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Metallic hydrogen results when hydrogen is sufficiently compressed and undergoes a phase change; it is an example of degenerate matter.

Metallic hydrogen consists of a crystal lattice of atomic nuclei (namely, protons...), with a spacing which is significantly smaller than a Bohr radius. Indeed, the spacing is more comparable with an electron wavelength (see De Broglie wavelength). The electrons are unbound and behave like the conduction electrons in a metal.

Contents 1 History 1.1 Predictions 1.2 Discovery 1.3 Contemporary research 2 Astrophysics 3 Applications 3.1 Nuclear power 3.2 Fuel 3.3 Superconductivity 4 References

Though topping the Periodic Table's alkali metal column, hydrogen is not, under ordinary conditions, an alkali metal. In 1935, however, physicists Eugene Wigner and H.B. Huntington predicted that under an immense pressure of two-hundred and fifty thousand atmospheres (~ 25 GPa), hydrogen atoms would indeed join their first-group kin, relinquishing their proprietary hold over their electrons [1]. The pressures necessary made experimental proof elusive, but their prediction, as to the pressure, was proven to be too low. [2]

In March of 1996, however, a group of scientists at Lawrence Livermore National Laboratory reported that they had serendipitously produced, for about a microsecond and at temperatures of thousands of kelvins and pressures of over a million atmospheres (>100 GPa), the first identifiably metallic hydrogen [3].

The Lawrence Livermore team did not expect to produce metallic hydrogen, as they were not using solid hydrogen, thought to be necessary, and were working at temperatures above those specified by metallization theory; furthermore, previous studies in which solid hydrogen was compressed inside diamond anvils to pressures of up to 2.5 million atmospheres (~253 GPa), did not confirm detectable metallization. The team had sought simply to measure the less extreme electrical conductivity changes which were expected to occur.

The researchers used a 1960s-era light gas gun, originally used in guided missile studies, to shoot an impactor-plate into a sealed container containing a half-millimetre thick sample of liquid hydrogen. First, at one end of the gun, the hydrogen was cooled to about 20 K inside a container which included a battery connected by wire to a Rogowski coil and an oscilloscope; the wires also made contact with the surface of the liquid-hydrogen in several places, so that the apparatus could be used to measure and record the electrical conductivity. At the opposite end, up to 3 kg (7 lb) of gunpowder was ignited and the resulting explosion pushed a piston through a pump tube, compressing the gas inside. Eventually, the hydrogen-gas reached a pressure high enough to throw a valve at the far end of the chamber. Entering the thin barrel, it propelled the plastic-covered metal impactor plate into the container at up to 8 km/s (18,000 mph), thus compressing the hydrogen inside.

The scientists were surpised to find that, as pressure rose to 1.4 million atmospheres (142 GPa), the electronic energy band gap, a measure of electrical resistance, fell to almost zero. The band-gap of hydrogen in its uncompressed state is about 15 eV, making it an "insulator" but, as the pressure increases significantly, the band-gap gradually falls to 0.3 eV and because the 0.3 eV is provided by the thermal energy of the fluid (the temperature became about 3000 K due to compression of the sample), the hydrogen may, at this point, effectively be considered metallic.

Many experiments are continuing in the production of metallic hydrogen in laboratory conditions. Arthur Ruoff and Chandrabhas Narayana from Cornell University in 1998 [4], and later Paul Loubeyre and René LeToullec from Commissariat à l'Énergie Atomique, France in 2002, have shown that at pressures close to those at the center of the Earth (3.2 to 3.4 million atmospheres or 324 to 345 GPa) and temperatures of 100 K–300 K, hydrogen is still not a true alkali metal, because of the non-zero band gap. The quest to see metallic hydrogen in the laboratory continues, over seventy years after its existence was predicted.

It may be noted that a problem with all of the theories of metallic hydrogen is the assumption that what is being dealt with is atomic hydrogen. If the hydrogen is in its molecular state, a possible explanation for the observed conductivity and the increase in density is for the compressed hydrogen gas to have phase-shifted into an ionic lattice state made up of "protonated hydrogen" and hydride ions, both of which are well known species. This lattice would be a good "conductor".

Metallic hydrogen is thought to be present in tremendous amounts in the gravitationally compressed interiors of Jupiter, Saturn, and some of the newly discovered extrasolar planets. Because previous predictions of the nature of those interiors had taken for granted metallization at a higher pressure than the one at which we now know it to happen, those predictions must now be adjusted. The new data indicates much more metallic hydrogen must exist inside Jupiter than previously thought, that it comes closer to the surface, and that therefore, Jupiter's tremendous magnetic field, the strongest of any planet in the solar system is, in turn, produced closer to the surface.

One method of producing nuclear fusion, called inertial confinement fusion, involves aiming laser beams at pellets of hydrogen isotopes. The increased understanding of the behavior of hydrogen in extreme conditions could help to increase energy yields.

It may be possible to produce substantial quantities of metallic hydrogen for practical purposes. The existence has been theorized of a form called 'Metastable Metallic Hydrogen', (abbreviated MSMH) which would not immediately revert to ordinary hydrogen upon the release of pressure.

In addition, 'MSMH' would make an efficient fuel itself and also a "clean" one, with only water as an end product. Nine times as dense as standard hydrogen, it would give off considerable energy when reverting to standard hydrogen. Burned more quickly, it could be a propellant with five times the efficiency of liquid H₂/O₂, the current Space Shuttle fuel. Unfortunately, the 'Lawrence Livermore' experiments produced metallic hydrogen too briefly to determine whether or not metastability is possible. [5]

Theory has been put forward that metallic hydrogen may be a superconductor as high as room temperature (290K), far higher than any other known candidate material. This stems from its extremely high speed of sound and the expected strong coupling between the conduction electrons and the lattice vibrations. [6]

[1] E. Wigner and H. B. Huntington, On the Possibility of a Metallic Modification of Hydrogen J. Chem. Phys. 3, 764 (1935).

[2] P. Loubeyre, R. LeToullec, D. Hausermann, M. Hanfland, R. J. Hemley, H. K. Mao, and L. W. Finger, X-ray diffraction and equation of state of hydrogen at megabar pressures Nature 383, 702 (1996).

[3] S. T. Weir, A. C. Mitchell, and W. J. Nellis, Metallization of Fluid Molecular Hydrogen at 140 GPa (1.4 Mbar) Physical Review Letters 76, 1860 - 1863 (1996).

[4] C. Narayana, H. Luo, J. Orloff, and A. L. Ruoff Solid hydrogen at 342 GPa: no evidence for an alkali metal Nature 393, 46-49 (1998).

[5] W. J. Nellis Metastable Metallic Hydrogen Glass Lawrence Livermore Preprint (1996).

[6]. N. W. Ashcroft Metallic Hydrogen: A High-Temperature Superconductor? Physical Review Letters 21 1748 - 1749 (1968).