Faculty Profiles - NIWA, Ken

High-pressure synthesis, crystal structure and anisotropic thermal/compressive behaviors of pseudo-ternary (V,Cr,Mo)P4 Reviewed

Chang, CC; Sasaki, T; Muto, S; Gaida, NA; Niwa, K; Hasegawa, M

JOURNAL OF SOLID STATE CHEMISTRY Vol. 340 2024.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Solid State Chemistry

Pseudo-ternary CrP4-type (V,Cr,Mo)P4 was successfully synthesized at the conditions of 4 GPa and 900 °C by using a large volume multi-anvil-type press. SEM-EDS and STEM analyses revealed that the synthesized pseudo-ternary (V,Cr,Mo)P4 contains almost equimolar amounts of transition metals. The lattice parameters and unit cell volume of (V,Cr,Mo)P4 yield the average value of its end-members. The low-temperature (133 K–333 K) in-situ synchrotron XRD measurements demonstrated that the c-axis showed the highest coefficient of thermal expansion, followed by the b-axis and the a-axis. These thermal behaviors are the same as the binary end-members and pseudo-binary (V,Cr)P4. While from the high-pressure (P < 10 GPa) in-situ synchrotron XRD measurements, it was found that (V,Cr,Mo)P4 showed no phase transition and compression behaviors in which the c-axis was more compressible than the other two axes (b and a) and the b-axis was slightly compressible more than the a-axis. The zero-pressure bulk modulus of 106 (1) GPa and 122.1 (7) GPa were obtained for (V,Cr)P4 and (V,Cr,Mo)P4, respectively. MoP6 octahedral is highly distorted in comparison with VP4 and CrP4, thus the incorporation of MoP4 greatly yields incompressible behaviors of CrP4-type phosphides with respect to temperature and pressure.

DOI： 10.1016/j.jssc.2024.125021

Emergence of New Nitrogen-Rich Compounds in Lead-Nitrogen Phase Diagram Under Pressure Reviewed

Pitié, S; Niwa, K; Frapper, G

JOURNAL OF PHYSICAL CHEMISTRY C Vol. 128 ( 45 ) page： 19410 - 19422 2024.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Physical Chemistry C

In this study, we employed a variable-composition evolutionary algorithm in conjunction with first-principles calculations to perform a comprehensive structural search for the Pb-N binary system under pressure and determine a hyperfine pressure-composition phase diagram. The findings revealed the existence of 11 new pressure-stabilized structures in the pressure range of 0-100 GPa and at 0 K. Six nitrogen-rich compositions, PbNz (z = 2-4, 6, 8, and 10), and seven thermodynamically stable phases were identified. The most prevalent nitrogen motif observed in the predicted PbNz phases is the N2 dumbbell (z = 2-4, 6, and 8). As z increases, the lowest N-N vibrational frequencies decrease from 2076 cm-1 in Im3̅ Pb(N2)4 to 1302 cm-1 in I4/mcm Pb(N2) at 25 GPa. This finding is related to a decrease in the charge transfer from Pb to N2 dimers. The νN-N frequencies act as a clear signature of the existence of high-pressure PbNz phases containing N2 dumbbells. In addition to N2 dimers, the aromatic pentazolate unit N5- is stabilized in the I4/m Pb(cyclo-N5)2 phase, which becomes thermodynamically stable at 25 GPa. This nitrogen-rich phase can be recoverable to ambient pressure and exhibits a high detonation velocity of 10.2 km/s. Additionally, one- and two-dimensional covalent nitrogen nets are also observed in PbNz (z = 6 and 8). Infinite polymeric chains are found in PbN6 and a two-dimensional nitrogen covalent net is encountered in Cc Pb@2D-N8, which is composed of fused aromatic 18-rings.

