Polar ordering and structural distortion in electronic domain-wall properties of BiFeO_3

摘要

Bandgaps, band edges, and structural properties of domain walls (DWs) in multiferroic BiFeO_3 (BFO) are studied using density functional theory. The ferroelectric DWs (namely, the 71°, 109°, and 180° DWs) in rhombohedral BFO are considered, with a view of critically examining the relationships between local bandgap, polar ordering, and octahedral distortion reported for this material. Our study reveals that the domain-wall electronic properties of BFO sensitively depend on the polarization behavior and structural distortion at the wall center. The value of the bandgap at the wall decreases with the weakening of ferroelectricity at the wall center. Compared to all the wall types, DWs with the paraelectric-like phase at the wall center exhibit a significantly reduced bandgap. The bandgap reduction is also found to be closely related to two key parameters of the octahedral distortion at the wall: Fe-O bonds and O-Fe-O angles. The reduction in the Fe-O bond deviation and the straightening in the O-Fe-O angles at the wall appear with a marked bandgap reduction in the BFO DWs. Such a structural change at BFO DWs is reminiscent of a cubic structure, which is consistent with the analysis of paraelectric-like behavior at DWs. However, our analysis reveals that the straightening of Fe-O-Fe does not have any obvious correlation with bandgap reduction as suggested in previous works. The influence of the octahedral distortion at DWs is further clarified by investigating the bandgap and structural properties of various BFO bulk phases including Pnma, Pna21, tetragonal (P4mm), and Amm2. Our result establishes important correlations between electronic properties, polar ordering, and structural distortion in BFO DWs. This study also suggests that photo nanocatalysts employing BFO DWs may be a promising strategy to enhance photocatalytic activity.