Hydrogen at extreme pressures (Review Article)

Abstract

Here we review recent experimental and theoretical studies of hydrogen approaching metallization regime. Experimental techniques have made great advances over the last several years making it possible to reach previously unachievable conditions of pressure and temperature and to probe hydrogen at these conditions. Theoretical methods have also greatly improved; exemplified through the prediction of new structural and ordered quantum states. Recently, a new solid phase of hydrogen, phase IV, has been discovered in a high-pressure high-temperature domain. This phase is quite unusual structurally and chemically as it represents an intermediate state between common molecular and monatomic configurations. Moreover, it shows remarkable fluxional characteristics related to its quantum nature, which makes it unique among the solid phases, even of light elements. However, phase IV shows the presence of a band gap and exhibits distinct phonon and libron characteristic of classical solids. The quantum behavior of hydrogen in the limit of very high pressure remains an open question. Prospects of studying hydrogen at more extreme conditions by static and combined static-dynamic methods are also presented. PACS: 64.30.Jk Equations of state of nonmetals; 67.80.F-Solids of hydrogen and isotopes.

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.…”

Hexagonal structure of phase III of solid hydrogen

“…The experimental x-ray data are consistent with a hexagonal space group, but not with the monoclinic space group of C2/c-24. 44 In this work we investigate this discrepancy using DFT methods. 45 We find a new hexagonal structure of high pressure hydrogen of P 6 1 22 symmetry, that is calculated to be more stable than the C2/c-24 structure below pressures of about 200 GPa, once the effects of quantum and thermal motion are incorporated.…”

Molecular to Atomic Phase Transition in Hydrogen under High Pressure

“…It has been claimed that metallic hydrogen may have been observed very recently by Eremets, et al [1]. While these findings have yet to be confirmed and are considered highly controversial, they resulted in a significant increase in attention on this regime of the phase diagram over the last several years [3,[20][21][22][23][24][25]. As a result, a new high pressure molecular phase was discovered at room temperature, phase IV [2,26,27], and the phase boundaries between the various molecular phases have been further clarified [4].…”