DOI： 10.1021/acs.jpcc.4c05540

Carbon Nitrides Synthesis using Supercritical Nitrogen Fluid under High-Pressure Reviewed

Niwa Ken, Fukui Riku, Fukai Toshifumi, Alexander Gaida Nico

SPring-8/SACLA Research Report Vol. 12 ( 5 ) page： 303 - 307 2024.10

2-2-2 High-pressure powder X-ray diffraction study of methane-methylcyclopentane hydrate Reviewed

OSAWA Kanta, OGISO Haruto, YONEZAWA Ryohei, SAKATA Masafumi, KIMURA Tomoaki, SASAKI Shigeo, NAGAE Takayuki, NIWA Ken

Proceedings of the Annual Conference of The Japan Institute of Energy Vol. 33 ( 0 ) page： 20 - 21 2024.7

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Institute of Energy

High-pressure powder X-ray diffraction measurements of methane-methylcyclopentane-water system have been performed in a diamond anvil cell up to 3.1 GPa at room temperature. As a result, a cubic structure II (sII) and a hexagonal structure H (sH) simultaneously form at 0.9 GPa. The sII hydrate which exists up to 2.9 GPa is considered to be composed of methane, methylcyclopentane and water, namely, methane-methylcyclopentane hydrate (MMCPH) because methane or methylcyclopentane with water cannot exist in clathrate hydrate under high pressure. On the other hand, the sH phase possibly comes from methane hydrate. The pressure region of the MMCPH-sII (< 2.9 GPa) is the widest for gas clathrate hydrates ever observed under high pressure at room temperature.

DOI： 10.20550/jietaikaiyoushi.33.0_20

High-pressure X-ray study of NbON oxynitride: direct transition from baddeleyite to cotunnite structure Reviewed

Kozaki, S; Yuan, Y; Ubukata, H; Wei, ZF; Tsumori, T; Asano, S; Niwa, K; Hasegawa, M; Kageyama, H

CHEMISTRY LETTERS Vol. 53 ( 6 ) 2024.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Chemistry Letters

The structural properties of NbON oxynitride under high pressure were investigated through in situ synchrotron X-ray diffraction (SXRD) up to 43 GPa. It was found that the bulk modulus of baddeleyite NbON (290 GPa) is larger than that of ZrO2 (150 GPa), indicating that the introduction of highly covalent nitrogen imparts greater stiffness. Furthermore, SXRD patterns reveal the emergence of a peak signaling a new crystalline phase above around 20 GPa. This is in contrast to TaON, where diffraction patterns only show an increase in background beyond 33 GPa. First-principle calculations suggest that the high-pressure phase adopts an orthorhombic cotunnite-type structure, distinguishing it from the oxide counterparts, wherein the ambient pressure phase transforms to a cotunnite structure via an orthorhombic-I structure.

DOI： 10.1093/chemle/upae113

Decompression-Induced In-Plane Lone Pair Electrons in BiSF Synthesized under High Pressure Reviewed

Synthesis and in-depth structure determination of a novel metastable high-pressure CrTe3 phase Reviewed Open Access

Voss, L; Gaida, NA; Hansen, AL; Etter, M; Wolff, N; Duppel, V; Lotnyk, A; Bensch, W; Ebert, H; Mankovsky, S; Polesya, S; Bhat, S; Farla, R; Hasegawa, M; Sasaki, T; Niwa, K; Kienle, L

JOURNAL OF APPLIED CRYSTALLOGRAPHY Vol. 57 ( Pt 3 ) page： 755 - 769 2024.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Applied Crystallography

