Thickness-confined metastable phase transitions drive large piezoelectricity in ultrathin BiFeO(3).

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 BiFeO layers within (BiFeO/CaCeMnO) multilayer films grown on LaAlO substrates. Atom-scale studies and quantitative electromechanical atomic force microscopy revealed that the transitional phase facilitates continuous polarization rotation in ultrathin BiFeO layers. This effect enhances the piezoelectric responses of the multilayer films and yields a giant piezoelectric coefficient ( ≈ 30 picometers per volt) for films containing 16-unit cell BiFeO layers, which is over four times higher than conventional rhombohedral BiFeO. 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 BiFeO layers. This work breaks the thickness limit of single-layer BiFeO for electromechanical applications and proposes a thickness-domain design strategy for lead-free piezoelectric heterostructures.