Ferroelectric polarization of tetragonal BiFeO3—an approach from DFT calculations for BiFeO3–BaTiO3 superlattices—

Abstract

Density functional theory calculations are conducted for tetragonal BiFeO3–BaTiO3 superlattices to investigate the influence of electronic structures on ferroelectric spontaneous polarization (P s). When the number of the perovskite unit cells in one layer (N) is decreased below 10, the P s starts to decrease from the volume-averaged one (50.9 μC cm−2) and eventually becomes half at N = 1. In the BiFeO3 cell (N = ∞) with a large P s (73.3 μC cm−2), 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.