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Neutron powder diffraction profiles were collected for iron deuteride (FeDx) while the temperature decreased from 1023 to 300 K for a pressure range of 4–6 gigapascal (GPa). The ε′ deuteride with a double hexagonal close-packed (dhcp) structure, which coexisted with other stable or metastable deutrides at each temperature and pressure condition, formed solid solutions with a composition of FeD0.68(1) at 673 K and 6.1 GPa and FeD0.74(1) at 603 K and 4.8 GPa. Upon stepwise cooling to 300 K, the D-content x increased to a stoichiometric value of 1.0 to form monodeuteride FeD1.0. In the dhcp FeD1.0 at 300 K and 4.2 GPa, dissolved D atoms fully occupied the octahedral interstitial sites, slightly displaced from the octahedral centers in the dhcp metal lattice, and the dhcp sequence of close-packed Fe planes contained hcp-stacking faults at 12%. Magnetic moments with 2.11 ± 0.06 μB/Fe-atom aligned ferromagnetically in parallel on the Fe planes.

We report on the formation of a new icosahedral quasicrystal (iQC) in the Au-Sn-Yb alloy system. This iQC has a primitive icosahedral lattice with a lattice constant aico of 0.5447(7) nm and a composition that was determined to be Au60.0Sn26.7Yb13.3. X-ray absorption spectroscopy measurement of the near Yb L3 edge demonstrates that the Yb valence in the iQC is an intermediate valence between divalent (4f14) and trivalent (4f13) at ambient pressure and was determined to be 2.18+. The results are compared to those for a corresponding 2/1 cubic approximant crystal. The formation of this new iQC is discussed in terms of the atomic size factor (δ) and the valence electron-to-atom ratio (e/a).

Perovskite-oxide Bi1–xPbxNiO3 for 0.60 ≤ x ≤ 0.80 was found to show a polar orthorhombic-to-nonpolar orthorhombic phase transition accompanied by negative thermal expansion. Bi1–xPbxNiO3 showed suc...

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.

The effect of interstitial atoms (nitrogen, carbon) on hydrogen storage properties in vanadium was investigated. When the N concentration was below 0.4 wt%, the plateau pressures increased with increasing N concentration during absorption and desorption and vanadium samples (body-centered cubic (BCC)) transformed to VH0.5 (body-centered tetragonal (BCT), c/a = 1.1) and then VH2 (face-centered cubic (FCC)). When the N concentration exceeded 0.6 wt%, a new single-phase region appeared in the pressure-composition isotherm, suggesting the formation of a new hydride phase. The X-ray diffraction data indicated that this new hydride phase was VH1.0 with a BCT structure and c/a = 1.24, and the phase transformation took place as V (BCC) became VH0.5 (BCT, c/a = 1.1), followed by VH1.0 (BCT, c/a = 1.24) and then VH2 (FCC). Density functional theory calculations indicated that the BCT structure model with hydrogen atoms fully occupying the octahedral sites (denoted as the Oz site) can explain the experimentally obtained crystal structure for VH1.0; they also indicated that the VH1.0 phase was stabilized by the addition of nitrogen. In addition, the N occupation site changed from the Oz site in VH0.5 and VH1.0 to the tetrahedral site (denoted as the T site) in VH2 in coordination with hydrogen during hydrogen absorption. A similar phenomenon was observed in carbon-containing vanadium. It can thus be concluded that the phase transformation pathway and stability of the hydride phases in the V–H system are highly sensitive to the addition of interstitial C and N atoms.

Ta2NiSe5 has recently attracted interest as an excitonic insulator. In this paper, crystallographic parameters of three non-ambient phases are reported from across the P–T phase diagram. A reversible pressure-induced structural transition above 3 GPa is associated with the coherent sliding of weakly coupled layers., The crystal structure of the excitonic insulator Ta2NiSe5 has been investigated under a range of pressures, as determined by the complementary analysis of both single-crystal and powder synchrotron X-ray diffraction measurements. The monoclinic ambient-pressure excitonic insulator phase II transforms upon warming or under a modest pressure to give the semiconducting C-centred orthorhombic phase I. At higher pressures (i.e. >3 GPa), transformation to the primitive orthorhombic semimetal phase III occurs. This transformation from phase I to phase III is a pressure-induced first-order phase transition, which takes place through coherent sliding between weakly coupled layers. This structural phase transition is significantly influenced by Coulombic interactions in the geometric arrangement between interlayer Se ions. Furthermore, upon cooling, phase III transforms into the monoclinic phase IV, which is analogous to the excitonic insulator phase II. Finally, the excitonic interactions appear to be retained despite the observed layer sliding transition.

