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.
Chen et al. (Fri,) studied this question.