Takuro Kawasaki

Current status of a time-of-flight single crystal neutron diffractometer SENJU at J-PARC

SENJU is a state-of-the-art single crystal time-of-flight Laue diffractometer in Materials and Life Science Experimental Facility at Japan Proton Accelerator Research Complex (J-PARC). The diffractometer is designed for precise crystal and magnetic structure analyses under multiple extreme conditions, such as low temperature, high-pressure, high-magnetic field. Measurements using small sample less than 1.0 mm3 will be also realized, which allows us to study wide variety of materials. SENJU is using a poisoned decoupled moderator to obtain peak profiles of Bragg reflection, and intensity distributions of superlattice reflections and diffuse scatterings with good accuracy. At the beginning, the diffractometer will have 31 two-dimensional scintillator detectors to cover wide area of reciprocal lattice space by a single measurement. The instrument is currently under construction and is scheduled to start on-beam commissioning in February 2012.

SENJU: a new time-of-flight single-crystal neutron diffractometer at J-PARC

SENJU, a time-of-flight Laue-type single-crystal neutron diffractometer, was developed at the Materials and Life Science Experimental Facility of the Japan Accelerator Research Complex (J-PARC). Molecular structure analysis of a sub-millimetre taurine crystal and magnetic structure analysis of an MnF2 crystal were performed to evaluate its performance.

Composite Behavior of Lath Martensite Steels Induced by Plastic Strain, a New Paradigm for the Elastic-Plastic Response of Martensitic Steels

Based on high-resolution neutron diffraction experiments, we will show that in lath martensite steels, the initially homogeneous dislocation structure, i.e., homogeneous on the length scale of grain size, is disrupted by plastic deformation, which, in turn, produces a composite on the length scale of martensite lath packets. The diffraction patterns of plastically strained martensitic steel reveal characteristically asymmetric peak profiles in the same way as has been observed in materials with heterogeneous dislocation structures. The quasi homogeneous lath structure, formed by quenching, is disrupted by plastic deformation producing a composite structure. Lath packets oriented favorably or unfavorably for dislocation glide become soft or hard. Two lath packet types develop by work softening or work hardening in which the dislocation densities become smaller or larger compared to the initial average dislocation density. The decomposition into soft and hard lath packets is accompanied by load redistribution and the formation of long-range internal stresses between the two lath packet types. The composite behavior of plastically deformed lath martensite opens a new way to understand the elastic-plastic response in this class of materials.

Crystal Structure Analysis of Layered Compounds CoxTiS2

A single-crystal X-ray structure analysis of Co x TiS 2 ( x = 0.26, 0.43, and 0.57) at room temperature has been performed to investigate both the ordered atomic arrangements of intercalated Co atoms and their site occupancies, as well as the nature of the chemical bond from the electron density distribution (EDD) obtained by the maximum entropy method. The superstructures of the Co atoms are observed to be 2 a ×2 a ×2 c unit cells for x = 0.26 and 0.57, and \(\sqrt{3}a{\times}\sqrt{3}a{\times}2c\) unit cells for x = 0.43. The overlapping of EDDs between Co and S atoms and Ti and S atoms corresponds to the covalent bonding in the van der Waals gap layer and TiS 2 layer. It is found that the nature of the covalent bonding between Co and S atoms causes a decrease in the interatomic distance. From in situ X-ray diffraction intensity measurements, the superstructures disappear at 510, 610, and 550 K for x = 0.26, 0.43, and 0.57, respectively. The type of transition for the three specimens is second-order-like.

Dislocation Characteristics in Lath Martensitic Steel by Neutron Diffraction

In situ neutron diffraction during tensile deformation of an as-quenched lath martensitic 22SiMn2TiB steel, was performed using a high resolution and high intensity time- of-flight neutron diffractometer. The characterizations of dislocations were analyzed using the classical Williamson-Hall (cWH) and modified Williamson-Hall (mWH) plots on the breadth method, and the convolutional multiple whole profile (CMWP) fitting method. As results, the dislocation density as high as 1015 m-2 in the as-quenched martensitic steel was determined. The dislocation density was found to decrease qualitatively with plastic deformation by the cWH and mWH plots, but hardly to change by the CMWP method. The scanning transmission electron microscopy observation supported the results of the latter method. In the CMWP method, the parameter M that represents the arrangement of dislocations was found to decrease rapidly where a very high work hardening was observed.

