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Evolution of ferroelastic domain walls during phase transitions in barium titanate nanoparticles
In this work, ferroelastic domain walls inside BaTiO3 (BTO) tetragonal nanocrystals are distinguished by Bragg peak position and studied with Bragg coherent X-ray diffraction imaging (BCDI). Convergence-related features of the BCDI method for strongly phased objects are reported. A ferroelastic domain wall inside a BTO crystal has been tracked and imaged across the tetragonal-cubic phase transition and proves to be reversible. The linear relationship of relative displacement between two twin domains with temperature is measured and shows a different slope for heating and cooling, while the tetragonality reproduces well over temperature changes in both directions. An edge dislocation is also observed and found to annihilate when heating the crystal close to the phase transition temperature. {I. Introduction} Perovskite transition-metal oxides have been studied for decades because of both their broad applications and fundamental scientific questions. The displacement of Ti and Ba ions relative to the oxygen in unit cell leads to local polarization, which gives rise to exotic electrical properties such as elevated dielectric susceptibility, ferroelectricity and piezoelectricity [1-4] By analogy with well-studied magnetic systems, it is believed that it is not the local polarization in unit cell level that directly links with these macroscopic electrical properties, but rather via the formation and rearrangement of polarized nanodomains. Therefore, the study of domain structures, preferably in three dimensions (3D), is important for understanding and improving these properties. BaTiO3 (BTO), for example, is frequently chosen as a lead-free functional material for both actuator and sensor applications [5-6] It goes through a series of crystal lattice systems: cubic, tetragonal, orthorhombic and rhombohedral upon cooling [7]. The corresponding transitions are first-order with critical temperatures of 393K, 278K and 183K, respectively, which can be adjusted by varying strain and sample size. The cubic-tetragonal phase transition temperature, for example, can be increased from 393K to 813K with 1.7{%} compressive strain [8] and can decrease to room temperature when the particle size is reduced to 3nm [9]. Recently, it was reported that the local structure remains locally rhombohedral throughout all phases [1011] The phase transition is also complex, demonstrating both order-disorder and displacive character [1213] From the high symmetry cubic phase to the lower symmetry tetragonal phase, the paraelectric ensemble breaks into ferroelectric domains of uniform electric polarization, driven by the minimization of the sum of electrostatic and elastic energy [1-4]. To accommodate local energy landscape and strain different types of domains could be formed by rotation or translation of crystal regions or domains into different locations with welldefined domain-wall interfaces. For example, there are 71, 109and 180domain walls in rhombohedral BTO [14]. In tetragonal BTO, the flipping of one region of a crystal along a face-diagonal leads to a ferroelectric and ferroelastic 90domain wall (twin boundary). While flipping along the long side of the tetragonal unit cell creates a ferroelectriconly 180domain wall instead, in which the a-domain and c-domain are …
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