Metallic hydrogen is a state of hydrogen in which it behaves as an electrical conductor. This state was predicted theoretically in 1935, but has not been reliably produced in laboratory experiments due to the requirement of high pressures, on the order of hundreds of kilobars. At these pressures, hydrogen might exist as a liquid rather than solid. Liquid metallic hydrogen is thought to be present in large amounts in the gravitationally compressed interiors of Jupiter, Saturn, and in some of the newly discovered extrasolar planets.
Theoretical predictions 
Metallization of hydrogen under pressure 
Though at the top of the alkali metal column in the periodic table, hydrogen is not, under ordinary conditions, an alkali metal. In 1935 however, physicists Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 25 GPa (250,000 atm or 3,500,000 psi), hydrogen atoms would display metallic properties, losing hold over their electrons. Since then, metallic hydrogen has been described as "the holy grail of high-pressure physics".
The initial prediction about the amount of pressure needed was eventually proven to be too low. Since the first work by Wigner and Huntington, the more modern theoretical calculations were pointing toward higher but nonetheless potentially accessible metallization pressures. Techniques are being developed for creating pressures of up to 500 GPa, higher than the pressure at the center of the Earth, in hopes of creating metallic hydrogen.
Liquid metallic hydrogen 
Helium-4 is a liquid at normal pressure and temperatures near absolute zero, a consequence of its high zero-point energy (ZPE). The ZPE of protons in a dense state is also high, and a decline in the ordering energy (relative to the ZPE) is expected at high pressures. Arguments have been advanced by Neil Ashcroft and others that there is a melting point maximum in compressed hydrogen, but also that there may be a range of densities (at pressures around 400 GPa) where hydrogen may be a liquid metal, even at low temperatures.
In 1968, Ashcroft put forward that metallic hydrogen may be a superconductor, up to room temperature (~290 K), 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.
Possibility of novel types of quantum fluid 
Presently known "super" states of matter are superconductors, superfluid liquids and gases, and supersolids. Egor Babaev predicted that if hydrogen and deuterium have liquid metallic states, they may have ordered states in quantum domains which cannot be classified as superconducting or superfluid in the usual sense. Instead, they may represent two possible novel types of quantum fluids: "superconducting superfluids" and "metallic superfluids". Such fluids were predicted to have highly unusual reactions to external magnetic fields and rotations, which might provide a means for experimental verification of Babaev's predictions. It has also been suggested that, under the influence of magnetic field, hydrogen may exhibit phase transitions from superconductivity to superfluidity and vice-versa.
Lithium doping reduces requisite pressure 
In 2009, Zurek et al. predicted that the alloy LiH6 would be a stable metal at only 1⁄4 of the pressure required to metallize hydrogen, and that similar effects should hold for alloys of type LiHn and possibly other alloys of type ?Lin.
Experimental pursuit before 2011 
Metallization of hydrogen in shock-wave compression 
In March 1996, a group of scientists at Lawrence Livermore National Laboratory reported that they had serendipitously produced, for about a microsecond at temperatures of thousands of kelvins, pressures of over a million atmospheres (>100 GPa) and density of approximately 0.6 g/cm3, the first identifiably metallic hydrogen. The team did not expect to produce metallic hydrogen, as it was not using solid hydrogen, thought to be necessary, and was working at temperatures above those specified by metallization theory. Previous studies in which solid hydrogen was compressed inside diamond anvils to pressures of up to 2,500,000 atm (250 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 employed in guided missile studies, to shoot an impactor plate into a sealed container containing a half-millimeter thick sample of liquid hydrogen. The liquid hydrogen was in contact with wires leading to a device measuring electrical resistance. The scientists found that, as pressure rose to 1,400,000 atm (140 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 fell to 0.3 eV. Because the thermal energy of the fluid (the temperature became about 3,000 K due to compression of the sample) was above 0.3 eV, the hydrogen might be considered metallic.
