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

Abstract

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.