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The stability of a zirconium (Zr)-substituted face-centered cubic (FCC) yttrium (Y) hydride (Y1-xZrx hydride) phase was investigated experimentally and theoretically. Two possible sites for hydrogen atoms exist in the FCC structure, namely, T- and O-sites, where hydrogen is present at the center of the tetrahedron and the octahedron composed of Y and/or Zr metals. The P-C isotherms revealed that the hydrogen content per metal (H/M) with 33% Zr-substituted YH3-δ was 2.2-2.3, which was lower than the expected value calculated from the starting composition of YH3-33% ZrH2 (Y0.67Zr0.33H2.67, H/M = 2.67). Hydrogen at the O-site in Y1-xZrx hydride mainly reacted during hydrogen desorption/absorption. On the basis of theoretical analyses, the hydrogen atoms do not occupy the center of the octahedron, when at least two of the six vertices of the octahedron were composed of Zr. The O-sites, where more than two Zr atoms coordinate, nonlinearly increased with the Zr content, and when the Zr content was >50%, almost no hydrogen atoms occupy the O-sites. The theoretical discussion supported the experimental results, and the Zr substitution was confirmed to reduce the occupancy of H at the O-site in the FCC YH3 significantly.

Sodium titanium oxide with a spinel-type structure is suitable for the stable sodium-intercalation host for the negative electrode of sodium-ion batteries, such as the spinel-type lithium titanium oxide (Li4Ti5O12, LTO) material for lithium-ion batteries. Recently, this has been partly discovered as the Na3LiTi5O12 (NTO) phase in the LTO particle. However, the single-phase NTO material has never been obtained, preventing accurate material characterizations and applications. Here, we successfully realized the NTO material with the single-phase by the chemical sodium insertion-extraction process. The chemical sodium-inserted LTO material is well converted to the pure NTO phase in the single particle level, via following chemical oxidation by water. The purified material was about 97 mol % of NTO as the single-phase spinel structure with a = 8.746 A. The basic lattice framework of the prepared NTO was confirmed to be the same as that of the LTO. The single-phase NTO electrode shows 0.8 V versus Na+/Na of the Na-insertion and extraction potential, and 99.4% of Na-insertion capacity with 99.7% of Coulombic efficiency during 200 cycles of the Na-ion half-cell experiment. Further, the Na2Fe2(SO4)3/NTO full-cell shows 3 V-class stable charge-discharge character during 100 cycles. This excellent stability of Na-insertion and extraction properties of single-phase NTO extends the range of constructing safe and stable high-voltage oxide-based sodium-ion battery cells for practical use.

A face-centered-cubic (FCC) YH3 phase is known to be stable only under high pressure (HP) of more than gigapascal order, and it reverts to the hexagonal YH3 ambient-pressure phase when the pressure is released. We previously found that the FCC YH3 can be stabilized even at ambient pressure by substituting Y for 10 mol % Li (LiH-stabilized YH3, LSY). The LSY was synthesized by heat treatment under gigapascal HP, but this process is unfavorable for mass production; that is, only a few tens of milligrams of a sample can be obtained in a single batch. In this study, we overcame this problem by applying a ball milling (BM) process for synthesizing the LSY phase, and the yield by the BM process reached on the order of grams. We confirmed that the structure of the BM sample was the same as that of the HP sample by X-ray diffractometry, Raman spectroscopy, and neutron total scattering pair distribution function analyses. The crystallinity of the BM sample, however, was lower than that of the HP sample. The difference in the crystallinity affects the thermal stability of the LSY. The BM sample with a lower crystallinity released hydrogen at a lower temperature. The BM sample was found to reversibly desorb/absorb hydrogen maintaining its initial FCC structure when the rehydrogenation temperature was at 423 K. However, when the rehydrogenation temperature of BM sample was more than 573 K, the FCC structure changed to the hexagonal ambient pressure phase due to thermal instability of FCC phase for the BM sample.

