Hydrogenation-induced insulating state in the intermetallic compound LaMg2Ni

First-principles investigation of LaMg2Ni and its hydrides

Using first-principles density functional theory calculations, the electronic structures of LaMg₂Ni and its hydrides LaMg₂NiH_4.5 (intermediate phase) and LaMg₂NiH₇ (fully hydrogenated phase), as well as the H adsorption on LaMg₂Ni (100) surface were investigated. For comparision, the atomic bonding characteristics of Co- and Pd-doped LaMg₂Ni, LaMg₂NiH_4.5 and LaMg₂NiH₇ compounds were also studied. Our aim is to provide new insights into the hydrogenation of LaMg₂Ni. The results show that the metallic intermediate hydride LaMg₂NiH_4.5 with Ni–H covalent bonds may act as the precursor state from the host compound LaMg₂Ni to the full hydride LaMg₂NiH₇. Upon LaMg₂Ni hydrogenation, the suppression of Mg–Ni and Ni–H interactions as well as the formation of La-H bonds favors for LaMg₂Ni–H formation.

Hydrogen-Release Reaction of a Complex Transition Metal Hydride with Covalently Bound Hydrogen and Hydride Ions

The hydrogen-release reaction of a complex transition metal hydride, LaMg₂ NiH₇ , composed of La³⁺ , 2×Mg²⁺ , [NiH₄ ]^4- and 3×H^- , was studied by thermal analyses, powder X-ray, and neutron diffraction and inelastic neutron scattering. Upon heating, LaMg₂ NiH₇ released hydrogen at approximately 567 K and decomposed into LaH_2-3 and Mg₂ Ni. Before the reaction, covalently bound hydrogen (H^c °^v. ) in [NiH₄ ]^4- exhibited a larger atomic displacement than H^- , although a weakening of the chemical bonds around [NiH₄ ]^4- and H^- was observed. These results indicate the precursor phenomenon of a hydrogen-release reaction, wherein there is a large atomic displacement of H^c °^v. that induces the hydrogen-release reaction rather than H^- . As an isothermal reaction, LaMg₂ NiH₇ formed LaMg₂ NiH_2.4 at 503 K in vacuum for 48 h, and LaMg₂ NiH_2.4 reacted with hydrogen to reform LaMg₂ NiH₇ at 473 K under 1 MPa of H₂ gas pressure for 10 h. These results revealed that LaMg₂ NiH₇ exhibited partially reversible hydrogen-release and uptake reactions.

Evidence of Intermediate Hydrogen States in the Formation of a Complex Hydride

A complex hydride (LaMg₂NiH₇) composed of La³⁺, two Mg²⁺, [NiH₄]^4- with a covalently bonded hydrogen, and three H^- was formed from an intermetallic LaMg₂Ni via an intermediate phase (LaMg₂NiH_4.6) composed of La, Mg, NiH₂, NiH₃ units, and H atoms at tetrahedral sites. The NiH₂ and NiH₃ units in LaMg₂NiH_4.6 were reported as precursors for [NiH₄]^4- in LaMg₂NiH₇ [ Miwa et al. J. Phys. Chem. C 2016 , 120 , 5926 - 5931 ]. To further understand the hydrogen states in the precursors (the NiH₂ and NiH₃ units) and H atoms at the tetrahedral sites in the intermediate phase, LaMg₂NiH_4.6, we observed the hydrogen vibrations in LaMg₂NiH_4.6 and LaMg₂NiH₇ by using inelastic neutron scattering. A comparison of the hydrogen vibrations of the NiH₂ and NiH₃ units with that of [NiH₄]^4- shows that the librational modes of the NiH₂ and NiH₃ units were nonexistent; librational modes are characteristic modes for complex anions, such as [NiH₄]^4-. Furthermore, the hydrogen vibrations for the H atoms in the tetrahedral sites showed a narrower wavenumber range than that for H^- and a wider range than that for typical interstitial hydrogen. The results indicated the presence of intermediate hydrogen states before the formation of [NiH₄]^4- and H^-.

Formation of novel transition metal hydride complexes with ninefold hydrogen coordination

Ninefold coordination of hydrogen is very rare, and has been observed in two different hydride complexes comprising rhenium and technetium. Herein, based on a theoretical/experimental approach, we present evidence for the formation of ninefold H- coordination hydride complexes of molybdenum ([MoH₉]³⁻), tungsten ([WH₉]³⁻), niobium ([NbH₉]⁴⁻) and tantalum ([TaH₉]⁴⁻) in novel complex transition-metal hydrides, Li₅MoH₁₁, Li₅WH₁₁, Li₆NbH₁₁ and Li₆TaH₁₁, respectively. All of the synthesized materials are insulated with band gaps of approximately 4 eV, but contain a sufficient amount of hydrogen to cause the H 1s-derived states to reach the Fermi level. Such hydrogen-rich materials might be of interest for high-critical-temperature superconductivity if the gaps close under compression. Furthermore, the hydride complexes exhibit significant rotational motions associated with anharmonic librations at room temperature, which are often discussed in relation to the translational diffusion of cations in alkali-metal dodecahydro-closo-dodecaborates and strongly point to the emergence of a fast lithium conduction even at room temperature.