Other experimental research since 1996 
Many experiments are continuing in the production of metallic hydrogen in laboratory conditions at static compression and low temperature. Arthur Ruoff and Chandrabhas Narayana from Cornell University in 1998, 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 100–300 K, hydrogen is still not a true alkali metal, because of the non-zero band gap. The quest to see metallic hydrogen in laboratory at low temperature and static compression continues. Studies are also undergoing on deuterium. Shahriar Badiei and Leif Holmlid from the University of Gothenburg have shown in 2004 that condensed metallic states made of excited hydrogen atoms (Rydberg matter) are effective promoters to metallic hydrogen.
Experimental results in 2008 
The theoretically predicted maximum of the melting curve (the prerequisite for the liquid metallic hydrogen) was discovered by Shanti Deemyad and Isaac F. Silvera by using pulsed laser heating. Hydrogen-rich alloy SiH4 was metalized and found to be superconducting by M.I. Eremets et al., confirming earlier theoretical prediction by Ashcroft. In this hydrogen rich alloy, even at moderate pressures (because of chemical precompression) the hydrogen forms a sub-lattice with density corresponding to metallic hydrogen. However, the claimed high-pressure metallic and superconducting phase of SiH4 was later identified as platinum hydride, that formed after the decomposition of SiH4.
Metallization of hydrogen in 2011 
Metallic hydrogen in other contexts 
Liquid metallic hydrogen is thought to be present in large 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 indicate 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.
Hydrogen permeation of metals 
As mentioned earlier, pressurized SiH4 forms a metal alloy. It is well known that hydrogen can permeate to a remarkable extent various ordinary metals under conditions of ordinary pressure. In some metals (e.g., lithium) a chemical reaction occurs that produces an ordinary non-metallic chemical compound (lithium hydride). In other cases it is possible that the hydrogen literally alloys itself with the metal (somewhat analogous to mercury amalgam formation). Certainly it is known that many metals remain metallic (e.g., palladium) after absorbing hydrogen—most become brittle, but many ordinary alloys are brittle, too.
In addition, metastable metallic hydrogen (MSMH) would make an efficient fuel and also a clean one, with only water as an end product (if burned in pure oxygen). About twelve times as dense as ordinary liquid molecular hydrogen, when recombining to molecular form it would release about 20 times the energy of hydrogen burning in oxygen. Burned more quickly, it could be a propellant with up to five times the efficiency of liquid H2/O2, the fuel that the Space Shuttle used. Unfortunately, the above-mentioned Lawrence Livermore experiments produced metallic hydrogen too briefly to determine whether metastability is possible.
See also 
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- "Peanut butter diamonds on display". BBC News. 27 June 2007. Retrieved 2010-01-02.
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- Bonev, S.A.; et al. (2004). "A quantum fluid of metallic hydrogen suggested by first-principles calculations". Nature 431 (7009): 669. arXiv:cond-mat/0410425. Bibcode:2004Natur.431..669B. doi:10.1038/nature02968.
- Ashcroft, N.W. (1968). "Metallic Hydrogen: A High-Temperature Superconductor?". Physical Review Letters 21 (26): 1748. Bibcode:1968PhRvL..21.1748A. doi:10.1103/PhysRevLett.21.1748.
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- Nellis, W.J. (2001). "Metastable Metallic Hydrogen Glass". Lawrence Livermore Preprint UCRL-JC-142360. OSTI 15005772. "minimum electrical conductivity of a metal at 140 GPa, 0.6 g/cm3, and 3000 K"
- Weir, S.T.; Mitchell, A.C.; Nellis, W. J. (1996). "Metallization of fluid molecular hydrogen at 140 GPa (1.4 Mbar)". Physical Review Letters 76 (11): 1860. Bibcode:1996PhRvL..76.1860W. doi:10.1103/PhysRevLett.76.1860. "0.28–0.36 mol/cm3 and 2200–4400 K"
- Ruoff, A.L.; et al. (1998). "Solid hydrogen at 342 GPa: No evidence for an alkali metal". Nature 393 (6680): 46. Bibcode:1998Natur.393...46N. doi:10.1038/29949.
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- Badiei, S.; Holmlid, L. (2004). "Experimental observation of an atomic hydrogen material with H–H bond distance of 150 pm suggesting metallic hydrogen". Journal of Physics: Condensed Matter 16 (39): 7017. Bibcode:2004JPCM...16.7017B. doi:10.1088/0953-8984/16/39/034.
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