This study reports the synthesis and crystal structure determination of a novel CrTe3 phase using various experimental and theoretical methods. The average stoichiometry and local phase separation of this quenched high-pressure phase were characterized by ex situ synchrotron powder X-ray diffraction and total scattering. Several structural models were obtained using simulated annealing, but all suffered from an imperfect Rietveld refinement, especially at higher diffraction angles. Finally, a novel stoichiometrically correct crystal structure model was proposed on the basis of electron diffraction data and refined against powder diffraction data using the Rietveld method. Scanning electron microscopy-energy-dispersive X-ray spectrometry (EDX) measurements verified the targeted 1:3 (Cr:Te) average stoichiometry for the starting compound and for the quenched high-pressure phase within experimental errors. Scanning transmission electron microscopy (STEM)-EDX was used to examine minute variations of the Cr-to-Te ratio at the nanoscale. Precession electron diffraction (PED) experiments were applied for the nanoscale structure analysis of the quenched high-pressure phase. The proposed monoclinic model from PED experiments provided an improved fit to the X-ray patterns, especially after introducing atomic anisotropic displacement parameters and partial occupancy of Cr atoms. Atomic resolution STEM and simulations were conducted to identify variations in the Cr-atom site-occupancy factor. No significant variations were observed experimentally for several zone axes. The magnetic properties of the novel CrTe3 phase were investigated through temperature- and field-dependent magnetization measurements. In order to understand these properties, auxiliary theoretical investigations have been performed by first-principles electronic structure calculations and Monte Carlo simulations. The obtained results allow the observed magnetization behavior to be interpreted as the consequence of competition between the applied magnetic field and the Cr-Cr exchange interactions, leading to a decrease of the magnetization towards T = 0K typical for antiferromagnetic systems, as well as a field-induced enhanced magnetization around the critical temperature due to the high magnetic susceptibility in this region.

DOI： 10.1107/S1600576724002711

Zhong, CC; Tassel, C; Kato, D; Ogawa, K; Niwa, K; Tomita, O; Ito, Y; Kawaguchi, S; Hasegawa, M; Abe, R; Kageyama, H

CHEMISTRY OF MATERIALS Vol. 36 ( 9 ) page： 4495 - 4501 2024.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Chemistry of Materials

Stereochemically active lone pair electrons (LPEs) associated with p-block cations, such as Pb2+ and Bi3+, possess the capability of causing a reduction in the symmetry of crystal structures. However, LPEs leading to steric hindrance may impede structural stabilization; compounds with Bi3+, akin to those containing La3+ with comparable ionic radii, are not always encountered. In this study, we synthesized BiSF using high-pressure (5 GPa) synthesis. BiSF crystallizes in a PbFCl-type structure, exhibiting orthorhombic distortion due to stereochemically active LPEs oriented along the b axis. This contrasts sharply with other PbFCl-type tetragonal compounds (e.g., BiOF), which have van der Waals gaps formed by interlayer LPEs. First-principles calculations and in situ XRD indicate that BiSF is of ideal tetragonal symmetry under high pressure, featuring inactive LPEs. Upon depressurization, a notable expansion of the soft S2- anions occurs, enhancing hybridization between the Bi-6s/6p and S-3s/3p orbitals. This activation of the stereochemistry of LPEs results in a symmetry reduction. BiSF exhibits anisotropic thermal expansion, which is attributed to the contraction of the “LPE-mediated” Bi-F bonds. Our findings illustrate that the direction and shape of LPEs can be tailored during the postsynthesis decompression, underlining high-pressure synthesis as an effective approach for obtaining compounds with stereochemically active LPEs at ambient pressure.

DOI： 10.1021/acs.chemmater.4c00114

High-pressure synthesis and low-temperature thermal expansion behavior of homogeneous phosphorus-rich pseudo-binary compound (V0.5Cr0.5)P4 Reviewed

High-pressure synthesis of a novel molybdenum–nitrogen compound and its anisotropic axial compression by nitrogen dimers Reviewed

Takuya SASAKI, Shuto ASANO, Ken NIWA, Masashi HASEGAWA

The Review of High Pressure Science and Technology Vol. 33 ( 4 ) 2024

Chang, CC; Sasaki, T; Gaida, NA; Niwa, K; Hasegawa, M

CERAMICS INTERNATIONAL Vol. 49 ( 24 ) page： 40990 - 40994 2023.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Ceramics International

A black phosphorus layered homogeneous phosphorus-rich compound – (V0.5Cr0.5)P4 is synthesized using a large volume press under conditions of P = 5 GPa, 1100 °C, and 1 h of heating. (V0.5Cr0.5)P4 has a monoclinic (C2/c) structure with lattice parameters and cell volume of a = 5.2307(3)Å, b = 10.861(4)Å, c = 5.8253(4)Å, β = 110.784°, and V = 309.41(4)Å3, respectively. The thermal expansion behavior above about 110K has been investigated by low-temperature synchrotron XRD. The coefficient of thermal expansion, α, was calculated to be αa = 0.57 × 10−5K−1, αb = 0.70 × 10−5K−1, αc = 0.92 × 10−5K−1, αβ = 0.13 × 10−5K−1 and αv = 2.10 × 10−5K−1 at 293K. This thermal expansion behavior presents an anisotropic expansion characteristic different from pure black phosphorus due to the V/Cr atom that bonded the black phosphorus layers together.

