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論文

Determination of site occupancy of boron in 6H-SiC by multiple-wavelength neutron holography

林好一*; Lederer, M.*; 福本陽平*; 後藤雅司*; 山本裕太*; 八方直久*; 原田正英; 稲村泰弘; 及川健一; 大山研司*; et al.

Applied Physics Letters, 120(13), p.132101_1 - 132101_6, 2022/03

https://doi.org/10.1063/5.0080895

被引用回数：0 パーセンタイル：0(Physics, Applied)

The local structure around boron doped in a 6H-type silicon carbide (SiC) was investigated using neutron holography. Three dimensional atomic images reconstructed from multiple-wavelength holograms revealed the boron substitution for both silicon and carbon. To determine the boron locations accurately, we calculated holograms with varying occupancies of six different sites and fitted the image intensities with those obtained from the experimental holograms by the steepest descent method. As a result, it was found that boron atoms were selectively located at the Si-C-cubic site layer. Furthermore, boundaries right above the boron locations were suggested from the absence of atomic images in the upper region of the reconstruction.

論文

Chemical-pressure-induced point defects enable low thermal conductivity for Mg $_{2}$ Sn and Mg $_{2}$ Si single crystals

齋藤亘*; 林慶*; Huang, Z.*; 杉本和哉*; 大山研司*; 八方直久*; 原田正英; 及川健一; 稲村泰弘; 林好一*; et al.

ACS Applied Energy Materials (Internet), 4(5), p.5123 - 5131, 2021/05

https://doi.org/10.1021/acsaem.1c00670

被引用回数：12 パーセンタイル：64.95(Chemistry, Physical)

The development of thermoelectric (TE) materials, which can directly convert waste heat into electricity, is vital to reduce the use of fossil fuels. Mg $_{2}$ Sn and Mg $_{2}$ Si are promising TE materials because of their superior TE performance. In this study, for future improvement of the TE performance, point defect engineering was applied to the Mg $_{2}$ Sn and Mg $_{2}$ Si single crystals (SCs) via boron (B) doping. Their crystal structures were analyzed via white neutron holography and SC X-ray diffraction. Moreover, nanostructures and TE properties of the B-doped Mg $_{2}$ Sn and Mg $_{2}$ Si SCs were investigated. The B-doping increased the chemical pressure on the Mg $_{2}$ Sn and Mg $_{2}$ Si SCs, leading to induce vacancy defects as a point defect. No apparent change was observed in electronic transport, but thermal transport was significantly prevented. This study demonstrates that the vacancy defects can be controlled by the chemical pressure, and can aid in achieving a high TE performance for the Mg $_{2}$ Sn and Mg $_{2}$ Si SCs.

論文

Behavior of Sm in the boron cage of Sm-doped B $_{6}$ ( =Yb, La) observed by multiple-wavelength neutron holography

上地昇一*; 大山研司*; 福本陽平*; 金澤雄輝*; 八方直久*; 原田正英; 稲村泰弘; 及川健一; 松浦航*; 伊賀文俊*; et al.

Physical Review B, 102(5), p.054104_1 - 054104_10, 2020/08

https://doi.org/10.1103/PhysRevB.102.054104

被引用回数：7 パーセンタイル：44.34(Materials Science, Multidisciplinary)

We have succeeded in visualizing the local atomic structures around Sm in B $_{6}$ ( =Yb, La), in which a rareearth atom is located in the boron cage, using the newly developed technique for local structure investigations, multiple-wavelength neutron holography in a pulsed neutron facility. From the local atomic structures, we were able to clarify the behavior of Sm in the boron cage. Doped Sm in YbB fluctuates within the boron cage with a mean displacement of 0.25(4) , while that of Sm in LaB is approximately 0.15 . The Sm doping causes the fluctuation of the first-nearest-neighbor B with a mean displacement of 0.28 , while its effect on La, Yb, and the rest of the boron atoms is negligible. These are the first observations of the local behavior of doped rare-earth atoms and B in rare-earth borides with boron cages.

