Faculty Profiles - NIWA, Ken

Papers - NIWA, Ken

Division display 41 - 60 of about 86 ／ All the affair displays >>

Lithium ionic conductivities of alpha-LiBO2 with two-dimensional Li-Li networks and gamma-LiBO2 with three-dimensional ones synthesized under high pressure Reviewed

Hirose Eiichi, Kataoka Kunimitsu, Nagata Hiroshi, Akimoto Junji, Sasaki Takuya, Niwa Ken, Hasegawa Masashi

JOURNAL OF SOLID STATE CHEMISTRY Vol. 274 page： 100 - 104 2019.6

Language：English Publishing type：Research paper (scientific journal)

© 2019 Elsevier Inc. Lithium ionic conductivities of two LiBO 2 polymorphs, i.e. the same composition Li borates: α-LiBO 2 and γ-LiBO 2 , which are ambient and high-pressure phases, have been measured by the AC impedance method between 450 and 520 K using their high-density polycrystalline samples synthesized under high pressures. It is found the intra-grain and total conductivities of the γ-LiBO 2 have been measured for the first time and be of the order 10 −5 ∼ 10 −6 Scm −1 and 10 −6 Scm −1 , respectively, both of which are higher in whole measured temperature range than those of the α-LiBO 2 . The higher lithium ionic conductivities of the former are discussed from the viewpoints of local structure and Li ion network in the crystal structures of the α-LiBO 2 and γ-LiBO 2 .

DOI： 10.1016/j.jssc.2019.02.045

Web of Science

Scopus

High-pressure synthesis, crystal growth, and compression behavior of hexagonal CrN2 having one-dimensionally aligned nitrogen dimer Reviewed

Niwa Ken, Yamamoto Takurou, Sasaki Takuya, Hasegawa Masashi

PHYSICAL REVIEW MATERIALS Vol. 3 ( 5 ) 2019.5

Infrared spectroscopy on electronic structures of platinum-group metal pernitrides MN2 (M = Ru, Rh, Ir, and Pt) Reviewed

Soda Kazuo, Komabuchi Mai, Maeguchi Keisuke, Kato Masahiko, Terabe Toshiki, Niwa Ken, Hasegawa Masashi, Ikemoto Yuka, Okamura Hidekazu

PHYSICA B-CONDENSED MATTER Vol. 558 page： 54 - 58 2019.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

The electronic structures of platinum-group metal pernitrides MN2 (M = Ru, Rh, Ir, and N) were investigated via synchrotron radiation infrared spectroscopy and first-principles calculations. Measured reflectance spectra of marcasite-type RuN2 and RhN(2 )showed Drude-like responses, approaching 1 as the photon energy was decreased, whereas reflectance of arsenopyrite-type IrN2 and pyrite-type PtN2 became similar to 0.3 in the low photon energy region with a few features. These findings agreed well with the predictions of the metallic nature of marcasite-type RuN2 and RhN2 and the semiconducting properties of arsenopyrite-type IrN2 and pyrite-type PtN2 , respectively. The measured reflectance spectra were also reasonably consistent with the calculated optical responses. The band gaps of IrN2 and PtN2 were estimated to be 0.8 and 2.1 eV, respectively, via first-principles calculation with a modified Becke-Johnson (MBJ) potential for the exchange potential.

DOI： 10.1016/j.physb.2019.01.017

Web of Science

Scopus

High-Pressure Synthesis, Crystal Chemistry, and Ionic Conductivity of a Structural Polymorph of Li3BP2O8 Reviewed

Hirose Eiichi, Kataoka Kunimitsu, Nagata Hiroshi, Akimoto Junji, Sasaki Takuya, Niwa Ken, Hasegawa Masashi

INORGANIC CHEMISTRY Vol. 57 ( 24 ) page： 15048 - 15050 2018.12

Language：English Publishing type：Research paper (scientific journal)

© 2018 American Chemical Society. A new structural polymorph of Li 3 BP 2 O 8 has been successfully synthesized via a solid-state reaction between Li 3 PO 4 and BPO 4 at 4 GPa and 600 °C. The high-pressure phase of Li 3 BP 2 O 8 (HP-Li 3 BP 2 O 8 ) was found to crystallize in monoclinic symmetry with the cell parameters of a = 8.57010(4) Å, b = 11.11812(5) Å, c = 5.55380(3) Å, and β = 97.7269(3)° [space group P2 1 /a (No. 14)]. HP-Li 3 BP 2 O 8 has a new crystal structure that has not been reported so far. The total ionic conductivities measured for the polycrystalline sample by alternating-currrent impedance were 3.4 × 10 -7 and 2.1 × 10 -6 S/cm at 399 and 456 K, respectively. The lithium ionic conductivity of HP-Li 3 BP 2 O 8 was higher than that of the low-pressure phase Li 3 BP 2 O 8 in the temperature range of 375-456 K. This is caused by the difference in the dimensions of the lithium arrangements between LP- and HP-Li 3 BP 2 O 8 .

