Thickness-confined metastable phase transitions drive large piezoelectricity in ultrathin BiFeO₃

Pursuing high-performance lead-free piezoelectrics beyond classical thickness limits remains challenging. This study identifies a transitional phase between rhombohedral and tetragonal structures in strained ultrathin BiFeO₃ layers within (BiFeO₃/Ca_0.96Ce_0.04MnO₃)₄ multilayer films grown on LaAlO₃ substrates. Atom-scale studies and quantitative electromechanical atomic force microscopy revealed that the transitional phase facilitates continuous polarization rotation in ultrathin BiFeO₃ layers. This effect enhances the piezoelectric responses of the multilayer films and yields a giant piezoelectric coefficient (d₃₃ ≈ 30 picometers per volt) for films containing 16-unit cell BiFeO₃ layers, which is over four times higher than conventional rhombohedral BiFeO₃. Phase-field simulations confirmed a thickness-dependent electromechanical coupling regularity, behaving as the coexistence of transitional/tetragonal mixed phases and dense nanodomains in strained ultrathin BiFeO₃ layers. This work breaks the thickness limit of single-layer BiFeO₃ for electromechanical applications and proposes a thickness-domain design strategy for lead-free piezoelectric heterostructures.