Piezoelectrics interconvert mechanical energy and electric charge and are widely used in actuators and sensors. The best performing materials are ferroelectrics at a morphotropic phase boundary, where several phases coexist. Switching between these phases by electric field produces a large electromechanical response. In ferroelectric BiFeO₃, strain can create a morphotropic-phase-boundary-like phase mixture and thus generate large electric-field-dependent strains. However, this enhanced response occurs at localized, randomly positioned regions of the film. Here, we use epitaxial strain and orientation engineering in tandem—anisotropic epitaxy—to craft a low-symmetry phase of BiFeO₃ that acts as a structural bridge between the rhombohedral-like and tetragonal-like polymorphs. Interferometric displacement sensor measurements reveal that this phase has an enhanced piezoelectric coefficient of ×2.4 compared with typical rhombohedral-like BiFeO₃. Band-excitation frequency response measurements and first-principles calculations provide evidence that this phase undergoes a transition to the tetragonal-like polymorph under electric field, generating an enhanced piezoelectric response throughout the film and associated field-induced reversible strains. These results offer a route to engineer thin-film piezoelectrics with improved functionalities, with broader perspectives for other functional oxides.

压电元件可相互转换机械能和电荷，广泛用于执行器和传感器。性能最好的材料是处于准同性相界的铁电体，其中多个相共存。通过电场在这些相位之间切换会产生较大的机电响应。在铁电 BiFeO ₃中，应变可以产生同向性相-边界状相混合物，从而产生大的电场相关应变。然而，这种增强的响应发生在薄膜的局部、随机定位的区域。_{在这里，我们串联使用外延应变和定向工程（各向异性外延）来制作 BiFeO 3}的低对称相它充当类菱形和类四方多晶型物之间的结构桥梁。干涉式位移传感器测量表明，与典型的类菱形 BiFeO ₃相比，该相具有 ×2.4 的增强压电系数。带激发频率响应测量和第一性原理计算提供的证据表明，该相在电场下经历了向四方晶型的转变，在整个薄膜和相关的场致可逆应变中产生增强的压电响应。这些结果为设计具有改进功能的薄膜压电材料提供了一条途径，并为其他功能性氧化物提供了更广阔的前景。