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

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 ₃ layers within (BiFeO ₃ /Ca _0.96 Ce _0.04 MnO ₃ ) ₄ multilayer films grown on LaAlO ₃ substrates. Atom-scale studies and quantitative electromechanical atomic force microscopy revealed that the transitional phase facilitates continuous polarization rotation in ultrathin BiFeO ₃ layers. This effect enhances the piezoelectric responses of the multilayer films and yields a giant piezoelectric coefficient ( d ₃₃ ≈ 30 picometers per volt) for films containing 16–unit cell BiFeO ₃ layers, which is over four times higher than conventional rhombohedral BiFeO ₃ . 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 ₃ layers. This work breaks the thickness limit of single-layer BiFeO ₃ for electromechanical applications and proposes a thickness-domain design strategy for lead-free piezoelectric heterostructures.