論文

Preparation, thermoelectric properties, and crystal structure of boron-doped Mg $_{2}$ Si single crystals

林慶*; 齋藤亘*; 杉本和哉*; 大山研司*; 林好一*; 八方直久*; 原田正英; 及川健一; 稲村泰弘; 宮崎譲*

AIP Advances (Internet), 10(3), p.035115_1 - 035115_7, 2020/03

https://doi.org/10.1063/1.5143839

被引用回数：16 パーセンタイル：73.04(Nanoscience & Nanotechnology)

Mg $_{2}$ Si is a potential thermoelectric (TE) material that can directly convert waste energy into electricity. In expectation of improving its TE performance by increasing electron carrier concentration, the element boron (B) is doped in Mg $_{2}$ Si single crystals (SCs). Their detailed crystal structures are definitely determined by using white neutron holography and single-crystal X-ray diffraction (SC-XRD) measurements. The white neutron holography measurement proves that the doped B atom successfully substitutes for the Mg site. The SC-XRD measurement confirms the B-doping site and also reveals the presence of the defect of Si vacancy (VSi) in the B-doped Mg $_{2}$ Si SCs. Regarding TE properties, the electrical conductivity, , and the Seebeck coefficient, S, decreases and increases, respectively, due to the decrease in the electron carrier concentration, contrary to the expectation. The power factor of the B-doped Mg $_{2}$ Si SCs evaluated from and S does not increase but rather decreases by the B-doping.

論文

Multiple-wavelength neutron holography with pulsed neutrons

林好一*; 大山研司*; 八方直久*; 松下智裕*; 細川伸也*; 原田正英; 稲村泰弘; 仁谷浩明*; 宍戸統悦*; 湯葢邦夫*

Science Advances (Internet), 3(8), p.e1700294_1 - e1700294_7, 2017/08

http://doi.org/10.1126/sciadv.1700294

Local structures around impurities in solids provide important information for understanding the mechanisms of material functions, because most of them are controlled by dopants. For this purpose, the X-ray absorption fine structure method, which provides radial distribution functions around specific elements, is most widely used. However, a similar method using neutron techniques has not yet been developed. If one can establish a method of local structural analysis with neutrons, then a new frontier of materials science can be explored owing to the specific nature of neutron scattering-that is, its high sensitivity to light elements and magnetic moments. Multiple-wavelength neutron holography using the time-of-flight technique with pulsed neutrons has great potential to realize this. We demonstrated multiple-wavelength neutron holography using a Eu-doped CaF $_{2}$ single crystal and obtained a clear three-dimensional atomic image around trivalent Eu substituted for divalent Ca, revealing an interesting feature of the local structure that allows it to maintain charge neutrality. The new holography technique is expected to provide new information on local structures using the neutron technique.

論文

Spontaneous formation of suboxidic coordination around Co in ferromagnetic rutile Ti $_{0.95}$ Co $_{0.05}$ O film

Hu, W.*; 林好一*; 福村知昭*; 赤木和人*; 塚田捷*; 八方直久*; 細川伸也*; 大和田謙二; 高橋正光; 鈴木基寛*; et al.

Applied Physics Letters, 106(22), p.222403_1 - 222403_5, 2015/06

https://doi.org/10.1063/1.4921847

被引用回数：39 パーセンタイル：81.57(Physics, Applied)

The local atomic structures around Co in high temperature diluted ferromagnetic semiconductor Co-doped TiO has been investigated using X-ray fluorescence holography and X-ray absorption fine structure experiments. While the Co atoms in the Ti $_{0.99}$ Co $_{0.01}$ O simply substituted for Ti sites in the rutile structure, a suboxidic arrangement of CoOTi was found to form around Co in the Ti $_{0.95}$ Co $_{0.05}$ O films. First-principles calculations supported the stability of the aggregated suboxidic clusters in the rutile TiO. The suboxidic coordination may be the source of strong exchange interaction, resulting in the high Curie temperature in Co-dopedTiO.

