Editing Metallic hydrogen (section)

==Theoretical predictions==
[[File:Jupiter diagram.svg|thumb|450px|A diagram of [[Jupiter]] showing a model of the planet's interior, with a rocky [[planetary core|core]] overlaid by a deep layer of liquid metallic hydrogen (shown as magenta) and an outer layer predominantly of [[molecular hydrogen]]. Jupiter's true interior composition is uncertain. For instance, the core may have shrunk as convection currents of hot liquid metallic hydrogen mixed with the molten core and carried its contents to higher levels in the planetary interior. Furthermore, there is no clear physical boundary between the hydrogen layers—with increasing depth the gas increases smoothly in temperature and density, ultimately becoming liquid. Features are shown to scale except for the aurorae and the orbits of the [[Galilean moons]].]]

===Hydrogen under pressure===
Though often placed at the top of the [[alkali metal]] column in the [[periodic table]], hydrogen does not, under ordinary conditions, exhibit the properties of an alkali metal. Instead, it forms [[Diatomic molecule|diatomic]] {{chem2|H2}} molecules, similar to [[halogens]] and some [[Nonmetal (chemistry)|nonmetal]]s in the second period of the periodic table, such as [[nitrogen]] and [[oxygen]]. Diatomic hydrogen is a gas that, at [[atmospheric pressure]], [[Liquid hydrogen|liquefies]] and [[Solid hydrogen|solidifies]] only at very low temperature (20&nbsp;[[kelvin (unit)|K]] and 14&nbsp;K respectively). 

In 1935, physicists [[Eugene Wigner]] and [[Hillard Bell Huntington]] predicted that under an immense [[pressure]] of around {{convert|25|GPa|atm psi|abbr=on}}, hydrogen would display [[metal]]lic properties: instead of discrete {{chem2|H2}} molecules (which consist of two electrons bound between two protons), a bulk phase would form with a solid lattice of protons and the electrons [[Delocalized electron|delocalized]] throughout.<ref name=Wigner1935/> Since then, producing metallic hydrogen in the laboratory has been described as "the holy grail of high-pressure physics".<ref>{{cite press release |date=6 May 1998 |title=High-pressure scientists 'journey' to the center of the Earth, but can't find elusive metallic hydrogen |url=https://www.sciencedaily.com/releases/1998/05/980512080541.htm |publisher=[[ScienceDaily]] |access-date=28 January 2017}}</ref>

The initial prediction about the amount of pressure needed was eventually shown to be too low.<ref>{{cite journal |last1=Loubeyre |first1=P. |display-authors=etal |year=1996 |title=X-ray diffraction and equation of state of hydrogen at megabar pressures |journal=[[Nature (journal)|Nature]] |volume=383 |issue=6602 |pages=702–704 |bibcode=1996Natur.383..702L |doi=10.1038/383702a0|s2cid=4372789 }}</ref> Since the first work by Wigner and Huntington, the more modern theoretical calculations point toward higher but potentially achievable metallization pressures of around {{convert|400|GPa|atm psi|abbr=on}}.<ref>{{cite journal |last1=Azadi |first1=S. |last2=Monserrat |first2=B. |last3=Foulkes |first3=W.M.C. |last4=Needs |first4=R.J. |year=2014 |title=Dissociation of High-Pressure Solid Molecular Hydrogen: A Quantum Monte Carlo and Anharmonic Vibrational Study |journal=[[Physical Review Letters]] |volume=112 |issue=16 |pages=165501 |doi=10.1103/PhysRevLett.112.165501 |pmid=24815656 |arxiv=1403.3681 |bibcode=2014PhRvL.112p5501A|s2cid=28888820 }}</ref><ref>{{cite journal |last1=McMinis |first1=J. |last2=Clay |first2=R.C. |last3=Lee |first3=D. |last4=Morales |first4=M.A. |year=2015 |title=Molecular to Atomic Phase Transition in Hydrogen under High Pressure |journal=[[Physical Review Letters]] |volume=114 |issue=10 |pages=105305 |doi=10.1103/PhysRevLett.114.105305 |pmid=25815944 |bibcode=2015PhRvL.114j5305M|doi-access=free }}</ref>

