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/Ca0.96Ce0.04MnO3)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.