Insights into high electric-field-induced strain in BiAlO₃ modified Bi_1/2Na_1/2TiO₃ films

The development of high-strain piezoelectric materials has presented a longstanding challenge, particularly in the development of high-strain polycrystalline lead-free piezoelectric thin films. In this work, we present a strategy for customizing the electrostrain in lead-free thin films through phase transition engineering. In this study, we achieved a high recoverable electrostrain in a Bi_1/2Na_1/2TiO₃–BiAlO₃ (BNT–BA) film. To accomplish this, ferroelectric BNT and BNT–BA films with identical thicknesses of 500 nm were fabricated on Pt(111)/TiO₂/SiO₂/Si(100) substrates via a sol-gel method. Compared with the BNT film, the BNT–BA film exhibited a greater polarization response and superior field strength endurance, maintaining the energy storage density beyond the breakdown field strength of the BNT. The BNT–BA film demonstrated a large unipolar strain of S = 0.43% with a normalized strain (maximum strain/maximum applied electric field (S_max/E_max)) of 203 pm/V, followed by an effective transverse piezoelectric coefficient ( ${\mathit{e}}_{31,\mathbf{f}}^{\mathit{\ast }}$) of ~2.48 C/m², which was more than two times greater than the value obtained for BNT (i.e., maximum strain/maximum applied electric field (S_max/E_max) = 72 pm/V and ${\mathit{e}}_{31,\mathbf{f}}^{\mathit{\ast }}$ of ~1.09 C/m²). This high strain response in the BNT–BA film can be attributed to the electric-field-induced phase transition of the mixed (i.e., cubic and rhombohedral) phases into rhombohedral and tetragonal phases (mainly the rhombohedral structure), which recover back to the original state when the electric field is removed. These findings suggest new pathways for achieving significant strain levels via alternative mechanisms, potentially enhancing the effectiveness and expanding the applications of piezoelectric materials.