ZHOU Shulan ¹, ZHAO Xian ², JIANG Xiangping ¹, HAN Xiaodong ¹

(1. Department of Material Science and Engineering, Jingdezhen Ceramics Institute, Jingdezhen 333403, Jiangxi China; 2. State Key Laboratory of Crystal Material, Institute of Crystal Material, Shandong University, Jinan 250100, Shandong China)

Abstract：The electronic structure, Born effective charge tensor and zone-center (Γ) phonon of rhomboherdral Na_1/2Bi_1/2TiO₃ were studied by employing the plane-wave pseudopentential method based on the density functional theory within the local density approximation. The Born effective charge tensor and Γ phonon were calculated using the linear response from density functional perturbation theory. The results of the electronic structure and effective charges indicate the strong covalent interactions of Ti-O and Bi-O bonds. On the other hand, the large Born effective charges of Ti, O and Bi show that the relative displacements of neighboring irons against each other giving rise to large polarization and therefore the ferroelectricity. The Γ phonon vibration modes of rhomboherdral Na_1/2Bi_1/2TiO₃ were assigned and compared with the experimental results. The calculated results are in good agreement with the experimental values. Also the splitting of frequencies of LO (longitudinal optical) and TO (transverse optical) phonons are studied in detail. The giant LO-TO splitting indicates that the vibration of TiO₆ can couple strongly with the electric field and that play an important role in the ferroelectric sensitivity to the size and formation of domains as well as electric boundary conditions.

Key words: electronic structure; Born effective charge tensor; Γ phonon; Na_1/2Bi_1/2TiO₃; first principl