Enhanced piezoelectric properties in Li_xBi_1-xNb_xFe_1-xO₃ films contributed by low-symmetry phase and self-polarization

Li_xBi_1-xNb_xFe_1-xO₃ films with various Li⁺ and Nb⁵⁺ substitutions were fabricated onto LaNiO₃-buffered Pt(111)/Ti/SiO₂/Si substrates through sol-gel methods. Owing to the substitution by smaller Li⁺ and Nb⁵⁺ for larger Bi³⁺ and Fe³⁺ respectively, the phase structure of Li_xBi_1-xNb_xFe_1-xO₃ films experiences a transition from rhombohedral phase (R phase, high symmetry) to tetragonal-like phase (T-like, low-symmetry) upon the chemical pressure with Li⁺ and Nb⁵⁺ substitution increases. Additionally, the dielectricity, ferroelectricity and leakage behaviors of substituted Li_xBi_1-xNb_xFe_1-xO₃ films are also significantly improved. More importantly, the substituted Li_xBi_1-xNb_xFe_1-xO₃ films show significantly enhanced piezoelectric responses, especially for the Li_0.05Bi_0.95Nb_0.05Fe_0.95O₃ film, whose d₃₃ with the value of 107.5 pm/V might be compared with some lead-based piezoelectric films. The enhanced piezoelectric response of the Li_0.05Bi_0.95Nb_0.05Fe_0.95O₃ film is contributed by synergistic effects of low-symmetry phase, improved ferroelectricity and downward self-polarization, where the electric field-triggered R-T phase transition and polarization vectors rotation can be easily took place upon small stimuli. Additionally, the reduction of leakage currents for substituted Li_xBi_1-xNb_xFe_1-xO₃ films also help to improve their dielectric, ferroelectric and piezoelectric properties.