The crystal structure changes in the hydrogenation and dehydrogenation process have been investigated by the in-situ synchrotron X-ray diffraction measurement to elucidate the mechanism of hysteresis phenomena in equilibrium hydrogen pressure. The hysteresis phenomena were observed on the crystal lattice. The lattice constants and volume of α-phase (hydrogen solid solution) and β-phase (hydride) in the dehydrogenation process with lower equilibrium pressure were smaller than those in the hydrogenation process with higher equilibrium pressure. This phenomenon is opposite to the general relationship between the lattice volume and the equilibrium pressure. The hysteresis phenomenon was also observed in the diffraction peak widths. The peak widths of β-phase in the dehydrogenation process were wider than those in the hydrogenation process. The hysteresis phenomena may be related to lattice defects and the hydrogen content in crystalline regions.

We have measured the volumes of dhcp- and fcc-FeH x at temperatures of 300–950 K and hydrogen pressures of 4–7 GPa using in situ X-ray diffraction. For fcc-FeH x , the expanded volumes per Fe atom with respect to fcc-Fe metal were converted to hydrogen compositions x using hydrogen-induced volume expansion v H . A miscibility gap in the x – T diagram of fcc-FeH x was confirmed to be in agreement with an earlier study, and the critical pressure was located in a rather narrow span of 4.0–4.5 GPa with a critical composition of x = ∼0.4. We examined Arrhenius plot analysis of the x – T relations to derive the enthalpy of solution of hydrogen ΔH s . However, an irregular behavior of the x – T relations did not allow derivation of a reliable value of ΔH s .

Hexagonal close-packed iron hydride, hcp FeHx, is absent from the conventional phase diagram of the Fe–H system, although hcp metallic Fe exists stably over extensive temperature (T) and pressure (P) conditions, including those corresponding to the Earth’s inner core. In situ X-ray and neutron diffraction measurements at temperatures ranging from 298 to 1073 K and H pressures ranging from 4 to 7 GPa revealed that the hcp hydride was formed for FeHx compositions when x 3/H-atom, which was larger than that of the face-centered cubic (fcc) hydride. The hcp hydride showed an increase in x with T, whereas the fcc hydride showed a corresponding decrease. The present study provides guidance for further investigations of the Fe–H system over an extensive x–T–P region.

Raman scattering, Mossbauer spectroscopy, and X-ray diffraction measurements have revealed that EuHx compressed in high-pressure H2 exhibits the crystal structure changes and the divalent (Eu2+) to trivalent (Eu3+) changes via two Eu2+/Eu3+ mixed-valence phases: EuHx-I (Pnma, x = 2, Eu2+) → EuHx-II (P63/mmc, x = 2, 7.0 GPa, Eu2+) → EuHx-III (I4/m, x ≃ 2.2, 7.0–9.0 GPa, Eu2+/Eu3+) → EuHx-V (x ≃ 2.8, 9.0–12.4 GPa, Eu2+/Eu3+) → EuHx-VI (x ≃ 3, 12.4–14.4 GPa, Eu3+) → EuHx-VII (x ≃ 3, 14.4–16 GPa, Eu3+) → EuHx-IV (I4/mmm, x ≃ 3, 16 GPa – , Eu3+). The EuHx-II transforms to EuHx-III at a constant pressure near 7.0 GPa due to the reaction with H2. Our results also demonstrate that Mossbauer spectroscopy can be a useful marker to detect the hydrogen storing in metal hydrides.

The volumetric hydrogen density of 160 kg H/m 3 in TiVCrNbH 8 is among the highest for interstitial hydrides, but the reported reversible capacity is only about 2/3 of the full theoretical capacity at room temperature. In the present work we have investigated the local structure in TiVCrNbD x , x = 0 , 2.2, 8 with the aim to unravel how the remaining sites can be destabilized with respect to hydrogen/deuterium occupation using total scattering measurements and Reverse Monte Carlo (RMC) structure modelling. Our analysis indicates that the partially desorbed deuteride ( x = 2.2 ) adopts a body-centred tetragonal structure ( I 4 / m m m ) where the deuterium atoms occupy both tetrahedral and octahedral interstices with low occupancies. There is a significantly higher portion of occupied sites with nearest-neighbour metals with low valence-electron concentration VEC. This observation is used to motivate strategies for further destabilization of the hydride. Inelastic neutron scattering (INS) and density functional theory (DFT) calculations indicate that the vibrational density of states is very diverse in TiVCrNbH 2.4 , and it is suggested that the hydrogen atoms might be mobile between nearby interstices.