DOI： 10.1021/acs.inorgchem.8b03155

Web of Science

Scopus

PubMed

Structural stability of the Li-ion conductor Li7La3Zr2O12 investigated by high-pressure in-situ X-ray diffraction and Raman spectroscopy

Hirose Eiichi, Niwa Ken, Kataoka Kunimitsu, Akimoto Junji, Hasegawa Masashi

MATERIALS RESEARCH BULLETIN Vol. 107 page： 361 - 365 2018.11

Language：English Publishing type：Research paper (scientific journal)

© 2018 Elsevier Ltd The high-pressure behavior of Li7La3Zr2O12 (LLZO) with a garnet-related structure has been investigated for the first time by high pressure in situ X-ray diffraction measurements and Raman spectroscopy using a diamond anvil cell. It was found that the tetragonal LLZO transforms to the high-pressure cubic phase (HP-c-LLZO) at 71.8 GPa in a N2 medium and at 25.8 GPa in the absence of pressure mediums. Uniaxial compression in the absence of pressure mediums induced the phase transition at much lower pressures than in the hydrostatic case when N2 was used as the pressure medium. High pressure in situ Raman spectroscopy suggested that the Li coordination in HP-c-LLZO was different from that reported in cubic LLZO, which had been stabilized by doping Al at ambient pressure and room temperature.

DOI： 10.1016/j.materresbull.2018.07.034

Web of Science

Scopus

Valence-band electronic structures of high-pressure-phase PdF2-type platinum-group metal dioxides MO2 (M = Ru, Rh, Ir, and Pt)

Kazuo Soda, Daichi Kobayashi, Tatsuya Mizui, Masahiko Kato, Yuichi Shirako, Ken Niwa, Masashi Hasegawa, Masaki Akaogi, Hiroshi Kojitani, Eiji Ikenaga, Takayuki Muro

Journal of the Physical Society of Japan Vol. 87 ( 4 ) 2018.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Physical Society of Japan

The valence-band electronic structures of high-pressure-phase PdF2-type (HP-PdF2-type) platinum-group metal dioxides MO2 (M = Ru, Rh, Ir, and Pt) were studied by synchrotron radiation photoelectron spectroscopy and first-principles calculations. The obtained photoelectron spectra for HP-PdF2-type RuO2, RhO2, and IrO2 agree well with the calculated valence-band densities of states (DOSs) for these compounds, indicating their metallic properties, whereas the DOS of HP-PdF2-type PtO2 (calculated in the presence and absence of spin–orbit interactions) predicts that this material may be metallic or semimetallic, which is inconsistent with the electric conductivity reported to date and the charging effect observed in current photoelectron measurements. Compared with the calculated results, the valence-band spectrum of PtO2 appears to have shifted toward the high-binding-energy side and reveals a gradual intensity decrease toward the Fermi energy EF, implying a semiconductor-like electronic structure. Spin-dependent calculations predict a ferromagnetic ground state with a magnetization of 0.475 μB per formula unit for HP-PdF2-type RhO2

DOI： 10.7566/JPSJ.87.044701

Web of Science

Scopus

High pressure in-situ synthesis and physical properties of Cr3C2-Ni cermets International coauthorship

Wenyan Zhai, Yimin Gao, Liang Sun, Yiran Wang, Ken Niwa, Masashi Hasegawa

CERAMICS INTERNATIONAL Vol. 43 ( 18 ) page： 17202 - 17205 2017.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

A novel Cr3C2-Ni cermets were successfully in-situ synthesized with Cr, C and Ni (20 wt%) mixed-powder at 1100 degrees C under 0-6 GPa isostatic pressure using a small-sized cubic-type multi-anvil press apparatus. High purity Cr3C2 phase was generated in the cermets. Comparing with the conventional and reactive sintering, the sintering temperature and the average grain size of Cr3C2 decreased by 200 degrees C and 50%, respectively. The physical properties, normalized lattice parameters and volumes of Cr3C2 20 wt%Ni cermet as a function of pressure had been studied both experimentally and by first-principles calculations for the first time.