Hydrogen induced site depopulation in the LaMgNi4-hydrogen system

The LaMgNi4-hydrogen system was investigated by in-situ neutron powder diffraction and pressure-composition isotherm measurements at 100 °C and hydrogen (deuterium) pressures of up 50 bar. The system displays three hydride phases that have distinctly different hydrogen plateau pressures and H atom distributions. The cubic α-LaMgNi4H0.75 phase forms below 0.01 bar hydrogen pressure and H atoms fill one type of tetrahedral Ni4 interstices. The orthorhombic distorted β-LaMgNiH3.7 phase forms at about 3 bar hydrogen pressure and H atoms fill both tetrahedral LaNi3 and triangular bi-pyramidal La2MgNi2 interstices. Interestingly, tetrahedral Ni4 interstices are no longer occupied. Finally, the most hydrogen rich γ-LaMgNi4H4.85 phase forms above 20 bar. It has again cubic symmetry and H atoms continue to occupy triangular bi-pyramidal La2MgNi2 interstices while filling a new type of tetrahedral Ni4 interstices that are neither occupied in the α- nor in the β-phase. The tetrahedral LaNi3 interstices occupied in the β-phase are empty. Hydrogen induced depopulations of interstitial sites in metal hydrides are relatively rare and consistent with, but not entirely due to, the onset of repulsive H–H interactions at increasing hydrogen concentrations.

Tetrahedral D Atom Coordination of Nickel and Evidence for Anti-isostructural Phase Transition in Orthorhombic Ce2Ni7D∼4

Hydrogenation of hexagonal Ce2Ni7 was investigated by synchrotron X-ray and neutron powder diffraction. In contrast to the recently investigated lanthanum analogue, which remains hexagonal (La2Ni7D6.5: space group P63/mmc), the cerium compound becomes orthorhombic (Ce2Ni7D approximately 4: space group Pmcn). As in the structurally related CeNi3D2.8, deuterium occupies CeNi2 slabs only, while the bulk of the CeNi5 slabs remains empty. A significant amount of deuterium is bonded in tetrahedral NiD4 units similar to those in nickel-based complex metal hydrides. These findings provide further evidence for directional bonding effects in hydrides that are traditionally considered as "interstitial". Ce2Ni7D approximately 4 displays various orthorhombic lattice distortions, delta = (b/ radical3 - a)/a. Hydrogen pressures of approximately 30 bar stabilize a phase having a negative distortion (delta < 0). Upon a decrease in the pressure, this phase transforms via a two-phase region into another phase having a positive distortion (delta > 0). Both phases are nearly isostructural and have the same space group symmetry and nearly the same composition. This situation is typical for a so-called anti-isostructural phase transition in which delta is considered to be an order parameter. Neither magnetic nor structural transitions have been detected down to 1.5 K.

Deuterium-induced copper pairing in Zr2CuD(approximately 5)

Deuteration of Zr2Cu leads to a reconstruction of its MoSi2-type metal atom arrangement. While the tetragonal alloy contains isolated copper atoms (Cu-Cu = 3.22 angstroms) in a cubic zirconium atom environment, the monoclinic deuteride Zr2CuD4.71 contains Cu2 dumbbells (Cu-Cu = 2.39 angstroms), of which each copper atom has a trigonal-prismatic zirconium atom environment. Deuterium occupies five sites, of which four have tetrahedral metal configurations (two Zr4-type and two Zr3Cu-type) and are fully occupied while one has a trigonal-bipyramidal metal configuration (Zr3Cu2-type) and is partially occupied (71%). Copper is bonded to four deuterium atoms in a saddle-like configuration (Cu-D = 1.73-1.87 angstroms). Two of the ligands connect copper in a nearly linear Cu-D-Cu arrangement to partially interrupted dimeric [Cu2D6](n)() ribbons running perpendicular to the Cu2 dumbbell direction. At the upper phase limit, the ribbons are presumably no longer interrupted and the deuteride can be described by the limiting ionic formula 2Zr2+[Cu(+)D3](2-)2D-.