The valence distribution and local structure of Bi1–xPbxNiO3 (x ≤ 0.25) were investigated by comprehensive studies of Rietveld analysis of synchrotron X-ray diffraction (SXRD) data, X-ray absorption spectroscopy (XAS), hard X-ray photoemission spectroscopy (HAXPES), and pair distribution function (PDF) analysis of synchrotron X-ray total scattering data. Disproportionation of Bi ions into Bi3+ and Bi5+ was observed for all the samples, but it was a long-ranged one with distinct crystallographic sites in the P1 triclinic structure for x ≤ 0.15, while the ordering was short-ranged for x = 0.20 and 0.25. An intermetallic charge transfer between Bi5+ and Ni2+, leading to large volume shrinkage, was observed for all the samples upon heating at ∼500 K.

A new PbTiO3-type compound, Bi2ZnVO6, with a giant tetragonal distortion of c/a = 1.26 (a = 3.7869(3) A, c = 4.7660(7) A) was synthesized under high pressure–high temperature conditions (9 GPa and 1373 K). A point charge model calculation based on the atomic positions refined by Rietveld analysis of synchrotron X-ray diffraction (SXRD) data gave an electrical ionic polarization of 126 μC/cm2, the largest value among PbTiO3-type perovskite compounds. The tetragonality (c/a) decreased with increasing temperature from 100 to 570 K without any trace of a phase transition. Instead, a pressure-induced transition from a polar tetragonal structure to a paraelectric GdFeO3 one accompanied by a 2.4% volume collapse was observed at 6.01 GPa. Bi2ZnVO6 showed paramagnetic behavior with S = 1/2 because of the random distribution of nonmagnetic Zn2+ and magnetic V4+ ions. Transport measurements indicated semiconductivity with an activation energy of 0.43 eV.

We investigated the hydrogenation of a ternary TiFe0.8Mn0.2 alloy under high pressure (up to 6.5 GPa) and at high temperature (up to 600 °C) by in situ synchrotron radiation X-ray diffraction measurements. A chemical order–disorder phase transition and the formation of body-centered cubic (BCC) hydride were observed. The phase-transition temperature was 400 °C, which is 200 °C lower than that of a binary TiFe alloy. A disproportionation reaction, which was observed for the binary TiFe alloy, did not occur at this temperature. The BCC hydride existed stably, and new hydrides were not formed after the BCC hydride became a single phase. Since the unit cell volume of the BCC hydride was almost equal to that of the γ hydride, TiFe0.8Mn0.2H1.8, under the same pressure–temperature conditions, the hydrogen content of the former was roughly estimated as the same as that of the latter (hydrogen to metal atom ratio = 0.9).

We investigated the hydrogenation of the L12-type AlNi3 alloy at 3 GPa and 500 °C using in situ synchrotron radiation X-ray diffraction measurements. An isotropic lattice expansion was observed to have occurred owing to the hydrogenation, and a novel hydride (AlNi3Hx) was formed. Because of such isotropic lattice expansion, the hydrogen-occupied sites were the body-center sites of the L12 structure (Pm-3m). In this case, the hydrogen content was calculated to be x = 1.0, which is consistent with the experimental results. The AlNi3Hx hydride was stable under ambient conditions.

Reduction in reversible hydrogen storage capacity with increasing hydrogenation and dehydrogenation cycle number is observed in numerous hydrogen storage materials, but the mechanism behind this unfavorable change has not been elucidated yet. In this study, we have investigated the development of structural defects or disorders in V1–xTixH2, x = 0, 0.2, and 0.5, during the first 15 hydrogen absorption and desorption cycles using the atomic pair distribution function (PDF) analysis of synchrotron X-ray total scattering data to find out the possible structural origin of the poor cyclic stability of V1–xTix alloys. While pure vanadium shows no significant change in the PDF, alloy samples subject to several hydrogenation and dehydrogenation cycles display fast decaying of the PDF profile due to a progressive increase in the PDF peak width with increasing r. This r-dependent PDF peak broadening effect becomes stronger with cycle number. Molecular dynamics (MD) simulations demonstrated that dislocation defects ...