Spinel-type Li4Ti5O12 (LTO) is expected as a safe and stable negative electrode material of sodium-ion batteries (SIBs). However, its small lattice volume is not sufficient as the Na+ intercalation...

Li2MnO3, a layered lithium-rich manganese oxide, is one of the potential high-capacity positive electrode materials among the Li containing transition metal oxide materials but its low electrical conductivity restricts its electrochemical activity. In our previous study, Li–Mn cation disorder in Li2MnO3 resulting in the formation of a NaCl-type structure was found to improve its electrochemical activity, i.e., a high initial discharge capacity of 320 mA h g−1. However, it still suffered from a poor cycling performance probably due to another problem, e.g., its structural instability, i.e., oxygen emission during charging. In this study, we found that the structural stability of NaCl-type Li2MnO3 was significantly improved by forming a composite with spinel-type LiMn2O4 by mechanical milling. The initial discharge capacity of the composite was close to 400 mA h g−1 and its 15th discharge capacity still showed more than 80% of the initial one, while less than 50% for NaCl-type Li2MnO3. The oxide ions in the composite were found to be more stable than those of NaCl-type Li2MnO3 during the electrochemical redox reaction, which was confirmed by O K-edge XAS measurements, resulting in the higher reversible capacity and better cycling performance.

The reaction mechanism of a spinel-type sodium titanium oxide (Na3LiTi5O12) was investigated using in situ XRD during the Na insertion/extraction process. The XRD profiles gradually shifted to lower and higher angles during the sodiation and de-sodiation processes, respectively, while maintaining a spinel-type structure and no new peaks were observed, indicating a solid-solution reaction according to the following reaction; (Na) 3 8 a (Li1Ti5)16dO 12 192 i + 3Na+ + 3e− ↔ (Na) 6 16 c (Li1Ti5)16dO 12 192 i . The evolution of the XRD profile was confirmed to be highly reversible, which explains the high cycling stability of the NTO electrode. There are two possible sites for Na to occupy: 8a and 16c. The Na ion initially occupies only the 8a site, as confirmed by the 220 peak. The intensity of the 220 peak gradually decreased with increasing SOC and was hardly detected at an SOC of 50%, indicating that all the Na ions at the 8a sites moved to the 16c sites. Topological analysis combined with XRD revealed that the Na ion at the 8a site is stable with one other Na ion at an adjacent 16c site despite the interatomic distance being too short.

Heavy rare-earth trihydrides such as GdH3, DyH3, and yttrium trihydide (YH3) usually show a hexagonal crystal structure under ambient pressure. This structure is known to transform to a face-centered cubic (FCC) one at higher pressures (at the order of GPa), and the FCC one returns to the hexagonal structure when the applied pressure is released. In this study, we investigated the structure of alkaline or alkaline-earth (A)-substituted REH3 (RE = Y, Gd, Dy; A = Li, K, or Mg) using X-ray diffraction, and measured the phase transition pressure. We found that this FCC high-pressure phase can be stabilized by 10-33 mol % A substitution for RE in the REH3. The mechanism of phase stabilization is simply explained by the ionic radius ratio between the cation and anion ( rcat/ rani), as well as the stabilities of other ionic crystals such as perovskite materials. For all considered REH3 samples, the FCC phase becomes stable when rcat/ rani > 0.856, such as in the case of 10 mol % Li-substituted YH3.

Silver titanium oxides in the Ag–Ti–O ternary system (ATO) have been reported as monoclinic phase structures, but no other types of structures have been reported to date. In this study, we report cubic-type silver titanium oxide with a spinel-based structure. Cubic-type ATO was successfully synthesized via the Ag+ ion-exchange reaction of spinel-type sodium titanium oxide (NTO). Ag+ preferentially substituted Na+ in the spinel 8a-site (oxygen tetrahedral site) of the NTO material by the AgNO3 aqueous solution process. Further, ion- exchange thoroughly proceeded via the thermal process of NTO by AgNO3 in the KNO3 melt. The X-ray diffraction pattern of the obtained powder clearly showed cubic-phase diffraction peaks (a ​= ​8.84 ​A) corresponding to the spinel-based structure with Ag+ occupying the 8a-site. Elemental analysis confirmed that pure ATO material could be synthesized with Ag2.86Ti5O11.43 formula.

