Breaking the H2 chemical bond in a crystalline environment

Through density functional theory and molecular dynamics calculations, we have analysed various metal polyhydrides to understand whether hydrogen is present in its molecular or atomic form - tetrahydrides of Ba,Sr,Ra, Cs and La; Ba$_8$H$_{46}$ and BaH$_{12}$. We show that, in experimentally reported binary barium hydrides (BaH$_x$), molecular H$_2$ and atomic H$^-$ can coexist with the metallic cations. In this thorough study of differences between BaH$_4$, higher barium hydrides, and other binary tetrahydrides we find the number of atomic hydrogens is equal to the formal charge of the cations. The remaining hydrogen forms molecules in proportions yielding, e.g. BaH$_2$(H$_2)_x$, at pressures as high as 200 GPa. At room temperature these are highly dynamic structures with the hydrogens switching between H$^-$ and H$_2$ while retaining the 2:x ratio. We find some qualitative differences between our static DFT calculations and previously reported structural and spectroscopic experimental results. Two factors allow us to resolve such discrepancies: Firstly, in static relaxation H$_2$ must be regarded as a non-spherical object, which breaks symmetry in a way invisible to X-rays; Secondly the required number of molecules $x$ may be incompatible with the experimental space group (e.g. $BaH_2(H_2)_5$). In molecular dynamics, bond-breaking transitions between various structural symmetry configurations happen on a picosecond timescale via an H$_3^-$ intermediate. Rebonding is slow enough to allow a spectroscopic signal but frequent enough to average out over the lengthscale involved in diffraction

Localisation effects on the Vibron Shifts in Helium-Hydrogen Mixtures

The vibrational frequency of hydrogen molecules has been observed to increase strongly with He concentration in helium hydrogen fluid mixtures. This has been associated with He-H interactions, either directly through chemical bonding, or indirectly through increased local pressure. Here, we demonstrate that the increase in the Raman frequency of the hydrogen molecule vibron is due to the number of H$_2$ molecules participating in the mode. There is no chemical bonding between He and H$_2$, helium acts only to separate the molecules. The variety of possible environments for H$_2$ gives rise to many Raman active modes, which causes broadening the vibron band. As the Raman active modes tend to be the lower frequency vibrons, these effects work together to produce the majority of the shift seen in experiment. We used Density Functional Theory (DFT) methods in both solid and fluid phases to demonstrate this effect. DFT also reveals that the pressure in these H$_2$-He mixture is primarily due to quantum nuclear effects, again the weak chemical bonding makes it a secondary effect

Morphological changes in carbon nanohorns under stress: a combined Raman spectroscopy and TEM study

In this work, we present the first study of highly compressed carbon nanohorns (CNHs).</p

High Pressure Insertion of Dense H2 into a Model Zeolite

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