H₂ Chemical Bond in a High-Pressure Crystalline Environment

Abstract

We show that the hydrogen in metal superhydride compounds can adopt two distinct states�atomic and molecular. At low pressures, the maximum number of atomic hydrogens is typically equal to the valency of the cation; additional hydrogens pair to form molecules with electronic states far below the Fermi energy causing low-symmetry structures with large unit cells. At high pressures, molecules become unstable, and all hydrogens become atomic. This study uses density functional theory, adopting BaH 4 as a reference compound, which is compared with other stoichiometries and other cations. Increased temperature and zero-point motion also favor high-symmetry atomic states, and picosecond-timescale breaking and remaking of the bond permutations via intermediate H 3 − units.

Pressure-Induced Metallization of BaH2 and the Effect of Hydrogenation

“…Additionally, modes around 1000 and 1250 cm –1 from hindered rotational modes (rotons) of BaH 4 25 are seen in the Raman spectrum, along with low-frequency Ba–Ba phonons. Upon laser heating the sample at 45 GPa to a temperature of 1200 K, the Raman spectrum resembles that previously reported for Ba 8 H 46 , 27 , 28 in which the low-frequency Ba–Ba phonons and H–H stretching modes of BaH 4 are no longer resolvable. Along with this, the intensity of the H 2 vibron decreases, suggesting that the amount of free molecular hydrogen in the sample chamber is reduced.…”

“…Additionally, it has been predicted that BaH 2 -III is metallic, but this has not been experimentally realized. , Furthermore, when BaH 2 is compressed in a H 2 medium, it incorporates H 2 molecules, forming BaH 4 by 40 GPa. , Here, the bonding nature of H 2 is altered by the pre-compression provided by Ba 2+ , inducing the elongation of the H 2 bond. Not only that, but BaH 2 undergoes further hydrogenation into a Weaire–Phelan barium polyhydride (Ba 8 H 46 ) and forms a clathrate by 50 GPa, which stabilizes tetrahedral cages that fit only one hydrogen atom, thus dissociating the H 2 molecules. ,, It has been proven that such structural motifs can be responsible for the metallic and accompanying superconducting character of hydrides. , The electronic properties of both BaH 4 and Ba 8 H 46 have not been previously studied.…”

“…Upon BaH 4 synthesis, there is an increase in resistance from 128 to 146 kΩ. Unlike BaH 2 , BaH 4 contains molecular hydrogen . The electronic structure of BaH 4 has distinct bands associated with Ba 5p , molecular H 2 , and atomic hydrogen, which gives a band structure of a small-gap semiconductor .…”

“… 12 SrH 22 is predicted to be superconducting at 200 GPa and 21 K, and Sr 3 H 13 is predicted to exhibit superconductivity at 150 GPa and 84 K. 12 Numerous barium polyhydride polymorphs have also been discovered, with Ba 8 H 46 stable at 50 GPa, 27 while BaH 4 , BaH 6 , BaH 10 , and BaH 12 are all stable above 90 GPa. 11 , 25 , 28 Most notably, BaH 12 has been demonstrated to exhibit superconductivity at 20 K and 140 GPa. 11 However, the electrical transport properties of the other barium hydrides have not yet been measured experimentally.…”

“…Not only that, but BaH 2 undergoes further hydrogenation into a Weaire–Phelan barium polyhydride (Ba 8 H 46 ) and forms a clathrate by 50 GPa, which stabilizes tetrahedral cages that fit only one hydrogen atom, thus dissociating the H 2 molecules. 25 , 27 , 28 It has been proven that such structural motifs can be responsible for the metallic and accompanying superconducting character of hydrides. 31 , 32 The electronic properties of both BaH 4 and Ba 8 H 46 have not been previously studied.…”