A surface-roughened clad (S-clad) current collector significantly extends the cycle-life of the lithium ion negative electrode composed of a silicon micropowder and an aqueous binder. The high tensile strength of the S-clad is also proved to be important for improving the battery performance of the electrode by comparison to a surface-roughened pure Cu current collector. Moreover, adding 10 vol.% fluoroethylene carbonate to the electrolyte further extends the cycle-life of the Si electrode. The synergic effect of the high adhesive and tensile strength of the S-clad current collector as well as the electrolyte additive results in maintaining the reversible capacity of 1000 mA h g−1 for more than 1000 cycles, in which 1.0–1.2 mg cm−2 of the active material is loaded on the electrode.

For the design of high-capacity and Co-free positive electrode material, stoichiometric LiNi1/2Mn1/2O2 and Li-excess Li1+x(NiyMn1-y)1-xO2 (0 3 ¯ m), but the sample has a biphasic composite of main Li-poor Li1-xTM1+xO2 (TM, transition metal) and minor Li-rich Li1+xTM1-xO2 phases after calcination below 950 °C. The fraction of the minor Li-rich phase decreased concomitantly with increasing calcination temperature. Only at 1000 °C was single-phase LiNi1/2Mn1/2O2 obtainable. The biphasic samples exhibited better electrochemical performance than that for the single-phase sample. Based on this result, two Li-excess Li1+x(NiyMn1-y)1-xO2 (0

The crystal structure of yttrium trihydride (YH 3 ) is usually characterized by hexagonal close-packed (HCP) metal ordering at ambient conditions and transforms into an FCC structure (FCC-YH 3 − δ ) at pressures greater than 8 GPa. FCC-YH 3 − δ was previously considered to only be stable under high pressures, but we found that it can be stabilized even at ambient pressures by the mechanical milling of hexagonal YH 3 (HCP-YH 3 ) in this study. The lattice constant of the FCC-“YH 3 − δ ” sample was evaluated to be a = 0.52801(2) nm, which corresponds to the expected value calculated in other studies. This structural change from HCP to FCC enlarged band gap of the YH 3 from 2.2 eV to 2.45 eV measured by UV–vis spectrometry. FCC-YH 3 was dehydrogenated to an FCC “YH 2 ” dihydride phase at 520 K, which is known to be conventionally present at ambient pressures. However, the YH 2 generated from FCC-YH 3 − δ maintained its FCC structure upon re-hydrogenation to trihydride, unlike how YH 2 usually changes its structure from FCC to HCP. The volume change after the dehydrogenation of FCC-YH 3 − δ was approximately 5%, which is smaller than that of HCP-YH 3 (approximately 10%).

Spinel-type sodium titanium oxide (Na3LiTi5O12, NTO), which has an analogous structure to Li4Ti5O12 (LTO), is prepared as a single-phase material for the negative electrode of Na-ion batteries. A superior Na insertion and extraction cycle performance is achievable based on its similarity with the LTO reaction mechanism. However, the detailed structure of the NTO material in the Na insertion state remains obscure. Consequently, the crystallographic features of the Na insertion mechanism have not been sufficiently elucidated. In this study, the structural analyses of NTO and Na-inserted NTO (Na-NTO) electrodes were performed via X-ray diffraction and Rietveld refinement. The Na occupation site of the NTO spinel lattice was altered from the oxygen tetrahedral to the octahedral site by the electrochemical Na insertion reaction. The lattice constants of NTO and Na-NTO were refined for aNTO = 8.73 A and aNa-NTO = 8.84 A; approximately 1% lattice expansion, which was caused by the local compressions of the Ti–O atomic distance existing in the NTO and their relaxation in the Na-NTO, was confirmed by Na insertion. Notably, Ti–O atomic distances in NTO are quite heterogeneous with values of 1.8 A and 2.2 A, while they are homogenized to 2.0 A, 2.1 A, and 2.2 A in the Na-NTO lattice. Furthermore, the oxygen position in the NTO lattice is modified to enhance lattice symmetry by Na insertion, similar to the Li insertion reaction of the LTO lattice. Thus, the Na-NTO lattice structure is considerably more stable than the NTO lattice, which promises stable Na insertion and extraction cycle performances for NTO materials toward Na-ion battery utilization.