DOI： 10.1016/j.ceramint.2023.09.176

High-Pressure Synthesis of a High-Pressure Phase of MnN Having NiAs-Type Structure. Reviewed

Asano S, Niwa K, Lawler KV, Kawaguchi-Imada S, Sasaki T, Hasegawa M

Inorganic chemistry Vol. 62 ( 49 ) page： 20271 - 20278 2023.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Inorganic Chemistry

A novel high-pressure phase of manganese mononitride, NiAs-type MnN, was successfully synthesized through a pressure-induced phase transition from a tetragonal distorted NaCl-type MnN at pressures above approximately 55 GPa. High-pressure experiments, including starting material preparation, were conducted using a laser-heated diamond anvil cell. This result is the first example of a nitride with a structural phase transition from the distorted NaCl-type to the NiAs-type structure. Upon decompression after the phase transition to NiAs-type structure, the NiAs-type MnN underwent a structural change to the distorted NaCl-type phase, indicating the phase transition was reversible. NiAs-type MnN has a higher density and bulk modulus in comparison to the distorted NaCl-type one. The phase transition pressure of MnN is lower than that of oxides, such as FeO and MnO, which show a structural phase transition from a NaCl-type to a NiAs-type structure. It is suggested that this is due to the lattice distortion caused by antiferromagnetic ordering.

DOI： 10.1021/acs.inorgchem.3c03241

Thermal expansion behavior of vanadium pernitride, CuAl2-type VN2, synthesized under high pressures Reviewed

PubMed

Asano, S; Niwa, K; Sasaki, T; Hasegawa, M

PROGRESS IN SOLID STATE CHEMISTRY Vol. 72 page： 100426 - 100426 2023.12

Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Solid State Chemistry

CuAl2-type VN2, which is synthesized under high pressure, is a recoverable material at ambient conditions and has a high bulk modulus. In this study, we investigated the thermal expansion behavior of CuAl2-type VN2 by low-temperature X-ray diffraction measurements between 109.3(5) K and 298.3(8) K. The axial thermal expansion coefficient of VN2 was determined to be αa = 2.7(9) × 10−6 K−1 and αc = 17.8(12) × 10−6 K−1 at 298.3(8) K, which has large anisotropy similar to that of compression behavior. It is found that the small coefficient of thermal expansion of the a-axis is due to the negative and positive effects on the a-axis length with increasing temperature of the bond angles and bond lengths of VN2, respectively. As a result, VN2 exhibits very large anisotropic thermal expansion behavior.

DOI： 10.1016/j.progsolidstchem.2023.100426

Synthesis of garnet-type Mn4+-activated Y3(Al,Ga)5O12 and (Y,Gd)3Al5O12 phosphors with controlled emission by cation substitution Reviewed