論文

Acute and obtuse rhombohedrons in the local structures of relaxor ferroelectric Pb(Mg $_{1/3}$ Nb $_{2/3}$ )O

Hu, W.*; 林好一*; 大和田謙二; Chen, J.*; 八方直久*; 細川伸也*; 高橋正光; Bokov, A.*; Ye, Z.-G*

Physical Review B, 89(14), p.140103_1 - 140103_5, 2014/04

https://doi.org/10.1103/PhysRevB.89.140103

被引用回数：48 パーセンタイル：85.62(Materials Science, Multidisciplinary)

The local structures around Nb and Pb in the relaxor ferroelectric Pb(Mg $_{1/3}$ Nb $_{2/3}$ )O were investigated by X-ray fluorescence holography. The Pb atomic images around Nb showed a rhombohedral distortion of the crystal unit cells. The Pb-Pb correlated images showed a local structure of body-center-like superlattice, proving a rigid three-dimensional network structural model combining the two kinds of rhombohedrons. This superstructure and the fluctuating Pb and Nb atoms play an important role in the relaxor behavior of this material.

論文

X-ray fluorescence holography

林好一*; 八方直久*; 細川伸也*; Hu, W.; 松下智裕*

Journal of Physics; Condensed Matter, 24(9), p.093201_1 - 093201_15, 2012/03

https://doi.org/10.1088/0953-8984/24/9/093201

被引用回数：86 パーセンタイル：60.71(Physics, Condensed Matter)

X-ray fluorescence holography (XFH) is a method of atomic resolution holography which utilizes fluorescing atoms as a wave source or a monitor of the interference field within a crystal sample. It provides three-dimensional atomic images around a specified element and has a range of up to a few nm in real space. In this article, we explain the theory of XFH including solutions to the twin-image problem, an advanced measuring system, and data processing for the reconstruction of atomic images. Then, we briefly introduce our recent applications of this technique to the analysis of local lattice distortions in mixed crystals and nanometer-size clusters appearing in the low-temperature phase of a shape-memory alloy.

論文

Three dimensional atomic image of TlInSe $_{2}$ by X-ray fluorescence holography

三村功次郎*; 細川伸也*; 八方直久*; Hu, W.; 林好一*; 脇田和樹*; 石井啓文*; 吉村政人*; Jeyakanthan, J.*; Mamedov, N.*

e-Journal of Surface Science and Nanotechnology (Internet), 9, p.273 - 276, 2011/07

https://doi.org/10.1380/ejssnt.2011.273

Three dimensional (3D) atomic images around the Tl atoms in single crystal TlInSe $_{2}$ thermoelectric material in the incommensurate phase were reconstructed by using Tl LIII X-ray fluorescence holography measured at room temperature. Comparison of the experimentally obtained 3D images with the theoretically calculated ones suggests that only the Tl atoms have large spatial fluctuations, which are related to the incommensurate phase of this material.

論文

Local structure around Ge atoms in IV-VI ferromagnetic semiconductor Ge $_{0.6}$ Mn $_{0.4}$ Te by X-ray fluorescence holography

八方直久*; 竹原祐紀*; 藤原真*; 田中公一*; 仙波伸也*; 細川伸也*; 林好一*; Hu, W.; 鈴木基寛*; 浅田裕法*

e-Journal of Surface Science and Nanotechnology (Internet), 9, p.247 - 250, 2011/06

https://doi.org/10.1380/ejssnt.2011.247

Local atomic structure around Ge atoms in Ge $_{1-x}$ Mn $_{x}$ Te thin film single crystal has been investigated by X-ray fluorescence holography (XFH) at room temperature. Obtained atomic image suggests that the Ge position is not stable in the exact positions of the anion fcc sublattice of the anion, and reveals the fluctuation of the Ge positions or the existence of cation vacancies.