We investigated the hydrogenation of a binary TiFe alloy at 5 GPa and 600 °C by in situ synchrotron radiation X-ray diffraction measurements. After formation of a solid solution of hydrogen in TiFe, an order–disorder phase transition in the metal lattice of TiFe occurred, which yielded a BCC TiFe hydride. The unit cell volume of the BCC hydride increased by 21.0% after the hydrogenation reaction. The volume expansion was larger than that of a γ -hydride TiFeH 1.9 prepared by hydrogenation near ambient conditions.

The novel complex hydride YLiFeH6 with the Fe–H complex anion ([FeH6]4−) was synthesized by hydrogenation of YFe2 together with additional Li. Li adjusts the original imbalanced charge between Y3+ and [FeH6]4− by donating an electron to convert into Li+ during the hydrogenation, resulting in the formation of the well charge-balanced state of Y3+Li+[FeH6]4−.

The compressibility of lanthanum (La) metal and its hydrides were measured at room temperature by high pressure synchrotron X-ray diffraction. La metal pressurized in a hydrogen medium forms a hydride with an fcc metal lattice, which likely contains hydrogen at a concentration close to 3.0 and persists over the measured pressure span up to 21 GPa. Equations of state have been determined by helium compression experiments for LaH2 with tetrahedral interstitial sites fully occupied with hydrogen atoms and for LaH2.46 with octahedral interstitial sites partially occupied with hydrogen atoms and tetrahedral sites fully occupied. Both hydrides possess fcc metal lattices. The bulk modulus values B 0 are 66.7 ± 1.2 GPa for LaH2 and 68.4±1.0 GPa for LaH2.46. These values are three times larger than that of La metal and are very close to each other despite the difference in hydrogen occupation states. The hardening of the metal lattice by hydrogenation is attributed predominantly to hydrogen–metal interactions at the tetrahedral sites and is most pronounced for La, which has the largest ionic radius among rare-earth metals.

We present the first successful in situ simultaneous measurement of the electrical resistance and X-ray diffraction of FeH x (x∼ 1) under high-pressure H2 up to 25.5 GPa and low temperatures down to 9 K. The electrical resistivity ρ showed a sharp increase with the formation of iron-hydride FeH x (x∼ 1) at 3.5 GPa. The ϵ′-phase of FeH x was found to be metallic up to 25.5 GPa. The ρ vs. T curves up to 16.5 GPa approximately follow Fermi-liquid law below 25 K. However, T 5 was found to be better fitting at 25.5 GPa. This change can be considered to be related to the previously reported ferromagnetism collapse at corresponding pressure.

The hydrogenation process of aluminum is observed at high pressure and temperature by angle-dispersive in situ X-ray diffraction method. At 10 GPa and 600 °C, single crystals of AlH3 about 20 μm in size are found to grow through three steps: self-pulverization of aluminum, hydrogenation of pulverized aluminum, and solid-state grain growth of hydride. The self-pulverization seems to be the precursor of the hydrogenation reaction because it occurs at pressure temperature conditions where AlH3 is thermodynamically stable.

Advanced Fundamental Research on Hydrogen Storage Materials (HYDRO☆STAR), was launched in 2007 to investigate the fundamental properties of hydrogen storage materials. The interest is focused on basic understanding of the hydrogen-metal interactions in metal hydride systems. Neutron scattering technique, which is capable of determining hydrogen positions and bonding states, is a key tool to investigate the interactions, and becomes significantly powerful when combined with high pressure technique, which realizes high hydrogen density states by compression of metal lattices. In this article, we present high-pressure neutron scattering techniques using Paris-Edinburgh cell and hydrogen-gas cell under development at J-PARC and JRR-3.

Pressure-induced structural transition of rare-earth metal hydride, YH3, has been investigated by synchrotron radiation X-ray diffraction experiments. We observed the pressure-induced structural transformation from the hexagonal metal lattice into an fcc one through an intermediate state, which appears in the wide pressure span of 12∼22 GPa. The obtained X-ray diffraction patterns in the intermediate state are represented by long-period structures, e.g. 27R, of the yttrium metal lattice. These long-period structures are interpreted in terms of the periodic arrangements of hexagonal-type and fcc-type stacking layers of the yttrium metals. Such structural transition is considered to be characteristic for rare-earth metal hydride.

