Multiphase cooperation for multilevel strain accommodation in a single-crystalline BiFeO3 thin film

The functionalities and diverse metastable phases of multiferroic BiFeO3 (BFO) thin films depend on the misfit strain. Although mixed-phase-induced strain relaxation in multiphase BFO thin films is well known, it is unclear whether a single-crystalline BFO thin film can accommodate misfit strain without the involvement of its polymorphs. Thus, understanding the strain relaxation behavior is key to elucidating the lattice strain–property relationship. In this study, a correlative strain analysis based on dark-field inline electron holography (DIH) and quantitative scanning transmission electron microscopy (STEM) was performed to reveal the structural mechanism for strain accommodation of a single-crystalline BFO thin film. The nanoscale DIH strain analysis results indicated a random combination of multiple strain states that acted as a primary strain relief, forming irregularly strained nanodomains. The STEM-based bond length measurement of the corresponding strained nanodomains revealed a unique strain accommodation behavior achieved by a statistical combination of multiple modes of distorted structures on the unit-cell scale. The globally integrated strain for each nanodomain was estimated to be close to -1.5%, irrespective of the nanoscale strain states, which was consistent with the fully strained BFO film on the SrTiO3 substrate. Density functional theory calculations suggested that strain accommodation by the combination of metastable phases was energetically favored in relation to single-phase-mediated relaxation. This discovery allows a comprehensive understanding of strain accommodation behavior in ferroelectric oxide films, such as BFO, with various low-symmetry polymorphs.