論文

Three-dimensional atomic images of TlInSe $_{2}$ thermoelectric material obtained by X-ray fluorescence holography

細川伸也*; 八方直久*; 林好一*; 三村功次郎*; 脇田和樹*; Hu, W.; 石井啓文*; 吉村政人*; Jeyakanthan, J.*; Mamedov, N.*

Japanese Journal of Applied Physics, 50(5), p.05FC06_1 - 05FC06_2, 2011/05

https://doi.org/10.1143/JJAP.50.05FC06

被引用回数：7 パーセンタイル：31.23(Physics, Applied)

Three dimensional (3D) atomic images around the Tl atoms in single crystal TlInSe $_{2}$ thermoelectric material in the incommensurate phase were reconstructed by using Tl $_{rm III}$ X-ray fluorescence holography measured at room temperature. In the obtained 3D image, the In atoms are clearly visible at the proper positions in the normal phase. However, the Tl atomic images are also observed but very weak. By comparing the experimental image with a theoretically calculated one, it is concluded that only the Tl atoms have large spatial fluctuations, which are related to the incommensurate phase of this material.

論文

Local structure around Mn atoms in IV-VI ferromagnetic semiconductor Ge $_{0.6}$ Mn $_{0.4}$ Te investigated by X-ray fluorescence holography

八方直久*; 竹原祐紀*; 藤原真*; 田中公一*; 仙波伸也*; 細川伸也*; 林好一*; Hu, W.; 鈴木基寛*; 浅田裕法*

Japanese Journal of Applied Physics, 50(5), p.05FC11_1 - 05FC11_2, 2011/05

https://doi.org/10.1143/JJAP.50.05FC11

被引用回数：5 パーセンタイル：23.4(Physics, Applied)

The local atomic structure around Mn atoms in Ge $_{1-x}$ Mn $_{x}$ Te thin-film single crystal has been investigated by X-ray fluorescence holography (XFH) at room temperature. The obtained atomic image suggests that the Mn atoms replace the Ge atoms in the host GeTe, and the Mn position is stable in the exact positions of the anion fcc sublattice. The XFH result also suggests the fluctuation of the Ge positions or the cation vacancies.

論文

Three dimensional local structure analysis of ZnSnAs $_{2}$ :Mn by X-ray fluorescence holography

林好一*; 内富直隆*; Asubar, J. T.*; 八方直久*; Hu, W.; 細川伸也*; 鈴木基寛*

Japanese Journal of Applied Physics, 50(1), p.01BF05_1 - 01BF05_4, 2011/01

https://doi.org/10.1143/JJAP.50.01BF05

被引用回数：1 パーセンタイル：4.79(Physics, Applied)

X-ray fluorescence holographic study on a room-temperature ferromagnetic semiconductor film of ZnSnAs $_{2}$ :Mn was performed using a strong X-ray beam of third generation synchrotron radiation of SPring-8. The real space reconstructions of the environments around Mn atoms were successfully visualized from the observed holograms despite the very small amount of Mn atoms. The reconstructions revealed that the Mn atomsoccupy the cation (Zn or Sn) site.

口頭

Direct observation of higher order atomistic structure in Ti $_{50}$ Ni $_{44}$ Fe $_{6}$ at lower temperature phase

Hu, W.; 林好一*; 八方直久*; 細川伸也*; 寺井智之*; 福田隆*; 掛下知行*; 高橋正光

no journal, ,

The Ti-Ni series shape memory alloys shows novel phase transition behaviours depending on the concentrations of doped third element Fe and only the precursor phenomena are observed when more than 6 at.% of Fe atoms are substituted for Ni. In the present work, we have focused on Ti $_{50}$ Ni $_{44}$ Fe $_{6}$ , which exhibits only a second-order-like parent (P) - incommensurate (IC) - commensurate (C) transition, and investigated the local structure around Fe and Ni by the X-ray fluorescence holography (XFH). We successfully obtained 3D real space local structures at both the P- and C-phases. Cluster-like structure appears around Fe and the direct evidence of the existence of supperlattice structure at C-phase appears around Ni. We discuss structural changes by comparing the microstructures obtained around Fe and Ni at two phases. The results provide hints on the lattice modulation related to precursors of the martensitic transition and direct evidence of superlattice phenomena.