We synthesized scandium hydrides by hydrogenation of a scandium foil with hydrogen fluid under high pressure at ambient temperature. Scandium dihydride (ScH 2 ) and trihydride (ScH 3 ) were prepared near 4 and 5 GPa, respectively. The hydrogenation process and pressure-induced structural changes in ScH 3 were investigated by synchrotron radiation X-ray diffraction measurements up to 54.7 GPa. A structural transition from hexagonal to the fcc lattice began at 30 GPa and was completed at 46 GPa via an intermediate state similar to those reported for other hexagonal trihydrides. The intermediate state was not interpreted in terms of a coexisting state for the low-pressure hexagonal and the high-pressure fcc structures. The onset transition pressure of ScH 3 supported the previously proposed relation that the hexagonal–fcc transition pressure is inversely proportional to the ionic radius of the trihydride.

We constructed a single-crystal synchrotron X-ray diffraction system to precisely study the structure under hydrostatic pressure conditions at low temperatures and applied it to a study on the phase transition phenomena of a Cd–Yb periodic approximant and a Cd–Yb quasicrystal. Four phases were newly observed for the 1/1 approximant crystal in a P–T range up to 5.2 GPa and down to 10 K. The innermost part of the atomic clusters of Cd4 tetrahedra exhibited various orientational ordering sensitively depending on pressure and temperature. High pressure diffraction measurements using a highly parallel synchrotron X-ray beam and a hydrostatically compressed single crystal enabled us to detect the weak diffractions due to the subtle structural changes.

The title compound is synthesized from a mixture of Bi2O3, ZnO, V2O3, and V2O5 with 10% excess of Bi and Zn (gold capsule, 9 GPa, 1373 K, 30 min).

Pressure induced structural transition of yttrium hydride has been investigated using synchrotron radiation X-ray diffraction measurement up to 24 GPa at room temperature. A reversible hexagonal–fcc transition with a wide intermediate region from 11 to 20 GPa is confirmed, which is consistent with previously reported X-ray results. The diffraction patterns measured for the intermediate state are not interpreted in terms of a hexagonal–fcc coexisting state or as rare-earth structures with various metal-layer stacking sequences. The equation of state determined for the hexagonal–YH 3 provides a bulk modulus B 0 of 71.9 GPa, which is nearly half of the previously reported value, but is within the range of values reported for other rare-earth metal trihydrides with hexagonal structures.

We have derived the magnetic phase diagram for Nd 1/2 Ca 1/2 (Mn 1- y M y )O 3 ( M = Ru, Cr and Sc) based on the neutron powder diffraction measurements. We found that the Ru- and Cr-doping induces an antiferromagnetic-to-ferromagnetic transition at y =0.01–0.02, while the Sc-doping does not induce the transition up to y =0.10. We have found significant dopant dependent effects on the distortion of the MnO 6 octahedra in the heavily-doped region of the long- c phase: the MnO 6 shape approached to the 2-long/4-short type with Ru-doping, while it does to the 4-long/2-short type with Sc-doping. Absence of the ferromagnetic phase in the Sc-doped system can be ascribed to (1) a strong charge-ordering instability due to the 4-long/2-short type distortion of the MnO 6 octahedra and (2) the reduced effective one-electron bandwidth due to the Sc 3+ ( d 0 ) impurities.

Lattice and magnetic properties are investigated for 3% Ru- and Cr-doped Nd 1/2 Ca 1/2 MnO 3 . The parent Nd 1/2 Ca 1/2 MnO 3 is a charge-ordered insulator ( T CO = 250 K). With decreasing temperature below ≈ 210 K, these compounds are separated into two perovskite phases, that is, the long- c and short- c phases. The long- c region shows a ferromagnetic transition at T C ≈210 K for the Ru-doped compound and ≈ 130 K for the Cr-doped compound, while the short- c region shows antiferromagnetic transition at T N ≈150 K for Ru and ≈ 110 K for Cr. We discuss the origin of the enhanced T C for the Ru-doped compound in terms of the effective one-electron bandwidth W of the e g -band.