DOI： 10.1016/j.ceramint.2017.09.145

Web of Science

Scopus

Pyrite form of group-14 element pernitrides synthesized at high pressure and high temperature

K. Niwa, H. Ogasawara, M. Hasegawa

DALTON TRANSACTIONS Vol. 46 ( 30 ) page： 9750 - 9754 2017.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

The chemical reaction of group-14 elements with molecular nitrogen at pressures above 60 GPa successfully leads to the formation of pyrite-type pernitrides. These new crystalline group-14 elemental pernitrides were found to show a bulk modulus higher than the other known group-14 elemental nitrides. Our results offer significant new progress in the solid-state chemistry of nitrides.

DOI： 10.1039/c7dt01583f

Web of Science

Scopus

PubMed

Microbeam Hard X-ray Photoemission Study on Platinum-Group Metal Pernitrides

Kazuo Soda, Tatsuya Mizui, Mai Komabuchi, Masahiko Kato, Toshiki Terabe, Kentaro Suzuki, Ken Niwa, Yuichi Shirako, Masashi Hasegawa, Masaki Akaogi, Hiroshi Kojitani, Eiji Ikenaga

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN Vol. 86 ( 6 ) page： 64804 - 64804 2017.6

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

Using microbeam hard X-ray photoelectron spectroscopy, we clarified the valence-band electronic structures and chemical states of platinum-group metal (Ru, Ir, and Pt) pernitrides, which have been synthesized in supercritical nitrogen fluid under extremely high pressures and temperatures. Their nitrogen contents relative to the platinum-group metal are estimated to be 2 from the photoemission intensity, which is consistent with the studies reported to date. The observed valence-band structures agree quite well with theoretically predicted structures for the pyrite-type PtN2, arsenopyrite-type IrN2, and marcasite-type RuN2. The origin of their extremely large bulk moduli is discussed based on the current results of the valence-band structures and core-level chemical shifts.

DOI： 10.7566/JPSJ.86.064804

Web of Science

Scopus

CiNii Research

Highly Coordinated Iron and Cobalt Nitrides Synthesized at High Pressures and High Temperatures

Ken Niwa, Toshiki Terabe, Daiki Kato, Shin Takayama, Masahiko Kato, Kazuo Soda, Masashi Hasegawa

INORGANIC CHEMISTRY Vol. 56 ( 11 ) page： 6410 - 6418 2017.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

Highly coordinated iron and cobalt nitrides were successfully synthesized via direct chemical reaction between a transition metal and molecular nitrogen at pressures above approximately 30 GPa using a laser-heated diamond anvil cell. The synthesized novel transition metal nitrides were found to crystallize into the NiAs-type or marcasite-type structure. NiAs-type FeN could be quenched at ambient pressure, although it was gradually converted to the ZnS-type structure after the pressure was released. On the other hand, CoN was recovered with ZnS-type structure through a phase transition from NiAs-type structure at approximately a few gigapascals during decompression. Marcasite-type CoN2 was also synthesized at pressures above approximately 30 GPa. High-pressure in situ X-ray diffraction measurement showed that the zero-pressure bulk modulus of marcasite-type CoN2 is 216(18) GPa, which is comparable to that of RhN2. This indicates that the interatomic distance of the N-N dimer in marcasite-type CoN2 is short because of weak orbital interaction between cobalt and nitrogen atoms, as in RhN2. Surprisingly, a first-principles electronic band calculation suggests that the NiAs-type FeN and CoN and marcasite-type CoN2 exhibit metallic characteristics with magnetic moments of 3.4, 0.6, and 1.2 mu(B), respectively. The ferromagnetic NiAs-type structure originates from the anisotropic arrangement of transition atoms stacked along the: c axis.

DOI： 10.1021/acs.inorgchem.7b00516

Synthesis, crystal structure and phase transition of a Xe-N-2 compound at high pressure: experimental indication of orbital interaction between xenon and nitrogen

K. Niwa, F. Matsuzaki, M. Hasegawa

PHYSICAL CHEMISTRY CHEMICAL PHYSICS Vol. 19 ( 1 ) page： 59 - 63 2017.1

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

The van der Waals compound Xe(N-2)(2) with a C15 Laves structure was successfully synthesised at pressures greater than 4.4 GPa. We found that, at 10 GPa, the structure reversibly transforms from a cubic to a tetragonal phase. Further compression results in changes of Xe-N compound, which could result in the enhancement of orbital interactions between the xenon and nitrogen atoms.

