[en] Solid-state metal hydrides display hydrogen densities close to that of liquid hydrogen and thus provide a safe and efficient way of storing hydrogen. As a result of recent neutron and synchrotron diffraction work some novel metal hydrides have been characterized that shed new light on the nature of metal-hydrogen interactions. While hydrogen appears as an anion surrounded by a large inventory of cation configurations in ionic hydrides such as Ca₄Mg₃H₁₄, Ca₁₉Mg₈H₅₄, Eu₂MgD₆, Eu₆Mg₇D₂₆ and Eu₂Mg₃D₁₀, it acts as a terminal ligand in covalently bonded hydride complexes based on p-elements such as [BH₄]^- and d-elements such as [IrH₅]^4- and [IrH₄]^5- in the complex hydrides LiBH₄ and Mg₆Ir₂H₁₁, respectively. Surprisingly, hydride complexes and hydride anions can also be discerned in typically metallic (interstitial) hydrides such as NdMgNi₄H₄ (= Nd³⁺Mg⁺².[Ni₄H₄]^5-) and LaMg₂NiD₇ (= La³⁺Mg⁺²₂.[NiH₄]^4-.3H^-). Some hydrides also reveal other interesting features such as a hydrogenation induced Ce⁴⁺→Ce³⁺ valence change in CeMn_1.8Al_0.2H_4.4 at room temperature that is accompanied by a Mn/Al metal atom exchange over distances of ∝2.6 A, and a hydrogen induced metal-to-nonmetal transition near ambient conditions that leads from the metallic compound Mg₃Ir to the red colored hydride Mg₆Ir₂H₁₁. In this article recent work and some methodological aspects are highlighted. (orig.)

[en] The partial loss of the cohesive metal-metal bonding in interstitial solid solutions of hydrogen in transition metals is compensated by Coulomb interactions. The hydrogen atoms occupy octahedral interstices at high electronegativity of the metal (PdH, NiH, β-VH). Otherwise tetrahedral sites are preferred by crystal field energy (delta-VH, β-TaH, NbH₂, etc.). The metal lattice can get distorted by the repulsion of neighbouring atoms, which can be shown by Madelung calculations on β, γ, zeta-NbH and NbH₂

No abstract available

[en] Photoelectron spectroscopy studies of hydrogen-bearing metals and alloys have provided fundamental information concerning the electronic interactions of hydrides. Studies of surface oxidation of several hydrogen storage materials (the LaNi₅-family) evaluated the role of surface oxidation on hydrogen uptake. Collaborative band theory studies were undertaken to support experimental studies of the metal-semiconductor transition in LaH₂-LaH₃ and of the refractory metal mono- and submonohydrides

[en] The electronic structures of representative metal hydrides have been examined through photoelectron and optical spectroscopies. The results of those studies ae compared with predictions of the one-electron band model for the various metal-hydrogen systems. Generally excellent agreement is found. The discussion emphasizes Nb₄H₃ and the dihydrides of Sc, Y, Nd, Gd, Tb, Er, Lu, Hf, and Th

[en] This chapter discusses metal-hydrogen electronic interactions in bulk hydrides by reviewing recent theoretical and experimental results for typical monohydrides (VH, NbH, and TaH), dihydrides (LaH₂, PrH₂, and NdH₂), and trihydrides (LaH₃). It attempts to highlight the basic physics behind the metal-hydrogen interaction in a variety of hydrides and compares experimental results with theoretical band structures and densities of states and finds generally good agreement, with exceptions noted. The chapter concludes that with the understanding of these relatively simple hydrides, it should be possible to examine the role of the electronic structure in the more complicated systems such as LaNi₅H /SUB 6.7/

[en] Ab initio molecular electronic structure theory has been applied to the family of transition metal tetrahydrides TiH₄ through NiH₄. For the TiH₄ molecule a wide range of contracted Gaussian basis sets has been tested at the self-consistent-field (SCF) level of theory. The largest basis, labeled M(14s 11p 6d/10s 8p 3d), H(5s 1p/3s 1p), was used for all members of the series and should yield wave functions approaching true Hartree-Fock quality. Predicted SCF dissociation energies (relative to M+4H) and M--H bond distances are TiH₄ 132 kcal, 1.70 A; VH₄ 86 kcal, 1.64 A; CrH₄ 65 kcal, 1.59 A; MnH₄ -- 36 kcal, 1.58 A; FeH₄ 0 kcal, 1.58 A; CoH₄ 27 kcal, 1.61 A; and NiH₄ 18 kcal, 1.75 A. It should be noted immediately that each of these SCF dissociation energies will be increased by electron correlation effects by perhaps as much as 90 kcal. For all of these molecules except TiH₄ excited states have also been studied. One of the most interesting trends seen for these excited states is the shortening of the M--H bond as electrons are transferred from the antibonding 4t₂ orbital to the nonbonding 1e orbitals

No abstract available

[en] Using a high pressure technique and the strong donating nature of H"-, a new series of tetragonal La_2Fe_2Se_2O_3-type layered mixed-anion arsenides, Ln_2M_2As_2H_x, was synthesized (Ln=La or Sm, M=Ti, V, Cr, or Mn; x∼3). In these compounds, an unusual M_2H square net, which has anti CuO_2 square net structures accompanying two As"3"- ions, is sandwiched by (LaH)_2 fluorite layers. Notably, strong metal-metal bonding with a distance of 2.80 Aa was confirmed in La_2Ti_2As_2H_2_._3, which has metallic properties. In fact, these compounds are situated near the boundary between salt-like ionic hydrides and transition-metal hydrides with metallic characters. (copyright 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] A particulate mixture of ceramic powder, boron and a hydride of a metal selected from the group consisting of hafnium, niobium, tantalum, titanium, vanadium, zirconium and mixtures thereof is hot pressed decomposing the hydride and reacting the resulting metal with boron producing a polycrystalline microcomposite comprised of a continuous phase of the boride of the metal which encapsulates at least about 20% by volume of the ceramic particles and which either encapsulates or is intermixed with the balance of said ceramic particles