Origin of large strain in BNKT-ST relaxor ferroelectrics based on atomic-scale structural analysis

In this work, (1−x)Bi_0·5(Na_0·8K_0.2)_0.5TiO₃-xSrTiO₃ ceramics were synthesized by the solid-phase method. The SrTiO₃ dopants cause a phase transition from of rhombohedral to tetragonal resulting in an enhanced strain of 0.49 % and normalized strain d₃₃* of 817 p.m./V under 60 kV/cm at a doping amount of 0.04. The microstructure of BNKT-0.04ST, including cation displacements δ_Ti-Bi/Na and the tilt of oxygen octahedron are researched by aberration corrected scanning transmission electron microscope (AC-STEM). The fast Fourier transform (FFT) of the annular dark field (ADF) STEM pattern along [110] axis reveals the 1/2 (ooo) superlattice reflection spots due to anti-phase a⁻a⁻a⁻ the tilt of oxygen octahedron. The cation displacement vector maps based on the ADF STEM and the enhanced annular bright fielde (eABF) STEM images along the [100] axis show the rhombohedral and tetragonal phase nanodomains, and the displacement decreases gradually from the inside of the nanodomains towards the domain walls. The tilt of oxygen octahedron has the same variation tendency indicating the regional coherence. Due to the flexible nanodomains based on the cooperative effect of the tilt of oxygen octahedron and cation displacement, which is thought to be the origin of the relaxor characteristic and strain.