Hydrogen composition and occupation state provide basic information for understanding various properties of the metal–hydrogen system, ranging from microscopic properties such as hydrogen diffusion to macroscopic properties such as phase stability. Here the deuterization process of face-centred cubic Fe to form solid-solution face-centred cubic FeDx is investigated using in situ neutron diffraction at high temperature and pressure. In a completely deuterized specimen at 988 K and 6.3 GPa, deuterium atoms occupy octahedral and tetrahedral interstitial sites with an occupancy of 0.532(9) and 0.056(5), respectively, giving a deuterium composition x of 0.64(1). During deuterization, the metal lattice expands approximately linearly with deuterium composition at a rate of 2.21 Å3 per deuterium atom. The minor occupation of the tetrahedral site is thermally driven by the intersite movement of deuterium atoms along the ‹111› direction in the face-centred cubic metal lattice., Knowledge of hydrogen content and coordination in deuterized metals is of importance. Here, the authors study the deuterization of face-centred cubic iron in-situ via neutron diffraction, observing the occupation by hydrogen of both octahedral and tetrahedral interstitial sites.

Temperature variation of the crystal structure has been investigated for Tb 2 Ba 2 Co 4 O 11 , which shows a metal-insulator (MI) transition at T MI = 340 K. The compound has a layered-type double-perovskite structure along the c -axis, with alternating the CoO 6 octahedron and the CoO 5 pyramids along the a -axis. With decrease of temperature below T MI , the distortion of the octahedral and pyramidal sites is fairly released, suggesting that spin state transition of the Co 3+ ions is the origin for the MI transition.

Destabilization of the charge-ordered (CO) state in half-doped manganites by chromium (Cr) doping in the manganese (Mn) sites was investigated. The Cr doping in the half-doped manganites makes the ...

Crystal and magnetic structures of Cr-doped manganites, Nd 1/2 Ca 1/2 Mn 1- y Cr y O 3 ( y =0.00 and 0.03), have been investigated by synchrotron radiation (SR) x-ray powder diffraction as well as neutron powder diffraction measurements. A detailed analysis of the high-resolution x-ray profile has revealed that the Cr-doped compound exhibits broad extra reflections, suggesting the formation of microdomains below the charge-ordering temperature T CO . The origin of the microdomain structure is discussed in terms of the charge separation.

Structural and magnetic properties of the phase separated state in ${\mathrm{Nd}}_{0.55}({\mathrm{Sr}}_{0.17}{\mathrm{Ca}}_{0.83}{)}_{0.45}{\mathrm{MnO}}_{3}$ have been investigated by means of synchrotron radiation x-ray as well as neutron powder diffraction experiments. We have observed a phase transformation into the phase separated (two-phase) state below ${T}_{\mathrm{CO}}\ensuremath{\sim}200$ K, and have observed that the lattice constants change discontinuously at the transformation. On the basis of the structural and magnetic data, origin of the insulator-metal behavior is discussed.

Temperature variation of the crystal structure has been investigated for ${\mathrm{TbBaCo}}_{2}{\mathrm{O}}_{5+\ensuremath{\delta}} (\ensuremath{\delta}=0.5),$ which shows a metal-insulator (MI) transition at ${T}_{\mathrm{MI}}\ensuremath{\approx}340 \mathrm{K}.$ The compound has an oxygen-deficient perovskite structure, with alternating ${\mathrm{CoO}}_{6}$ octahedral and ${\mathrm{CoO}}_{5}$ pyramidal chains along the crystallographic $b$ axis. We have found that both the pyramids and octahedra are significantly distorted in the insulating phase below ${T}_{\mathrm{MI}},$ suggesting a ${d}_{{3x}^{2}\ensuremath{-}{r}^{2}}{/d}_{{3y}^{2}\ensuremath{-}{r}^{2}}\ensuremath{-}\mathrm{type}$ orbital ordering.

Electronic structure has been investigated by reflectivity measurement for ordered-perovskite transition-metal oxides, ${\mathrm{Sr}}_{2}M{\mathrm{MoO}}_{6}$ $(M=\mathrm{Cr},$ Mn, Fe, and Co). The ground states of the Co and Mn compounds are paramagnetic insulators, making a sharp contrast with the ferrimagnetic metallic ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}.$ For all the compounds, we have observed intense optical transition at $\ensuremath{\sim}2\mathrm{eV},$ which has been ascribed to the charge-transfer excitation from the $O2p$ state to the $\mathrm{Mo}4d$ level. Based on the optical spectra, we argue the electronic structure of this class of compounds.

Previous x-ray diffraction measurements revealed the pressure-induced decomposition of an fcc ${\mathrm{LaH}}_{2.3}$ into H-rich and H-poor fcc phases around 11 GPa. The present neutron diffraction measurements on ${\mathrm{LaD}}_{2}$ confirm the formation of NaCl-type LaD as a counterpart of the D-rich ${\mathrm{LaD}}_{2+\ensuremath{\delta}}$ by disproportionation. First-principles enthalpy and lattice dynamic calculations demonstrate that the NaCl-type LaH is stabilized at high pressures and can be recovered at ambient conditions. Finding the NaCl-type LaH will pave the way for investigations on the site-dependent nature of hydrogen-metal interactions.