Yttrium in YH3 was shown to be substituted by zirconium in a ball milling process, yielding FCC-YH3, a zirconium-hydride-stabilized YH3 (ZSY) structure. FCC-YH3, which is stable only under the pressure greater than 7.8 GPa, has previously been reported to be stabilized at ambient pressure by alkaline or alkaline earth metal substitution, but only in a limited compositional range. In contrast, we demonstrated solid solubility of Y and Zr in ZSY in a wide compositional range, with a Y/Zr molar ratio between 4 and 0.333, as confirmed by changes in the lattice constants of ZSY with various amounts of Zr. A color of ZSY changed from yellowish (transparent) to black by Zr substitution. The UV–vis spectroscopy revealed that Zr substitution narrowed the bandgap of YH3. At a molar fraction of 33 mol%, Zr-substituted ZSY was shown to maintain its FCC structure during dehydrogenation and rehydrogenation cyclirng, and its volume change ( Δ V ) was estimated to be 1%, with an accompanying H2 emission of 0.5 wt%. This volume change was significantly lower than that of conventional hydrogen storage alloys such as LaNi5 and Mg–La–Ni alloys, i.e., Δ V is about 10 ∼ 20% when they desorb same amount of hydrogen.

A potential positive electrode material, α-NaFeO2, can be viewed one of the base material for rechargeable Na batteries. In this study, we found that its structural evolution during charge process differentiates depending on its synthesis route. The α-NaFeO2 is prepared by three methods: one is by the hydrothermal technique (HT), and the other two by solid-state reactions, SSn and SSμ, in which nano-sized Fe3O4 and micro-sized Fe3O4 are used as the Fe source, respectively. The synchrotron radiation X-ray diffraction profiles of the initial state suggest that all three samples are apparently identified as the target product, α-NaFeO2, with rhombohedral structures. Though all three samples remain stable for more than 50 cycles under the cycling conditions of a constant amount of charging (Na extraction) to 70 mA h g−1, their structural evolution differs during the sodium extraction. The SSμ transitions from the rhombohedral structure to a spinel-like cubic structure, while the HT transforms into a monoclinic structure. On the other hand, both the monoclinic and cubic phases co-exist in SSn. What produces these differences among the seemingly identical materials is discussed.

Spinel lithium titanate (LTO; Li4Ti5O12) is one of the promising materials for negative electrodes of sodium-ion batteries (SIBs). The stable charge–discharge performance of SIB cells using LTO electrodes depends on the reversible Na insertion–extraction mechanism of LTO, where the spinel lattice is expanded with Na insertion, and two phases, Na-inserted LTO (Na-LTO) and Li-inserted LTO (Li-LTO) phases, are generated. These phases are confirmed using X-ray diffraction (XRD), while the mechanism of the two-phase coexistence with different lattice volumes is yet unclear. Here, we investigate the detailed morphology of the coexisting Na-LTO and Li-LTO phases using in situ XRD measurements and high-resolution transmission electron microscopy (TEM) observation. Na-LTO (a = 8.74 A) and Li-LTO (a = 8.36 A) phases are confirmed in both the electrochemically formed Na-inserted LTO electrode and the single-crystalline LTO thin specimen. We observed that the Na-LTO/Li-LTO interface is parallel to the (001) plane, and contains an inevitable lattice mismatch along the interface, while the expansion of the Na-LTO phase can be partially relaxed normal to the interface. We observed that the Na-LTO/Li-LTO interface has interface layers of lattice disordering with a 1–2 nm width, relaxing the lattice mismatch, as opposed to results from the previous scanning TEM observation. How the different lattice volumes at the two-phase interface are relaxed should be the key issue in investigation of the mechanism of Na insertion and extraction in LTO electrodes.

