Hiroo Tajiri

We applied x-ray fluorescence holography (XFH) to a Li0.15La0.85/3NbO3 Li ion conductor to investigate the La3+ ionic arrangement. Thus far, this compound has been believed to have the A-site-deficient perovskite structure with the alternately arranged La-/Li-rich and La-/Li-empty layers (A1 and A2 layers, respectively) along the c axis. However, our XFH results revealed that A sites in the A2 layers are partially occupied by La3+ ions. From the detailed analysis of the obtained atomic images, it was indicated that the La3+ ions are not randomly distributed but form a column-like arrangement, where the La3+ ions in the A2 layers (La2 ions) are aligned in the c-axis direction and those in the A1 layers are aggregated near these La2 ions. We discuss the reason why the La3+ ions are located in the A2 layers in terms of crystal growth processes, and we also examine the structural origin of the enhanced ionic conductivity of the present sample.

This study reports the first application of X-ray fluorescence holography (XFH) under high-pressure conditions. We integrated XFH with a diamond anvil cell to investigate the local structure around Sr atoms in single-crystal SrTiO ₃ under high pressure. By utilizing nano-polycrystalline diamond anvils and a yttrium filter, we effectively eliminated significant background noise from both the anvils and the gasket. This optimized experimental configuration enabled the measurement of Sr K α holograms of the SrTiO ₃ sample at pressures up to 13.3 GPa. The variation of lattice constants with pressure was calculated by the shifts of Kossel lines, and real-space images of the atomic structures were reconstructed from the Sr K α holograms at different pressures. This work successfully demonstrates the feasibility of employing XFH under high-pressure conditions as a novel method for visualizing pressure-induced changes in the three-dimensional local structure around the specified element.

We observed surface X-ray diffraction from He-4 submonolayers adsorbed on a single-surface graphite using synchrotron X-rays. Time evolutions of scattering intensities along the crystal truncation rod (CTR) were observed even after reaching the base low temperature in a selected condition of sample preparation. Our simulations for CTR scatterings based on the random double-layer model, in which helium atoms are distributed randomly in the first and second layers with a certain occupancy ratio, can consistently explain the observed intensity changes. These results support the scenario that He atoms are stratified initially as a nonequilibrium state and then relaxed into a uniform monolayer by surface diffusion, where the relaxation process was observed as a decrease in CTR scattering intensity. The observed time constant was, however, much longer than those estimated from quantum and thermal surface diffusions. This implies homogeneous processes in surface diffusions were strongly suppressed by local potentials in such as atomic steps or microcrystalline boundaries.

X-ray fluorescence holography (XFH) was, for the first time, applied to a polycrystalline system in order to visualize local structures within a single grain in the system using an x-ray microbeam. XFH was performed on (Ba,Ca)(Zr,Ti)O3 (BCZT) piezoelectric ceramic as the polycrystalline system in the normal mode with a two-dimensional detector. After data acquisition and processing, we obtained a Zr-Kα hologram from the BCZT ceramic. The Kossel lines were clearly observed, which were in good agreement with our simulated results, although there is a possibility that the hologram is contaminated by the signals from other surrounding grains. Furthermore, we confirmed that several atomic images reconstructed from the obtained Zr-Kα hologram can be related to the Ti atoms around Zr in the real space plane. These results clearly demonstrate the applicability of XFH to polycrystalline samples, which significantly expands the potential use of this technique for micro- or mesoscopic research in materials science.

To tackle disorder in crystals and short- and intermediate-range order in amorphous materials, such as glass, we developed a carry-in diffractometer to utilise X-ray fluorescence holography (XFH) and anomalous X-ray scattering (AXS), facilitating element-specific analyses with atomic resolution using the wavelength tunability of a synchrotron X-ray source. Our diffractometer unifies XFH and AXS configurations to determine the crystal orientation via diffractometry. In particular, XFH was realised even for a crystal with blurred emission lines by a standing wave in a hologram, and high-throughput AXS with sufficient count statistics and energy resolution was achieved using three multi-array detectors with crystal analysers. These features increase tractable targets by XFH and AXS, which have novel functionalities.

To supply the growing demand for high photon flux in synchrotron science including surface diffraction, a middle energy-bandwidth monochromator covering the 10⁻⁴ to 10⁻³ range has been adapted by applying an asymmetric diffraction geometry to a cryogenically cooled silicon 111 double-crystal monochromator used as a standard for the undulator source at SPring-8. The asymmetric geometry provides a great advantage with its ability to configure flux gains over a wide energy range by simply changing the asymmetry angle, while the angular divergence of the exit beam remains unchanged. A monolithic design with three faces has been employed, having one symmetrically cut and another two asymmetrically cut surfaces relative to the same atomic plane, maintaining cooling efficiency and the capability of quickly changing the reflection surface. With the asymmetric geometry, an X-ray flux greater than 10¹⁴ photons s⁻¹ was available around 12 keV. A maximum gain of 2.5 was obtained relative to the standard symmetric condition.

A new beam profile monitoring system for the small X-ray beam exiting from the SPring-8 front-end was developed and tested at BL13XU. This system is intended as a screen monitor and also as a position monitor even at beam currents of 100 mA by using photoluminescence of a chemical vapor deposition-grown diamond film. To cope with the challenge that the spatial distribution of the photoluminescence in the vertical direction is too flat to detect the beam centroid within a limited narrow aperture, a filter was installed that absorbs the fundamental harmonic concentrated in the beam center, which resulted in “de-flattening” of the vertical distribution. For the measurement, the filter crossed the photon beam vertically at high speed to withstand the intense heat flux of the undulator pink-beam. A transient thermal analysis, which can simulate the movement of the irradiation position with time, was conducted to determine the appropriate configuration and the required moving speed of the filter to avoid accidental melting. In a demonstration experiment, the vertically separated beam profile could be successfully observed for a 0.8 × 0.8 mm² beam shaped by an XY slit and with a fundamental energy of 18.48 keV. The vertical beam centroid could be detected with a resolution of less than 0.1 mm.