SPring-8 Angstrom Compact free electron LAser（SACLA）is the world second X-ray laser which started user operation in 2012. As an introduction of the special issue, here we show how the SACLA project got started. While originated as a tiny R&D program within RIKEN, the SACLA project has been grown to one of the five National Key Technology projects in Japan. SACLA is the first step of the Compact X-ray free electron laser. Further downsizing of the facility is envisaged.
An overview of the SACLA facility is described. An accelerator and beamline system,and the performance of XFEL light are introduced. Experimental stations with a synchronized optical laser and 2-dimensional detectors are summarized. As a key experimental instrument, DAPHNIS system to perform shot-to-shot diffraction measurements is reported. A future perspective is summarized.
We are developing an ultra-short pulse powder X-ray diffraction method using X-ray pulses of high brilliance and femtosecond pulse width from the X-ray free-electron laser apparatus in SACLA, Japan. The distinctive feature of this method is detection of anisotropic crystal structure deformation during non-thermal process. We have applied this method to some materials, including chalcogenide Ge-Sb-Te, porous coordination polymers, hydroxyhydrides, etc. In this review, we will introduce some preliminary results of observation of photo-induced anisotropic lattice expansion in a non-thermal process in a metastable face-centered cubic Ge2Sb2Te5 crystal.
Laser-induced plastic deformation imparts compressive residual stress on materials and enhances reliability of components. The dynamic behavior of the phenomena was studied for A6061 aluminum alloy with XFEL at SACLA. A foil of A6061 was stuck on an acrylic plate with vacuum grease, through which a laser pulse of an Nd：YAG laser was irradiated. XFEL impinged on the opposite free surface of the foil with the various delay time and the diffraction of XFEL was stored with a two dimensional detector（MPCCD）. When the impulsive wave arrived at the opposite surface, the diffraction pattern changed from spotty to a smoother ring pattern, suggesting the fragmentation of coarse grains. Shifts of diffraction angles were also observed coinciding with the pressure profile due to the laser irradiation.
Coherent X-ray diffraction imaging（CXDI）is a lens-less imaging technique that can visualize the structures of non-crystalline particles with micro- to sub-micrometer dimensions. In CXDI experiments, spatially isolated particles are irradiated by X-ray beam with high transverse coherence. Projection images of the particles along the incident X-ray direction are then directly reconstructed from the diffraction amplitude using a phase retrieval algorithm. Here we report CXDI experiments carried out at the X-ray free electron laser（XFEL）facility SACLA, and describe the theoretical background of CXDI, development of a diffractometer and data processing.
We have performed resonant X-ray diffraction experiments at the Ir L absorption edges for a post-perovskite compound CaIrO3 with a Ir4+:（t2g）5 electronic configuration. By observing the magnetic signals, we could clearly see that the magnetic structure was a striped ordering with antiferromagnetic moments along the c axis and that the wave function of a t2g hole is strongly spin-orbit entangled, the Jeff＝1/2 state. The observed spin arrangement is consistent with a theoretical work predicting a unique superexchange interaction called the quantum compass model. Our studies stimulate further studies for developing novel quantum states in iridium oxides.
We have successfully prepared the three types of highly c-axis-oriented polycrystalline materials of lanthanum silicate apatite（La9.33Si6O26, La9.50Si6O26.25 and La9.50Si5.87O26）by isothermal heating of the diffusion couples consisting of La2SiO5 and La2Si2O7 at 1873 K for 50-100 h. The resulting polycrystals were subsequently characterized using polarizing microscopy, X-ray diffration and impedance spectroscopy. The annealed couples were mechanically processed, and the thin-plate electrolytes consisting of the grain-aligned polycrystals were obtained. The oxide-ion conductivity along the c-axis of oxygen hyper-stoichiometric apatite, La9.50Si6O26.25,was ca. 2.5 times higher than that of oxygen stoichiometric apatite, La9.33Si6O26, at 723-973 K. The polycrystalline material of Si-deficient apatite, La9.50Si5.87O26, showed, among the three types of apatite polycrystals, the highest conductivity above 850 K; the conductivity at 873 K was 4.2×10−2 S/cm.
A new air-stable cyclohexasulfur（cyclo-S6）was discovered in the crystal of 3,5-diphenyl-1,2,4-dithiazol-1-ium（dpdti）iodide, formed by cocrystallization. The dpdti was synthesized via an oxidation reaction of thiobenzamide with iodine in benzene. Two kinds of crystal habits—brock-shaped（crystal-A）and needle-shaped（crystal-B）—were obtained following recrystallization from acetonitrile solvent. All molecules in crystal-A and -B were identified through single-crystal X-ray structure analysis using a synchrotron X-ray. The cyclic-S6 molecule in the crystal-A is a self-assembly enclosed by the dpdti cation and iodide anion, and is assumed to contribute to their stability in the crystal, akin to the Sn ring in zeolites and sodalites. In the context of cyclo-S6, cocrystallization is constituted by weak intermolecular effects such as the van der Waals force. Crystal engineering of cocrystals resulted in air-stable cyclo-S6. This study presents a method to turn an unstable molecule into a stable crystalline molecule that maintains its origin structure, with an aim of predicting its stability in future studies.