High-pressure synthesis has been widely used for exploration of novel materials. In this study, Li-Y hydrides were explored under GPaorder pressure by using a cubic-anvil-type apparatus. Unfortunately, a novel hydride in Li-Y system was not synthesizedin this study, but it is found that 10 at% addition of Li can stabilize FCC-YH3 high pressure phase even under ambient pressure. The Li added FCC-YH3 have a CeH3type structure judging from the results of XRD and raman spectrometry and its lattice constant was estimated to be 0.52666(1) nm. Total hydrogen content of the hydride was estimated to be 3.52 mass%. This hydride was partially dehydrogenated at 575 K with decreasing its lattice constant down to 0.5206(1) nm. Then, the sample could absorb hydrogen again under 5 MPa-H2 at 623 K. Hydrogenation enthalpy of the hydride

MgCu, 14) and Mg50Cu21 14) by using this method. In our previous study, MgNi2 (C36) was hydrogenated under a hydrogen pressure of the order of gigapascals rather than ambient hydrogen pressure. 15) The hydrogen content of a novel MgNi2 hydride was estimated to be 2.23 mass% and their dehydrogenation temperature was approximately 460 K, which is approximately 100 K lower than those of MgH2 and Mg2NiH4. Moreover, Mg(Ni1� xCux)2 (x ¼ 0{0:2) was found to be hydrogenated, 14) but the optimum condition of hydrogenation of Mg(Ni1� xCux)2 was not investigated. Therefore, in this study, the optimum condition for hydrogenating Mg(Ni1� xCux)2 (x ¼ 0:0{0:3) intermetallics was investigated. The effects of Cu substitution on their crystal structures, thermal stabilities and hydrogen contents of Mg(Ni1� xCux)2 (x ¼ 0:0{0:3) hydrides were also investigated. 2. Experimental Procedures

High-pressure works are attractive techniques to obtain new compounds, such as alkali or alkaline earth metal-based systems. The atomic radius of Mg under GPa pressure is considerably smaller compared with transition metals; as such, it may be preferable to synthesize novel intermetallic compounds and hydrides by using high-pressure techniques. In this study, novel compounds were synthesized in an Mg-Ni system by a high-pressure technique using a cubic-anvil-type apparatus. A novel Mg 6 Ni intermetallic compound was obtained by exposing a mixture of Mg and Ni to 6 GPa at 900 °C for 2h. The crystal structure of the compound is a tetragonal F-43m structure with a lattice parameter of a = 1.9987(1) nm. This compound decomposed to Mg and Mg 2 Ni phases at 278 °C with exothermic reaction. As is well known, MgNi 2 does not form hydrides under conventional hydrogenation conditions, hence we investigated the reactivity of MgNi 2 with highly pressurized hydrogen. It was found that the MgNi 2 was able to form MgNi 2 H 3.2 2 by treatment at 700 °C for 2 h under 5 GPa with a hydrogen source, leading to a hydrogen capacity of 2.23 mass%. This novel hydride was found to be a tetragonal MoSi 2 -type structure (14/mmm) with lattice parameters of a = 0.327(3) nm and c = 0.878(9) nm. The dehydrogenation of this hydride occurred at 187 °C with endothermic reaction, and caused decomposition into C36-type MgNi 2 . This hydride had solubility of Ni content and its thermal stability decreased with increasing Ni content.

