We report the improvement of an apparatus for Bragg coherent X-ray diffraction imaging (Bragg-CDI) at BL22XU in SPring-8 to expand the applicable particle size and the application of the Bragg-CDI technique for Pd and ferroelectric barium titanate (BaTiO 3 ) fine crystals with particle sizes of 40-500 nm. Preparing a vacuum environment around the sample enabled us to obtain the high-contrast diffraction pattern of a 40 nm particle. The reconstructed three-dimensional image showed the outer shape, size, and internal phase (strain) for a single particle. A single 500 nm BaTiO 3 particle showed a straight and sharp antiphase-boundary shape, whereas smaller BaTiO 3 particles showed different phase boundary shapes. The present Bragg-CDI apparatus, thus, allows the observation of the outer shape, size, and inner phase distribution for a single particle with a size of 40-500 nm.

The shapes and sizes of grains influence the anisotropy of crystal structures and the configuration of ferroelectric domains. In order to better understand these effects, we introduce a method to observe a ferroelectric phase transition in a single ferroelectric particle of sub-micrometer size. The phase transition was observed by cooling the sample through its Curie temperature, and studying the pattern variations of Bragg coherent X-ray diffraction from a single particle of 500-nm-sized BaTiO3. A change from a single 200 peak (cubic phase) to both 200 and 002 peaks (tetragonal phase); was seen, with fringes connecting them like a bridge. The pattern from the BaTiO3 particle in the cubic phase was also imaged using Bragg coherent diffraction imaging. The apparent strain distribution caused by dislocation internal to the BaTiO3 crystal was visible in the reconstructed image.

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (ftJs). nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric Batio 3 thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depthresolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. in the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.