DOI： 10.1039/c6cp06552j

Web of Science

PubMed

High-Pressure Synthesis and Magnetic Behavior of A-Site Columnar-Ordered Double Perovskites, LnMn(Ga0.5Ti0.5)(2)O-6 (Ln = Sm, Gd)

Gen Shimura, Ken Niwa, Yuichi Shirako, Masashi Hasegawa

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY Vol. 2017 ( 4 ) page： 835 - 839 2017.1

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-V C H VERLAG GMBH

A-site columnar-ordered double perovskites, LnMn(Ga0.5Ti0.5)(2)O-6 (Ln = Sm, Gd), were successfully synthesized under high pressure and high temperature (6 GPa, 1375 K). From the synchrotron powder X-ray diffraction patterns, all of the diffraction peaks can be indexed by the P4(2)/nmc space group with lattice parameters a, c approximate to 2a(p) (a(p): primitive cubic perovskite lattice) and no ordering of the B-site cations. Rietveld analysis of the synchrotron powder X-ray diffraction patterns and Curie-Weiss fitting of their magnetizations reveal that the ionic formulae of these perovskites are Ln(3+) Mn2+(Ga0.53+Ti0.54+)(2)O-6(2-). SmMn(Ga0.5Ti0.5)(2)O-6 shows canted-antiferromagnetic behavior, whereas GdMn(Ga0.5Ti0.5)(2)O-6 exhibits two different magnetic states at low temperature depending on the applied magnetic field and shows an unusual magnetization curve. These magnetic behaviors originate by decreasing the antiferromagnetic interaction by substituting Ga3+(d(10)) for Ti4+(d(0)) and by decreasing the ferromagnetic interaction between columnar-ordered Ln(3+) and Mn2+.

DOI： 10.1002/ejic.201601392

Web of Science

Scopus

CiNii Research

Microbeam hard X-ray photoelectron and ion-beam analyses of Nb hydrides formed in supercritical water

Kazuo Soda, Shinya Sugiura, Kanta Yamaguchi, Masahiko Kato, Ken Niwa, Masashi Hasegawa, Kohtaku Suzuki, Ryoya Ishigami, Eiji Ikenaga

International Symposium on Atomic Level Characterizations for New Materials and Devices ‘17 Proceedings 2017

High pressure stability and ambient metastability of marcasite-type rhodium pernitride Reviewed

K. Niwa, T. Terabe, K. Suzuki, Y. Shirako, M. Hasegawa

Journal of Applied Physics Vol. 119 page： 065901 2016

Multicomponent A-site ordered perovskite BiMn3(Fe0.25Ti0.75)4O12: High-pressure synthesis, crystal chemistry and magnetic behavior Reviewed

G. Shimura, K. Niwa, Y. Shirako, S. Muto, K. Kusaba, M. Hasegawa

Solid State Communications Vol. 234-235 page： 40-44 2016

Molecular routes syntheses of graphite-like C-N compounds with various N/C ratios in high pressure and temperature Reviewed

T. HORIBE, K. KUSABA, K. NIWA, M. HASEGAWA, K. YASUDA, R. ISHIGAMI

Journal of the Ceramic Society of Japan Vol. 124 ( 10 ) page： 1013-1016 2016

High-pressure synthesis and relationship between A-site ordering and local structure of multicomponent perovskites(Ln0.25Mn0.75) (Al0.25Ti0. 75)O3, Ln=La, Pr, Nd, Sm, Gd, Tb, Dy, Y Reviewed

G. Shimura, Y. Shirako, K. Niwa, M. Hasegawa

Journal of Solid State Chemistry Vol. 242 page： 55-62 2016

Angle-resolved hard x-ray photoemission study of Nb hydride formation in high-pressure supercritical water Reviewed

Kazuo Soda, Hiroki Kondo, Kanta Yamaguchi, Masahiko Kato, Tatsuhito Shirak, Ken Niwa, Keiji Kusaba, Masashi Hasegawa, Kozina Xeniya, Eiji Ikenaga

Journal of Alloys and Compounds Vol. 643 page： 195-200 2015

Growth of rectangular hollow tube crystals with rutile-type structure in supercritical fluids Reviewed

K. Niwa, H. Ikegaya, T. Taguchi, S. Muto, T. Tokunaga and M. Hasegawa

Journal of Physics: Conference Series (JPCS) Vol. 500 page： 022007 2014

High pressure synthesis of marcasite-type rhodium pernitride Reviewed International coauthorship

K. Niwa, D. Dzivenko, K. Suzuki, R. Riedel, I. Troyan, M. Eremets and M. Hasegawa

Inorganic Chemistry Vol. 53 page： 697-699 2014

　 PREV - NEXT 　

<NIWA, Ken>

Papers - NIWA, Ken