A novel intermetallic compound of MgNi was synthesized from amorphous MgNi by high pressure technique. Crystal structure, thermal stability and hydrogenation property of the compound were studied. The crystallized MgNi was synthesized at 573 K for 2 h under the pressure of more than 5 GPa, and was found to exhibit a CuTi-type structure with lattice parameters of a=0.2997(1) nm, c=0.3166(2) nm. The MgNi was decomposed into Mg2Ni and amorphous phase at 575 K and decomposed into Mg2Ni and MgNi2 over 682 K. Moreover the MgNi was hydrogenated into novel hydride with a CsCl-type structure at 355 K under 2 MPa hydrogen. This novel hydride with a lattice parameter of a=0.3215(1) nm was decomposed into Mg2Ni and MgNi2 over 428 K under an Ar atmosphere.

Novel hydrides were synthesized in Mg–Ni–H and Mg–Ni–Cu–H systems by high-pressure technique using a cubic-anvil-type apparatus. Crystal structure and thermal stability of the hydrides were studied. The novel hydride with a composition of MgNi 2 H y was synthesized at 973 K for 2 h under the pressure of over 2 GPa. This hydride could be synthesized from each starting material, such as MgNi 2 (C36) and mixture of MgH 2 – x mol% Ni ( x = 65–70). From fusion analysis, hydrogen content of the sample of MgH 2 –67 at.% Ni prepared with hydrogen source at 973 K for 2 h under 5 GPa was estimated to be 2.23 mass% and chemical formula was corresponding to be MgNi 2 H 3.2 . The novel hydride was found to exhibit the body-centered tetragonal structure (space group I 4/ mmm , No. 139) with lattice parameters of a = 0.327(3) nm, c = 0.878(9) nm. Moreover, crystal structure of novel hydride was distorted with varying Ni content, then it was found to exhibit C-faced based-centered orthorhombic structure (space group Pmmm , No. 47) with lattice parameters of a = 0.460(1) nm, b = 0.468(1) nm, c = 0.893(1) nm. In Mg–Ni–Cu–H system, crystal structure and thermal stability of Mg(Ni 1− x Cu x ) 2 H y ( x = 0.05, 0.10, 0.15, 0.20) were investigated. The sample of Mg(Ni 0.95 Cu 0.05 ) 2 prepared with hydrogen source at 973 K for 8 h under 5 GPa was dehydrogenated at 446 K, which is 15 K lower than that of MgNi 2 H 3.2 . Moreover, Mg(Ni 0.95 Cu 0.05 ) 2 prepared with hydrogen source at 973 K for 2 h under 5 GPa,which did not react hydrogen sufficiently, was decomposed at 409 K.

High-pressure synthesis has been widely used for exploration of novel materials. In this study, new Mg-based ternary hydrides in Mg–TM–H systems (TM = Zr, Nb, Hf and Ta) have been prepared under a pressure of 5 GPa by using a cubic-anvil-type apparatus. In TM = Zr, Nb and Hf, MgTM2Hy with a monoclinic-type structure were synthesized at 923 K under 5 GPa. Lattice parameters of MgTM2Hy were a = 5.816 A, b = 3.3569 A, c = 8.519 A, β = 103.06° (for TM = Zr); a = 5.70 A, b = 3.2888 A, c = 7.927 A, β = 103.06° (for TM = Nb); a = 5.782 A, b = 3.317 A, c = 8.507 A, β = 103.06° (for TM = Hf). Results from fusion extraction analysis for MgTM2Hy (TM = Zr, Nb and Hf) showed their hydrogen contents, giving chemical formula of MgTM2H5.9. In TM = Nb and Ta, ternary hydrides with an FCC-type structure were synthesized under 5 GPa. These FCC hydrides had lattice parameters of a = 9.5526 A (TM = Nb) and 9.5376 A (TM = Ta). For TM = Nb, hydrogen content was determined from the fusion extraction analysis to be 5.8 mass%, corresponding to Mg6.2NbH14. Among the newly found hydrides, Mg6.2NbH14 showed the lowest dehydrogenation temperature of 595 K with endothermic reaction. Crystal structures and thermal stabilities of these hydrides were also investigated.

