Metallization of solid hydrogen: the challenge and possible solutions

Abstract

The search for the means to convert molecular hydrogen to a metal under static conditions at high pressure is reviewed with emphasis on selected recent developments in both experimental studies and theoretical approaches. One approach suggested recently makes use of mixtures of hydrogen and suitable impurities. In these materials hydrogen is perturbed by impurities with the goal of obtaining the metallization of hydrogen at moderate pressures. This approach has also been extensively examined through the use of first-principles methods and we review this recently explored experimental approach and several theoretical studies that have provided an atomic-scale picture of the interaction of hydrogen with impurities under pressure. The objective of this novel approach is to help determine if metallization of hydrogen at pressures is attainable with currently available experimental techniques.

Rubidium Polyhydrides Under Pressure: Emergence of the Linear H3− Species

“…One strategy to achieve metallization at experimentally achievable pressures is to perturb the electronic structure of H 2 by the addition of impurities 18. Doping with an electropositive element can lead to the partial filling of the H 2 σ u * bands and metallicity already at 1 atm as in LiH 6 28.…”

“…But in experiments this does not happen below 342 GPa and temperatures below 100 K. [17] One strategy to achieve metallization at experimentally achievable pressures is to perturb the electronic structure of H 2 by the addition of impurities. [18] Doping with an electropositive element can lead to the partial filling of the H 2 s u * bands and metallicity already at 1 atm as in LiH 6 . [28] Unfortunately, LiH 6 remains unstable with respect to decomposition into H 2 and LiH below 110 GPa.…”

“…At temperatures lower than 100 K, hydrogen turns black at 320 GPa16 and does not become metallic even at pressures approaching those found in the Earth’s core 17. Various attempts to dope hydrogen by an impurity capable of promoting metallization have also been carried out 18. Band‐gap closure and concomitant superconductivity in various metal hydrides such as GeH 4 ,19 SnH 4 ,20 AlH 3 ,21, 22 and GaH 3 23 have been computed at pressures attainable in diamond anvil cells.…”

“…[17] Various attempts to dope hydrogen by an impurity capable of promoting metallization have also been carried out. [18] Band-gap closure and concomitant superconductivity in various metal hydrides such as GeH 4 , [19] SnH 4 , [20] AlH 3 , [21,22] and GaH 3 [23] have been computed at pressures attainable in diamond anvil cells. Compression not only affects the electronic structure and proper-ties of extended systems, but it also modifies their chemistry: it may entice elements which would not normally combine to do so, [24] or to mix in nontraditional proportions.…”

The Search for Superconductivity in High Pressure Hydrides

“…Pressure can coerce compounds to assume stoichiometries and geometric arrangements that would not be accessible at atmospheric conditions. [6][7][8][9][10][11][12][13][14][15][16][17] Because experimental trial-and-error high pressure syntheses can be expensive to carry out and the results difficult to analyze, it is desirable to predict which elemental combinations and pressures could be used to synthesize compounds with useful properties. However, neither chemical intuition nor data-mining techniques are typically useful for these purposes because they have been developed based upon information gathered at atmospheric conditions.…”