Strain performance and thickness-induced asymmetry in SrTiO3 doped BNT-based lead-free ferroelectrics from nonergodic to ergodic phase

Abstract

Changes in the relaxation state can greatly affect the electric-field induced strain response in bismuth sodium titanate-based materials. In this work, the strain performances from nonergodic relaxation (NER) to ergodic relaxation (ER) phase transition are investigated by introducing SrTiO3 in (Bi0·5Na0.5)0.98(La0·5Li0.5)0.02TiO3. The solid solutions of perovskite are achieved in all compositions. The ferroelectric to the relaxation phase transition is revealed based on the characteristic peak shape in XRD and temperature dependence of the dielectric properties. The weakened hysteresis of electric-field induced polarization and strain curves results from the creation of localized random fields due to the coexistence of multiple A-site ions, disrupting the ferroelectric long-range ordering at a higher SrTiO3 dopant. The formation of polar nanodomain is promoted and accompanied by the transition from the NER to the ER state. Meanwhile, chemical inhomogeneity is deduced by the anomalous variation in the conduction mechanism and surface state of elemental environments. Asymmetric strain behavior is obtained upon reducing the sample thickness to 0.2 mm, especially pronounced in components with stronger ferroelectricity, where highly asymmetric strain is dominated by surface effect and generated by the non-180° domain switching of the upper and lower surface layers. These features provide insights into the design of strain properties in lead-free piezoelectric ceramics.