Europium hydride ${\mathrm{EuH}}_{x}$, when exposed to high-pressure ${\mathrm{H}}_{2}$, has been found to exhibit the following structural and valence changes: $Pnma(x=2,\mathrm{\text{divalent}})\ensuremath{\rightarrow}P{6}_{3}/mmc(x=2,7.2--8.7\text{ }\text{ }\mathrm{GPa})\ensuremath{\rightarrow}I4/m(xg2,8.7--9.7\text{ }\text{ }\mathrm{GPa})\ensuremath{\rightarrow}I4/mmm(xg2,9.7\text{ }\text{ }\mathrm{GPa}\ensuremath{-},\mathrm{\text{trivalent}})$. With a trivalent character and a distorted cubic fcc structure, the $I4/mmm$ structure is the $\ensuremath{\beta}$ phase commonly observed for other rare-earth metal hydrides. Our study clearly demonstrates that ${\mathrm{EuH}}_{x}$ is no longer an irregular member of the rare-earth metal hydrides.

Structural change of lanthanum dihydride LaH${}_{2.3}$, which has a face-centered-cubic (fcc) metal lattice with tetrahedral interstitial sites fully occupied with hydrogen atoms and partially occupied octahedral sites, has been investigated at high pressures up to 20 GPa at ambient temperature by synchrotron radiation x-ray diffraction. Additional Bragg reflections appear just on higher angle sides of the original ones at $~11$ GPa and their peak intensities increase gradually on further compression. The coexistence state of two fcc metal lattices thus observed above 11 GPa is interpreted in terms of phase separation or disproportionation reaction from the dihydride toward a solid solution and trihydride states, in both of which the octahedral interstitial sites are partially occupied with hydrogen atoms. A gradual distortion from the cubic to a tetragonal lattice is observed prior to the phase separation. The coexistence phase goes back to the dihydride fcc phase via the lattice distorted phase with decreasing pressure.

Interrelation between the distortion of the ${\mathrm{MnO}}_{6}$ octahedra and spin ordering was investigated for ${\mathrm{Nd}}_{0.45}{\mathrm{Sr}}_{0.55}{\mathrm{MnO}}_{3}$ and ${\mathrm{Nd}}_{0.35}{\mathrm{Sr}}_{0.65}{\mathrm{MnO}}_{3}$ by means of synchrotron-radiation x-ray powder diffraction and neutron powder diffraction. The ground state of the former (latter) compounds is the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}{(d}_{{3z}^{2}\ensuremath{-}{r}^{2}})$ orbital-ordered state. A strong orbital-spin coupling is observed in ${\mathrm{Nd}}_{0.45}{\mathrm{Sr}}_{0.55}{\mathrm{MnO}}_{3},$ making a sharp contrast with the weak coupling in ${\mathrm{Nd}}_{0.35}{\mathrm{Sr}}_{0.65}{\mathrm{MnO}}_{3}.$ We interpret the strong coupling in terms of the kinetic energy gain of the ${e}_{g}$ carriers in the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital-ordered state.

$B$-site substitution effects on the structural and magnetic properties are investigated in ${\mathrm{Nd}}_{1/2}{\mathrm{Ca}}_{1/2}$$({\mathrm{Mn}}_{0.97}{M}_{0.03})$${\mathrm{O}}_{3}$ $(M=\mathrm{Al},$ Zn, Sc, Fe, Ni, Co, and Cr). With decreases of temperature below $\ensuremath{\sim}200 \mathrm{K},$ these compounds transform into a two-phase state. With further decrease of temperature below $\ensuremath{\sim}100 \mathrm{K},$ the Cr-, Co-, and Ni-doped compounds show the ferromagnetic transition, while the other compounds show only the antiferromagnetic component. We discuss these dopant-dependent ferromagnetic transition in terms of the effective one-electron bandwidth of the ${e}_{g}$ band.