新規な14族元素窒化物の超高圧高温合成と結晶化学

丹羽健, 佐々木拓也, 長谷川正

日本結晶学会誌 Vol. 65 ( Supplement ) page： s31 - s31 2023.10

Sasaki Takuya, Asaka Ryota, Niwa Ken, Gaida Nico Alexander, Hasegawa Masashi

Journal of the Ceramic Society of Japan Vol. 131 ( 10 ) page： 696 - 700 2023.10

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

Ga- and Gd-substituted garnet-type Y3Al5O12:Mn4+ phosphors, Y3(Al1−xGax)5O12:Mn4+ and (Y1−yGdy)3Al5O12:Mn4+, were synthesized by a conventional solid-state reaction to elucidate the relationship between the emission and the local structure of the activator in the host material. Completely substituted Y3(Al1−xGax)5O12 and ∼80 % substituted (Y1−yGdy)3Al5O12 (y ≤ ∼0.8) solid solutions have been produced. The lattice parameters of the garnet-type solid solutions increased linearly with the substitution of Ga and Gd for Al and Y sites, respectively. The emission peak attributed to the Mn4+ luminescence center of their garnet-type solid solutions shifted toward a longer wavelength side by substituting Ga and Gd. The emission peak wavelength of the Mn4+-activated garnet-type solid solutions increased almost linearly with increasing the average bond length between the cation and anion in the dodecahedral sites. The dodecahedral site in the garnet structure is edge-sharing with the octahedral site where Mn4+ exists. These results indicate that the emission wavelength can be controlled by changing the local structure around Mn4+ due to the cation substitution of coordination polyhedra neighboring Mn4+ coordination polyhedra.

DOI： 10.2109/jcersj2.23063

Low-temperature thermal expansion behavior of the entropy-stabilized oxide (MgCoNiCuZn)O Reviewed

Chang C.C., Sasaki T., Alexander Gaida N., Niwa K., Hasegawa M.

Materials Letters Vol. 347 2023.9

Language：English Publishing type：Research paper (scientific journal) Publisher：Materials Letters

The coefficient of thermal expansion (CTE) of entropy-stabilized oxide (MgCoNiCuZn)O has been firstly experimentally measured and studied at low temperatures below room temperature. The temperature-dependent lattice parameters of (MgCoNiCuZn)O were determined by synchrotron X-ray diffractometry. A CTE value of α = 2.2780x10-5 K−1 at 113 K for (MgCoNiCuZn)O was calculated using a second-degree polynomial fitting. The quenched homogeneous NaCl-type (MgCoNiCuZn)O structure has the highest CTE value below 133 K among all its constituent oxides. An unquenched (MgCoNiCuZn)O was also investigated, and the homogeneity was found to affect CTE values. Finally, the anisotropic behavior of the constituent oxides and (MgCoNiCuZn)O were also compared.

DOI： 10.1016/j.matlet.2023.134601

High Temperature and Pressure Synthesis, Compression Behaviors and Thermal Expansion of Novel Transition Metal Nitrides Reviewed

Hasegawa Masashi, Asano Shuto, Chang Chung-Ching, Sasaki Takuyta, Niwa Ken

Netsu Sokutei Vol. 50 ( 1 ) page： 13 - 19 2023.1

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Calorimetry and Thermal Analysis

Pressure is one of the thermodynamic parameters together with the temperature to control the stability of materials. Ultra-high pressure in more than GPa regions allows us to obtain novel transition metal nitrides. This review shows a synthesis system under ultra-high pressures and high temperatures called laser-heating diamond anvil cell system in the first part. Then, our recent research results of the first synthesis of a novel niobium nitride, U2S3-type Nb2N3 are described. In the second part, compression and thermal expansion behaviors of the U2S3-type Nb2N3 are shown and compared with each other. Besides, the high pressure in-situ XRD and low temperature in-situ XRD measurement systems in the Aichi Synchrotron Radiation Center (Aichi SR) are also described briefly.

DOI： 10.11311/jscta.50.1_13

High-pressure synthesis and crystal structures of molybdenum nitride Mo3N5 with anisotropic compressibility by a nitrogen dimer Reviewed

Sasaki, T; Yamamoto, T; Asano, S; Niwa, K; Hasegawa, M

DALTON TRANSACTIONS Vol. 52 ( 2 ) page： 469 - 475 2023.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Dalton Transactions

A novel high-pressure molybdenum nitride phase, Mo3N5, was synthesized at above 45 GPa via a nitridation reaction of molybdenum with nitrogen under high pressure using a laser-heated diamond anvil cell. Mo3N5, having an N-N dimer and 7-coordinated Mo sites, crystallizes in an orthorhombic structure with a space group of Cmcm (No. 63) without other prototype structures. The refined lattice parameters for Mo3N5 were a = 2.86201(2) Å, b = 7.07401(6) Å, and c = 14.59687(13) Å. The DFT enthalpy calculation suggested that Mo3N5 is a high-pressure stable phase, which is also consistent with an increasing coordination number compared to ambient- and low-pressure phases. The zero-pressure bulk modulus of Mo3N5 was determined to be K0 = 328(4) GPa with K′0 = 10.1(6) by the fitting for the compression curve, which is almost consistent with the theoretical E-V curve and elastic stiffness constants. The compressibility of Mo3N5 has axial anisotropy corresponding to the N-N dimer direction in the crystal structure.

