[20p-A304-1~15] 9.1 誘電材料・誘電体

[20p-A304-5] BiFeO₃の強誘電性—BiFeO₃-BaTiO₃超格子のDFT計算からのアプローチ—

〇野口 祐二¹、松尾 拓紀¹ (1.熊本大学)

キーワード：強誘電体、分極、第一原理計算

We report the origin of a large ferroelectric polarization in BiFeO₃ in rhombohedral R3c and tetragonal P4mm. Density functional theory calculations are conducted for tetragonal BiFeO₃–BaTiO₃ superlattices¹⁾ to investigate the influence of electronic structures on ferroelectric spontaneous polarization (P_s) ²⁾. When the number of the perovskite unit cell in one layer (N) is decreased below 10, the P_s starts to decrease from the volume-averaged one (50.9 mC/cm²) and eventually becomes half at N = 1. In the BiFeO₃ cell (N = ∞) with a large P_s (73.3 mC/cm²), a covalent bond arising from a Bi_6p-O_2p orbital interaction is extended through a Bi-O network, and stereo-chemical lone-pair electrons of Bi are accommodated in the opposite direction of the polar c axis. In the superlattice with N = 1, the Bi-O network cannot be developed by the presence of Ba and then the Bi-O bond becomes ionic. We show that the large P_s of the BiFeO₃ cell originates from the Bi_6p-O_2p mixing superimposed on the stereo-chemical nature of the lone-pair electrons of Bi.