口頭

Direct observation of local structure distortion in Pb(Mg $_{1/3}$ Nb $_{2/3}$ )O $_{3}$ by X-ray fluorescence holography

Hu, W.; 林好一*; 八方直久*; 大和田謙二; Chen, J.*; Ye, Z.-G*; 細川伸也*; 高橋正光

no journal, ,

The chemical ordering regions (CORs) and polar nanoregions (PNRs) in relaxor ferroelectrics have been long believed to the origin of their high and temperature-independent permittivities. In the present work, we carried out X-ray fluorescence holography (XFH) experiments of Pb(Mg $_{1/3}$ Nb $_{2/3}$ )O $_{3}$ (PMN) and La-doped PMN (2 at% La, PLMN) single crystals, which are prototype relaxor ferroelectric material, and investigated the local structure around Nb atoms at BL22XU of SPring-8. The La-doped PMN single crystal shows much bigger CORs than pure PMN. Nb K (16.6 keV) fluorescent X-rays were analyzed and focused on an avalanche photodiode detector with a cylindrical graphite-crystal energy analyzer. The results indicate that both Pb-site and Mg/Nb-site atoms have big fluctuations in PMN system and 1st neigher Pb atoms tend to shift toward central Nb atom along [111] direction.

口頭

Precursor phenomena of martensitic transformation in Fe-doped Ti-Ni alloys studied by X-ray fluorescence holography

Hu, W.; 林好一*; 八方直久*; 寺井智之*; 福田隆*; 掛下知行*; 高橋正光

no journal, ,

The gradual changes in the lattice modulation, phonon dispersion and elastic modulus, which are attracting much interest as precursors of the martensitic transition, have been experimentally reported for many materials. However, studying the precursors is difficult because they are immediately followed by the main martensitic transition. Ti $_{50}$ Ni $_{44}$ Fe $_{6}$ , which shows only parent - incommensurate - commensurate phase transition, is a suitable sample for investigating the lattice modulation accompanied by the appearance of the superlattice structure. In the present work, X-ray fluorescence holography technique have been applied for investigating the 3D local structure around Fe and Ni at both the parent and commensurate phases. We will discuss the dynamical structural changes at both phases, and the results will provide hints to the lattice modulation related to precursors of the martensitic transformation.

口頭

3D local structure analysis of relaxor ferroelectrics by X-ray fluorescence holography

Hu, W.; 林好一*; 八方直久*; 大和田謙二; Chen, J.*; Ye, Z.-G*; 細川伸也*; 高橋正光

no journal, ,

Relaxor ferroelectrics as a class of disordered crystals possessing peculiar structure and properties have attracted significant research attention. However, quantitative analysis of its local structure and lattice distortion is difficult by conventional diffraction methods. In the present work, the authors carried out X-ray fluorescence holography experiments on PMN and La-doped PMN (PLMN) single crystals at BL22XU of SPring-8 and successfully obtained 3D atomic images around Nb and Pb elements up to 20 with atomic resolution, respectively. The local distortion of Pb atoms was directly revealed by comparing the atomic images with the reference sample SrTiO $_{3}$ (ideal perovskite structure). We will discuss the differences between PMN and PLMN by comparing the obtained middle range local structures around Nb and Pb, and the pure structure of the chemical ordered domains on the atomic scale.

口頭

蛍光X線ホログラフィーによるPb(Mg $_{1/3}$ Nb $_{2/3}$ )O $_{3}$ リラクサー強誘電体の三次元局所構造解析

Hu, W.; 林好一*; 八方直久*; 大和田謙二; Chen, J.*; Ye, Z.-G*; 細川伸也*; 高橋正光

no journal, ,

Relaxor ferroelectrics as a class of disordered crystals possessing peculiar structure and properties have attracted significant research attention. However, quantitative analysis of its local structure and lattice distortion is difficult, and there is no report to our knowledge of direct observation of its three-dimensional local structure. In the present work, we carried our X-ray fluorescence holography experiments on Pb(Mg $_{1/3}$ Nb $_{2/3}$ )O $_{3}$ (PMN) single crystal at BL22XU of SPring-8 and successfully obtained 3D atomic images around Nb and Pb elements up to 2 nm with atomic resolution. Our result shows the direct evidence of the rhombohedral distortion in PMN. Comparing with theoretical calculation, coexistence of two rhombohedral distortions was confirmed. In this presentation, we will discuss the short-range distortion of Pb atoms by comparing the obtained 3D atomic images around Nb at room temperature and 100 K.