Instrument Design and Performance Evaluation of a New Single Crystal Neutron Diffractometer SENJU at J-PARC

A new single crystal time-of-flight neutron diffractometer has been installed at BL18 of the Materials and Life Science Experimental Facility of J-PARC. The diffractometer “SENJU” was designed for precise crystal and magnetic structure analyses using a small crystal less than 1 mm under multiple extreme environments such as low-temperature and high-magnetic field [1]. The first neutron beam was delivered to the sample position on March 5, 2012. Subsequently, the hardware and software of SENJU have been checked and confirmed that they worked fine. At the same time, diffraction experiments of several organic and inorganic crystals have been done using NaCl, 12CaO!7Al2O3 (Fig. 1), and so on. In these measurements, Bragg reflections in the high-Q region (d-spacing < 0.5 A) were clearly observed and analyzed successfully. In this presentation, we will show the instrument design and performance evaluation of SENJU in detail.

X-ray Structural Study of Layered Compounds MnxTiS2

An X-ray structural study of single crystals of Mn x TiS 2 ( x =0.13, 0.18, and 0.26) was performed to investigate both the nature of the chemical bond from the electron density distribution (EDD) obtained by the maximum entropy method and local atomic arrangements of intercalated Mn atoms. Mn atoms were intercalated into octahedral sites in the van der Waals gap layer, and interlayer spacing and the distance between each layer were increased by increasing the number of Mn atoms without any change in layered structure. The EDD revealed that covalent bonding exists between Ti and S atoms, and no covalent electrons were found between Mn and S atoms. It is further realized by X-ray diffuse scattering measurements that intercalated Mn atoms are correlated with each other three-dimensionally. In Mn 0.13 TiS 2 and Mn 0.18 TiS 2 , an in-plane ordered structure for the low-temperature phase is expected to be of the 2×2-type in a Wood notation. On the other hand, a \(\sqrt{3}\times\sqrt{3}R30\)°-type ordered struc...

Single Crystal Neutron Diffraction Study of High Neutron Absorbing Compound EuGa4

Single crystal neutron diffraction measurement of EuGa4 has been carried out using time-of-flight single crystal neutron diffractometer SENJU at BL18 in MLF/J-PARC. In spite of extremely high neutron absorption of natural Eu (not enriched by Eu isotope), significant numbers of Bragg reflection were observed. Lattice parameters and lattice symmetry above TN (= 15 K) obtained from the positions and systematic extinction of the reflections were well agreed with the predefined results of single crystal X-ray diffraction. Furthermore, magnetic reflections were clearly observed below TN at positions of h+k+l = odd. Thus, antiferromagnetic structure of EuGa4 can be described with a propagation vector q = (0 0 0), which breaks a body-centered symmetry

Utilization of an Event-Recording System for Neutron Diffraction Experiments

We present effectivity of an event-recording system in neutron diffraction experiments at the Engineering Materials Diffractometer TAKUMI at the Materials and Life Science Experimental Facility in J-PARC. We developed an event-recording system for recording information of neutron generation and detection and sample environmental conditions like load, strain, etc. as events with synchronized timing tag across whole system, and developed a software to handle the event data. It gave us several advantages by allowing us flexible data reduction depending on users’ purposes, especially for time resolved experiments. We demonstrated in situ neutron diffraction measurements using these new recording system and software to show the advantages. These new system and software are very attractive to be applied for measurements that need stroboscopic data collection such as fatigue tests.

High-pressure environments of single crystal neutron diffractometer SENJU/J-PARC

High-pressure technique is a powerful tool for physical property measurements and structural analyses as well as other external conditions, such as magnetic fields and temperatures. In the field of neutron experiments, measurements under high-pressure conditions are also useful and attractive; because pressure is one of important thermodynamic parameter that can be used to tune magnetic property, crystal field, and other parameters to obtain insight into the microscopic physics of many phenomena. SENJU have been constructed for a single crystal time-of-flight neutron Laue diffractometer at beamline 18 (BL18) of Materials and Life Science Experimental Facility (MLF) at Japan Proton Accelerator Research Complex (J-PARC) [1], designed in consideration of precise crystal and magnetic structure analyses for small size single crystals, 1 mm3 or less in volume: and also taking account of the neutron diffraction measurements under multiple extreme conditions. In this research we are planning to introduce high-pressure sample environments into SENJU. Two types of compact high-pressure cells have been prepared, one is clamp type piston-cylinder cell made of copper-beryllium alloy (< 2 GPa), and the other is clamp type opposite anvil cell, can be expected to reach maximum pressure of 10 GPa. A taurine single crystal (3 mm3) was enclosed in the piston-cylinder pressure cell together with deuterated glycerol (pressure transmitting medium) and pressurized up to 1 GPa. Accelerator power of J-PARC was 300 kW and the exposure time was 6 hours. We can observe many distinct Bragg reflections from the sample crystal (taurine) even through the pressure cell body, as shown in the figure. In this presentation, we will show more details and current status of high-pressure sample environments in SENJU.