Large Piezoelectric Response Induced by the Coexistence of Low-Symmetry and Self-Polarization in Li⁺-Nb⁵⁺-Doped BiFeO₃ Polycrystalline Films

Pure BiFeO₃ (S1) and Li⁺-Nb⁵⁺-pair-doped BiFeO₃ (S2, S3, and S4) films were prepared by the radio-frequency magnetron sputtering onto the LaNiO₃-buffered Pt/Ti/SiO₂/Si substrate. The phase structure of the doped BiFeO₃ films is low-symmetry T-like phase owing to the chemical pressure induced by the substitution of larger Li⁺ and Nb⁵⁺ for smaller Fe³⁺, which is different from the pure BiFeO₃ film (high-symmetry R phase). Compared with the pure BiFeO₃ film, the dielectric, ferroelectric, and piezoelectric performances are greatly enhanced, especially for the piezoelectric response of 9 wt % Li⁺-Nb⁵⁺-doped BiFeO₃ (S3) film. The converse piezoelectric coefficient d₃₃ of S3 film is 114.1 pm/V, which is comparable to that of lead-based piezoelectric films. The excellent piezoelectricity of S3 film is considered as the coupling results of low-symmetry T-like phase BiFeO₃ and upward self-polarization because the polarization vectors of the T-like phase and nanodomains rotate easily under small external stimuli. In addition, the reduced leakage currents in the doped BiFeO₃ films also contribute to the enhanced dielectric, ferroelectric, and piezoelectric performances.