===Liquid metallic hydrogen===
[[Helium-4]] is a [[liquid helium|liquid]] at [[standard pressure|normal pressure]] near [[absolute zero]], a consequence of its high [[zero-point energy]] (ZPE). The ZPE of protons in a dense state is also high,<ref>{{Cite journal |last=Geng |first=Hua Y. |date=2022-11-17 |title=Full Temperature-Dependent Potential and Anharmonicity in Metallic Hydrogen: Colossal NQE and the Consequences |url=https://pubs.acs.org/doi/10.1021/acs.jpcc.2c05027 |journal=The Journal of Physical Chemistry C |language=en |volume=126 |issue=45 |pages=19355–19366 |doi=10.1021/acs.jpcc.2c05027 |issn=1932-7447|arxiv=2211.14474 }}</ref> 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 might be a range of densities, at pressures around 400 GPa, where hydrogen would be a liquid metal, even at low temperatures.<ref>{{cite journal |last1=Ashcroft |first1=N. W. |year=2000 |title=The hydrogen liquids |journal=[[Journal of Physics: Condensed Matter]] |volume=12 |issue=8A |pages=A129–A137 |bibcode=2000JPCM...12..129A |doi=10.1088/0953-8984/12/8A/314|s2cid=250917368 }}</ref><ref>{{cite journal |last1=Bonev |first1=S. A. |display-authors=etal |year=2004 |title=A quantum fluid of metallic hydrogen suggested by first-principles calculations |journal=[[Nature (journal)|Nature]] |volume=431 |issue=7009 |pages=669–672 |arxiv=cond-mat/0410425 |bibcode=2004Natur.431..669B |doi=10.1038/nature02968 |pmid=15470423|s2cid=4352456 }}</ref>

Geng predicted that the ZPE of protons indeed lowers the melting temperature of hydrogen to a minimum of {{convert|200|to|250|K|C}} at pressures of {{convert|500|-|1500|GPa|atm psi|abbr=on}}.<ref>{{cite journal |last1=Geng |first1=H. Y. |display-authors=etal |year=2015 |title=Lattice stability and high-pressure melting mechanism of dense hydrogen up to 1.5 TPa |journal=[[Physical Review B]] |volume=92 |issue=10 |pages=104103 |doi=10.1103/PhysRevB.92.104103|arxiv=1607.00572 |bibcode=2015PhRvB..92j4103G |s2cid=118358601 }}</ref><ref>{{cite journal |last1=Geng |first1=H. Y. |display-authors=etal |year=2016 |title=Predicted reentrant melting of dense hydrogen at ultra-high pressures |journal=[[Scientific Reports]] |volume=6 |pages=36745 |doi=10.1038/srep36745|pmid=27834405 |pmc=5105149 |arxiv=1611.01418 |bibcode=2016NatSR...636745G }}</ref>

Within this flat region there might be an elemental [[mesophase]] intermediate between the liquid and solid state, which could be [[Metastability|metastably]] stabilized down to low temperature and enter a [[supersolid]] state.<ref>{{cite journal |last1=Geng |first1=H. Y. |display-authors=etal |year=2017 |title=Prediction of a mobile solid state in dense hydrogen under high pressures |journal=[[J. Phys. Chem. Lett.]] |volume=8 |issue=1 |pages=223–228 |doi=10.1021/acs.jpclett.6b02453|pmid=27973848 |arxiv=1702.00211 |s2cid=46843598 }}</ref>

===Superconductivity===
{{Main|Superconductivity}}
{{further|Room-temperature superconductor}}
In 1968, [[Neil Ashcroft]] suggested that metallic hydrogen might be a [[superconductivity|superconductor]], up to [[room temperature]] ({{convert|290|K|C|abbr=on|disp=or}}). This hypothesis is based on an expected strong [[Coupling (physics)|coupling]] between conduction electrons and [[lattice vibration]]s.<ref>{{cite journal |last1=Ashcroft |first1=N. W. |year=1968 |title=Metallic Hydrogen: A High-Temperature Superconductor? |journal=[[Physical Review Letters]] |volume=21 |issue=26 |pages=1748–1749 |bibcode=1968PhRvL..21.1748A |doi=10.1103/PhysRevLett.21.1748}}</ref>

