Mixed phases with stripe patterns are considered as a prerequisite for remarkable piezoelectric properties in epitaxial BiFeO₃ (BFO) thin films. Among the mixed phases in BFO epitaxial thin films, the vital role in achieving high piezoelectric response is attributed to the intermediate S-polymorph that bridges the transition between rhombohedral and tetragonal phases. However, the correlation between the S-phase and the piezoelectric strain in mixed-phase BFO has not been understood well due to the lack of a probe technique that can investigate the structural and electromechanical variations simultaneously. Here, using the time-resolved X-ray microdiffraction technique, we investigate the electric field induced phase transformation dynamics of a mixed-phase epitaxial La-doped BFO (BLFO) thin film. The BLFO thin film exhibits phase transformations between monoclinic M_C and tilted M_C, S/tilted S (S_tilt) phases under an applied electric field. Among the competing phases, we found that the S- or tilted S-phases exhibit the highest piezoelectric strain variation up to 0.7% at an electric field pulse of strength 1.3 MV/cm and width 5 μs. The present study correlates the high piezoelectric response in mixed-phase BLFO thin films with the presence of S/S_tilt -phases, which offers a useful pathway into designing strain-engineered ultra-high piezoelectric BFO thin films.

具有条纹图案的混合相被认为是外延BiFeO _3中具有卓越压电性能的前提（BFO）薄膜。在BFO外延薄膜的混合相中，实现高压电响应的重要作用归因于中间S-多晶型物，它桥接了菱形和四方相之间的过渡。但是，由于缺乏能够同时研究结构和机电变化的探针技术，人们对混合相BFO中S相和压电应变之间的相关性还没有很好的理解。在这里，使用时间分辨的X射线微衍射技术，我们研究了混合外延掺La的BFO（BLFO）薄膜的电场诱导的相变动力学。BLFO薄膜在单斜M _C和倾斜M _C，S /倾斜S（S外加电场下的_倾斜相。在竞争相中，我们发现在强度为1.3 MV / cm和宽度为5μs的电场脉冲下，S相或倾斜S相表现出最高的压电应变变化，最高可达0.7％。本研究将混合相BLFO薄膜中的高压电响应与S / S_倾斜相的存在相关联，这为设计应变工程超高压电BFO薄膜提供了有用的途径。