Inelastic neutron scattering evidence for anomalous H-H distances in metal hydrides

Chemically Assisted Precompression of Hydrogen Molecules in Alkaline-Earth Tetrahydrides

Through a series of high pressure diamond anvil experiments, we report the synthesis of alkaline earth (Ca, Sr, Ba) tetrahydrides, and investigate their properties through Raman spectroscopy, X-ray diffraction, and density functional theory calculations. The tetrahydrides incorporate both atomic and quasi-molecular hydrogen, and we find that the frequency of the intramolecular stretching mode of the H2δ- units downshifts from Ca to Sr and to Ba upon compression. The experimental results indicate that the larger the host cation, the longer the H2δ- bond. Analysis of the electron localization function (ELF) demonstrates that the lengthening of the H-H bond is caused by the charge transfer from the metal to H2δ- and by the steric effect of the metal host on the H-H bond. This effect is most prominent for BaH₄, where the precompression of H2δ- units at 50 GPa results in bond lengths comparable to that of pure H₂ above 275 GPa.

Exploring the use of neutrons to detect hydrogen embrittlement in high strength steel

With the aim of exploring neutron techniques for the non-destructive detection of hydrogen in embrittled steel, three sets of steel samples were studied with neutron scattering: Ni coated, Cd coated, and Cr coated. Each set contained a non-embrittled or low-hydrogen concentration reference and one or two embrittled and high-hydrogen concentration samples. It is observed that the incoherent scattering, when normalized by the intensity of the Bragg peak, is significantly higher for high-hydrogen concentration or embrittled samples than in the reference. Although the difference is small, this represents a non-destructive technique of detecting hydrogen embrittlement. Neutron radiography, and inelastic or small-angle scattering could not distinguish between embrittled and reference samples.

The Crystal Structures in Hydrogen Absorption Reactions of REMgNi4-Based Alloys (RE: Rare-Earth Metals)

REMgNi4-based alloys, RE(2−x)MgxNi4 (RE: rare-earth metals; 0 < x < 2), with a AuBe5-type crystal structure, exhibit reversible hydrogen absorption and desorption reactions, which are known as hydrogen storage properties. These reactions involve formation of three hydride phases. The hydride formation pressures and hydrogen storage capacities are related to the radii of the RE(2−x)MgxNi4, which in turn are dependent on the radii and compositional ratios of the RE and Mg atoms. The crystal structures formed during hydrogen absorption reactions are the key to understanding the hydrogen storage properties of RE(2−x)MgxNi4. Therefore, in this review, we provide an overview of the crystal structures in the hydrogen absorption reactions focusing on RE(2−x)MgxNi4.

Interstitial Hydrogen in Fe/V Superstructures: Lattice Site Location and Thermal Vibration

We report real space location of hydrogen in single crystalline Fe/V superstructures. Anisotropic strain is quantified versus hydrogen concentration by using the yield of backscattered primary 2 MeV ^{4}He ions for incidence in different crystallographic directions. From a comparison of ion channeling in combination with ^{1}H(^{15}N,αγ)^{12}C nuclear reaction analysis and Monte Carlo simulations we show that hydrogen is located in octahedral z sites and quantify its vibrational amplitude of 0.2 Å.

Crystal Structural Investigations for Understanding the Hydrogen Storage Properties of YMgNi4-Based Alloys

The hydrogen storage properties and crystal structures of YMgNi₄-based alloys, which were synthesized from (2 - x)YNi₂ and xMgNi₂ (0.6 ≤ x ≤ 1.2), were investigated by pressure-composition-temperature measurements and powder neutron diffraction at a deuterium gas pressure to understand the hydrogen absorption and desorption reactions viewed from atomic arrangements around H atoms. Reducing the amounts of MgNi₂, which was utilized as a Mg source in YMgNi₄-based alloys, has been observed to lower the hydrogen absorption and desorption pressures and increase the hydrogen storage capacities. However, the reversible hydrogen capacity attained a maximum value of 1.2 mass % at x = 0.8 because of the formation of a thermodynamically stable hydride in which hydrogen was not released at x = 0.6. In the case of x = 0.6, the presence of excessive Y atoms around the H atoms in the hydrogen-absorbed phase would lead to the formation of a hydride with stronger interaction between Y and H because of the affinity between them. Moreover, the presence of small amounts of D atoms with short interatomic D-D distances (1.6 and 1.9 Å) in the deuterium-absorbed phase (Y_0.81Mg_1.19Ni_4.00D_3.35 and Y_1.06Mg_0.94Ni_4.00D_3.86) at <5 MPa and 323 K was proposed by the crystal structural investigations. The D atoms with short D-D interatomic distances were located in the same local atomic arrangements of D atoms in a deuterium-absorbed phase, which were formed at a higher-pressure range, and had higher hydrogen storage capacities than the deuterium-absorbed phases in this study.

Molecular designs for expanding the limits of ultralong C-C bonds and ultrashort HH non-bonded contacts

Recent experiments have reported the formation of very long C-C bonds (d_C-C > 1.80 Å) and very short HH non-bonded contacts (d_HH < 1.5 Å) in several sets of molecules. Both these rare phenomena arise due to specific donor-acceptor interactions and London dispersion interactions respectively. Favorable negative hyperconjugation, namely H₂N(lone-pair) →σ*(C-C), creates an ultralong C-C bond in diamino-o-carborane with d_C-C > 1.829 Å and a planar amine reminiscent of a transition-state like structure for ammonia inversion. The small and narrow barrier favours rapid inversion through quantum mechanical tunnelling (QMT) and produces a translationally averaged planar amine as observed in the experiments. On the other hand, designing specific confined molecular cavities or chambers like in,in-bis(hydrosilane) or its germanane analogs furnishes an ultrashort HH distance = 1.47 Å and 1.38 Å respectively. The predisposition of such closely placed HH contacts arises from the rather effective attractive dispersion interactions between them. Controlling the strength of the dispersion interactions provides a rich landscape for realizing such close HH distances. Molecular design ably assisted by computational modeling to further tune these interactions provides new avenues to break the glass-ceilings of ultralong C-C bonds or ultrashort HH contacts. Dispersion-corrected DFT calculations and ab initio molecular dynamics simulations generate a large library of such unique features in a diverse class of molecules. This feature article highlights the design principles to realize hitherto longest C-C bonds/shortest HH contacts.