DOI： 10.1039/d2dt03433f

High-Pressure Synthesis of a Novel Molybdenum–Nitrogen Compound and its Anisotropic Axial Compression by Nitrogen Dimers Open Access

PubMed

SASAKI Takuya, ASANO Shuto, NIWA Ken, HASEGAWA Masashi

The Review of High Pressure Science and Technology Vol. 33 ( 4 ) page： 254 - 260 2023

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of High Pressure Science and Technology

Pernitrides and polynitrides synthesized under high pressure, containing nitrogen dimers and molecular nitrogen, respectively, are a group of compounds quite different from conventional nitrides synthesized under ambient pressure. Transition-metal polynitrides composed of transition-metal and molecular nitrogen have been synthesized above about 100 GPa, and various types of molecular nitrogen have been discovered. Pernitrides containing nitrogen dimers have anisotropic axial compressibility in the bonding direction of the dimers due to the low compressibility of the dimers. In this article, we review recent progress in molecular nitrogen-containing metal compounds and the anisotropic compression behavior of a novel molybdenum–nitrogen compound, Mo3N5, containing nitrogen dimers.

DOI： 10.4131/jshpreview.33.254

Open Access

Ultra-High-Pressure Synthesis of Transition Metal – Nitrogen Binary Compounds Open Access

NIWA Ken, ASANO Shuto, CHANG Chung-Ching, SASAKI Takuya, HASEGAWA Masashi

The Review of High Pressure Science and Technology Vol. 33 ( 1 ) page： 26 - 33 2023

Authorship：Lead author,　Corresponding author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of High Pressure Science and Technology

The recent development of high-pressure experimental techniques combined with advanced characterization and analytical methods allows us to reveal the crystal chemistry of the transition metal-nitrogen binary compounds up to the megabar pressure regime. Novel early transition metal-nitrogen compounds, VN2, Nb2N3, and MoC-type WN0.6, have been synthesized by using a laser-heated diamond-anvil cell or DIA-type high-pressure apparatus. In addition to conventional high-pressure in-situ X-ray diffraction measurements, the low-temperature thermal expansion behavior and the valance state of transition metal ions of these newly synthesized compounds were revealed. In this article, the details of our experiments, and recent progress in high-pressure synthesis and crystal chemistry of transition metal nitrogen binary compounds are reviewed.

DOI： 10.4131/jshpreview.33.26

Open Access

High-pressure polycrystalline thin-film synthesis and semiconducting property of platinum pernitride Reviewed Open Access

Ken Niwa, Tomoki Iizuka, Masashi Kurosawa, Yuto Nakamura, Hubert Okadome Valencia, Hideo Kishida, Osamu Nakatsuka, Takuya Sasaki, Nico Alexander Gaida, Masashi Hasegawa

AIP Advances Vol. 12 ( 5 ) page： 055318 - 055318 2022.5

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

A polycrystalline platinum pernitride (PtN2) thin-film was successfully synthesized via nitridation of a platinum thin-film deposited on α-Al2O3 substrate at the pressure of ∼50 GPa by using the laser-heated diamond anvil cell. The current–voltage characteristic and optical reflectance of the synthesized PtN2 thin-film were measured under ambient conditions. Combined with first-principles calculations, these experimental results have revealed that PtN2 exhibits semiconducting property with a bandgap of ∼2 eV. This high-pressure thin-film synthesis technique could also be applied for revealing the physical properties of other novel pernitrides synthesized under ultra-high pressure, which can offer new insights into the physical properties and functionality of the pernitrides and related nitrides.

DOI： 10.1063/5.0090089

Open Access