Structural change of LaH2 has been investigated at high pressure and low temperature by synchrotron radiation X-ray diffraction. LaH2 has an fcc metal lattice with interstitial hydrogen atoms located at tetrahedral sites. The fcc-LaH2 decomposes into two phases, which have different hydrogen compositions such as hydrogen-poor LaH and hydrogen -rich LaH2+δ phases, at 11 GPa at room temperature. In spite of largely suppressed diffusivity of the interstitial hydrogen atoms at low temperature, we have found the disproportionation reaction around 14 GPa at 200 K. Difference in the molar volume between each phases suggests monohydride LaH is formed as the disproportionation product even at low temperature. The volume fraction of the hydrogen-poor phase relative to the hydrogen-rich one at 200 K rapidly rises against pressure and reaches to ~0.45 at ~17 GPa.

Structural parameters are systematically investigated for Mo pyrochlore ${R}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ $(R=\mathrm{Dy},$ Gd, Sm, and Nd) by means of synchrotron radiation x-ray powder diffraction. This system shows a crossover from the spin-glass (SG) state to the ferromagnetic metallic (FM) state as the averaged ionic radius ${r}_{\mathrm{R}}$ of the rare-earth ion increases. We have found the Mo-O-Mo bond angle increases with ${r}_{\mathrm{R}}$ from $129.7\ifmmode^\circ\else\textdegree\fi{}$ for ${\mathrm{Dy}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ (SG) to $131.5\ifmmode^\circ\else\textdegree\fi{}$ for ${\mathrm{Nd}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ (FM). This behavior is opposite to the so-called Kanamori-Goodenough rule.

Aluminum-based alloy hydride Al2CuHx (x ∼ 1) is synthesized by hydrogenating Al2Cu alloy using high-temperature and high-pressure hydrogen atmosphere. Al8Cu square antiprisms in Al2Cu twist around the c axis of a tetragonal unit cell by hydrogenation. The twist enlarges the interstitial spaces for accommodating hydrogen atoms which align linearly parallel to the c axis in Al2CuHx. Thermodynamic stability of Al2CuHx results from the balance of stabilization by H 1s and Al 3sp hybridization and destabilization owing to the Fermi-level lifting upon hydrogenation. The crystal and electronic structures of Al2CuHx illustrate the formation of an interstitial hydride of aluminum-based alloy.

Powder x-ray diffraction experiments have been done on Ne up to 237 GPa at room temperature. Ne remains in the fcc phase up to the highest pressure. In order to observe the weak 200 reflection at ultrahigh pressures, we have used the tube-cross slit at the exit side of x-rays, which effectively reduced the scattered x-rays by the diamond anvil. The lattice parameters obtained from the 111 and 200 reflections significantly differ from each other, implying the existence of sizable uniaxial stress in Ne at ultrahigh pressures. On the basis of the calculated elastic constants of Ne at high pressures, we conclude that the 111 reflection is least affected by the uniaxial stress. The equation of state constructed from the 111 reflection agrees well with previous data.

We have investigated the effect of hydrogenation on La 5d and 6p electronic states in metallic LaHx (x = 0.0, 2.0, 2.3, and 2.6) by X-ray absorption near edge structure at the La L-edges. As the hydrogen content x increases from 0 to 2.6, white-line intensity at the La L2,3-edges shows a remarkable increase in the range of x>2.0. This is interpreted as the increase in La 5d hole induced by interstitial H atoms on the octahedral sites. On the other hand, the shoulder structure at the La L1-edge disappears in the process of x = 0.0rightarrow;2.0, indicating that the p-d hybridization is weakened by H atoms on the tetrahedral sites. This study demonstrates that H atoms on the two interstitial H sites provide different contribution to the modification of the electronic states.

The pressure-temperature diagram of hydrogen-aluminum system was determined for a pressure range of 0–10GPa and a temperature range of 27–800°C by in situ x-ray diffraction measurements. Pristine aluminum was hydrogenated to trihydride at 8.9GPa and 600°C. The cyclic formation and decomposition of the hydride resulted in lowering of the hydrogenation conditions down to 4.9GPa and 330°C. Transparent single crystals were recovered at ambient conditions.

2 p , c, and of Tb1 xCaxMnO3 (0:5 x 1:0) against Ca concentration x at 10K. The powder patterns for the lower and higher x-range can be well reproduced by the single orthorhombic (Pnma: open symbols) model. In the concentration range of 0:65 x 0:85, however, the powder patterns were reproduced by the twophase model with orthorhombic and monoclinic (P21=m: filled symbols) phases. Note that the room temperature structure was the single-phase even in this x region. The lattice constants, a, b= ffiffiffi