Origin of the Enhanced Piezoelectricity of Vanadium-Doped La₂Ti₂O₇ Ceramics

Abstract

In recent years, La2Ti2O7 (LTO)-based piezoelectric materials, which belong to the perovskite-like layered structure (PLS) materials, have become candidates in the elevated temperature piezoelectric field due to their high Curie temperature, high resistivity, and gradually improved piezoelectric performance. In our previous work (Li, Y.; Lee, T.; Jiang, L.; Wang, W.; Jiao, Z.; Liang, D.; Yan, X.; Xu, M.; Chen, Q.; Pan, X.; Zhu, J. Improved Electrical Properties of Layer Structured La2Ti1.96V0.04O7 Ceramics. J. Electron. Mater. 2020, 49 (4), 2584–2595), the piezoelectric coefficient d33 = 4.8 pC/N of La2Ti1.96V0.04O7+0.02 (LTVO-0.04) could be enhanced ∼2.5 times that of pure LTO (d33 = 1.9 pC/N). Thereby, the improvement of the electrical properties of LTO by doping 2% vanadium has enhanced the application potential of LTO-based ceramics in the elevated temperature piezoelectric field. To explore the reason that the piezoelectric coefficient d33 of LTO could be increased significantly by doping 2% vanadium, X-ray diffraction (XRD) Rietveld refinement was performed on LTVO-0.04 ceramics. The refinement results predicted that the doped vanadium ions would enter into the LTO lattice to replace the Ti ions at the B sites and attract the surrounding oxygen ions to cause the lattice distortion. However, we did not directly observe this phenomenon experimentally at that time. In this work, we used scanning transmission electron microscopy (STEM) with double spherical aberration correctors to directly observe the atomic resolution images of the LTVO-0.04 lattice and confirmed the hypothesis that the doped vanadium ions replaced the Ti ions in the LTO lattice. On the basis of this result, the first-principles calculation results suggest that a possible reason for the vanadium dopant-induced piezoelectricity enhancement is because of the decrease of the oxygen vacancies near the B sites in the LTO lattice by the vanadium substitution, and the subsequent distorted lattice has a large influence on the intrinsic spontaneous polarization (Ps) of LTO along the c axis.