Hydrogen storage materials are attracting much attention as media of storing hydrogen. High-pressure synthesis has been widely used for exploration of novel materials. We have reported that many new Mg-based hydrides or alloys have been synthesized by anvil-type apparatus under the pressure of GPa-order. In Mg - TM ( TM = Nb, Ta ) - H systems, it was reported that novel FCC-type hydride which is similar in crystal structure and composition to Mg7TiH13-16 was synthesized under 8 GPa. On the other hand, there is few reports of novel Ca-based hydrides to be synthesized under high pressure. However, the compressibility of calcium is higher than that of magnesium. Thus, there is a tendency for Ca compounds to be synthesized by lower pressure than Mg ones. This study describes the synthesis of new Ca-based hydrides by this high-pressure techniques. In Ca - TM ( TM = Ti, Hf, V, Nb and Ta ) systems, the influence of applied pressure on present phases were investigated. For the composition of CaH2 - 14.3 mol%ZrH2 in Ca - Zr - H system, novel hydride was synthesized at 1073 K for 2 h under 5 GPa. Crystal structure of the novel hydride was found to be FCC-type with a lattice parameter of a = 0.531 nm. In addition, the thermal stability and hydrogen contents of this novel hydride were investigated. In Ca - Hf - H system, the unknown phase was observed in the sample of CaH2 - 12.5 mol%HfH2 prepared 1073 K for 2 h under 5 GPa. This unknown phase is FCC structure with lattice parameter of a = 0.528(2) nm.

A novel hydride was synthesized in Mg-Ni-H system by high pressure technique using a cubic-anvil-type apparatus. Crystal structure, thermal stability and hydrogen content of the hydride were studied. The novel hydride with a composition of MgNi2Hy was synthesized at 973 K for 2 h under the pressure of more than 2 GPa, and was found to exhibit the body-centered tetragonal structure (space group I4=mmm, No.139) with lattice parameters of a ¼ 0:327ð3Þ nm, c ¼ 0:878ð9Þ nm. Moreover, this hydride could be synthesized from each starting material, such as MgNi2 high pressure phase, MgNi2 (C36) and mixture of MgH2-x mol% Ni (x ¼ 65{70). Its hydrogen content was estimated to be 2.23 mass% from fusion analysis and corresponding chemical formula was found to be MgNi2H3:2. This hydride was decomposed into MgNi2 (C36) over 460 K, and has solubility of Ni content and thermal stability of the novel hydride was decreased with increasing Ni content. [doi:10.2320/matertrans.47.1957]

High-pressure synthesis has been widely used for exploration of novel materials. In this study, new Li-based hydrides in Li-Nb and Li-Ta systems have been successfully synthesized under GPa-order by using a cubic-anvil-type apparatus. And crystal structures, thermal stabilities of novel hydrides were investigated.For Li-Nb-H system, novel hydride with primitive hexagonal structure was synthesized at 973 K under 5 GPa. Its lattice parameter was a = 0.56250(5) nm, c = 0.57416(4) nm. A structural model of the hydride was CaCu5-type structure ( P 6/mmm, No. 191). For Li-Ta system, novel hydride with C-face centered monoclinic structure was obtained at 973 K under 5 GPa. Its lattice parameter was a = 0.7918(5) nm, b = 0.8012(4) nm, c = 1.030(7) nm, β =111.27 (6)˚. These novel hydrides in Li-Nb and Li-Ta systems were decomposed at 510 K and 573 K, respectively, and these values were about 400 K lower than that of LiH (>953 K).