method based on density functional theory (DFT). The band structures and the density of states of different compounds were calculated based on their optimized structures. The enthalpy of formation and band gaps of iBTN, BTN and BiT are -56.21, -30.72, -43.32 eV and 0.796, 2.535, 2.436 eV, respectively. The analysis shows that the intergrowth structure iBTN stays at a thermodynamic metastable state in relative to BTN and BiT. The electronic conductivity of iBTN is mainly depended on the perovskite layers, and the narrowed band gap is attributed to the down shift of the bottom of the conduction band. Detailed partial density of states of iBTN shows that different perovskite layers have different effects on the conduction band which suggests that the electrons choose their transportation channels in this material. This study can improve the understanding of the intergrowth mechanism and the influence between the structure and the electronic properties of iBTN.