===As a rocket propellant===
[[metastability|Metastable]] metallic hydrogen may have potential as a highly efficient rocket propellant; the metallic form would be stored, and the energy of its decompression and conversion to the diatomic gaseous form when released through a nozzle used to generate thrust, with a theoretical [[specific impulse]] of up to 1700 seconds (for reference, the current most efficient chemical rocket propellants have an {{math|''I''{{sub|sp}}}} less than 500 s<ref name=":3">{{Cite journal |last1=Silvera |first1=Isaac F. |last2=Cole |first2=John W. |date=2010 |title=Metallic Hydrogen: The Most Powerful Rocket Fuel Yet To Exist |url=https://dash.harvard.edu/handle/1/9569212 |journal=Journal of Physics: Conference Series |volume=215 |issue=1 |page=012194 |doi=10.1088/1742-6596/215/1/012194 |bibcode=2010JPhCS.215a2194S |s2cid=250688957 |language=en-US |issn=1742-6596|doi-access=free }}</ref>), although a metastable form suitable for mass-production and conventional high-volume storage may not exist.<ref>{{cite conference|title=Metallic Hydrogen: The Most Powerful Rocket Fuel Yet To Exist|conference=Proceedings of the International Conference on High Pressure Science and Technology|date=July 2009|url=https://dash.harvard.edu/bitstream/handle/1/9569212/Silvera_Metallic.pdf|last1=Silvera|first1=Isaac F.|last2=Cole|first2=John W.|journal=Journal of Physics: Conference Series |volume=215|issue=1|page=012194|doi=10.1088/1742-6596/215/1/012194|bibcode=2010JPhCS.215a2194S|doi-access=free}}</ref><ref>{{cite journal|title=On the lifetime of metastable metallic hydrogen|journal=Low Temperature Physics|volume=43|issue=10|date=29 December 2017|last1=Burmistrov|first1=S.N.|last2=Dubovskii|first2=L.B.|pages=1152–1162|doi=10.1063/1.5008406|arxiv=1611.02593|bibcode=2017LTP....43.1152B|s2cid=119020689}}</ref> Another significant issue is the heat of the reaction, which at over 6000 K is too high for any known engine materials to be used. This would necessitate diluting the metallic hydrogen with water or liquid hydrogen, a mixture that would still provide a significant performance boost over current propellants.<ref name=":3" />

===Possibility of novel types of quantum fluid===
Presently known "super" states of matter are [[superconductor]]s, [[superfluid]] liquids and gases, and [[supersolid]]s. [[Egor Babaev]] predicted that if hydrogen and [[deuterium]] have liquid metallic states, they might have quantum ordered states that cannot be classified as superconducting or superfluid in the usual sense. Instead, they might represent two possible novel types of [[quantum fluid]]s: ''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 a magnetic field, hydrogen might exhibit [[phase transition]]s from superconductivity to superfluidity and vice versa.<ref>{{cite journal |last1=Babaev |first1=E. |last2=Ashcroft |first2=N. W. |year=2007 |title=Violation of the London law and Onsager–Feynman quantization in multicomponent superconductors |journal=[[Nature Physics]] |volume=3 |issue=8 |pages=530–533 |arxiv=0706.2411 |bibcode=2007NatPh...3..530B |doi=10.1038/nphys646|s2cid=119155265 }}</ref><ref>{{cite journal |last1=Babaev |first1=E. |last2=Sudbø |first2=A. |last3=Ashcroft |first3=N. W. |year=2004 |title=A superconductor to superfluid phase transition in liquid metallic hydrogen |journal=[[Nature (journal)|Nature]] |volume=431 |issue=7009 |pages=666–668 |arxiv=cond-mat/0410408 |bibcode=2004Natur.431..666B |doi=10.1038/nature02910 |pmid=15470422|s2cid=4414631 }}</ref><ref>{{cite journal |last1=Babaev |first1=E. |year=2002 |title=Vortices with fractional flux in two-gap superconductors and in extended Faddeev model |journal=[[Physical Review Letters]] |volume=89 |issue=6 |page=067001 |arxiv=cond-mat/0111192 |bibcode=2002PhRvL..89f7001B |doi=10.1103/PhysRevLett.89.067001 |pmid=12190602|s2cid=36484094 }}</ref>

===Lithium alloying reduces requisite pressure===
In 2009, Zurek ''et al.'' predicted that the [[alloy]] {{chem2|LiH6}} would be a stable metal at only one quarter of the pressure required to metallize hydrogen, and that similar effects should hold for alloys of type LiH<sub>''n''</sub> and possibly "other [[Polyhydride|alkali high-hydride systems]]", i.e. alloys of type XH<sub>''n''</sub>, where X is an [[alkali metal]].<ref>{{cite journal |last1=Zurek |first1=E. |display-authors=etal |year=2009 |title=A little bit of lithium does a lot for hydrogen |journal=[[Proceedings of the National Academy of Sciences]] |volume=106 |issue=42 |pages=17640–17643 |bibcode=2009PNAS..10617640Z |doi=10.1073/pnas.0908262106 |doi-access=free |pmc=2764941 |pmid=19805046}}</ref> This was later verified in AcH<sub>8</sub> and [[Lanthanum decahydride|LaH<sub>10</sub>]] with ''T''<sub>c</sub> approaching 270&nbsp;K<ref>{{Cite journal |doi = 10.1063/PT.6.1.20180823b|title = Pressurized superconductors approach room-temperature realm|journal = Physics Today|year = 2018|s2cid = 240297717 | last1=Grant | first1=Andrew | issue=8 | page=30438 | bibcode=2018PhT..2018h0438G }}</ref> leading to speculation that other compounds may even be stable at mere MPa pressures with room-temperature superconductivity.