Metallic hydrogen: Difference between revisions

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'''Metallic hydrogen''' is a [[phase (matter)|phase]] of [[hydrogen]] in which it behaves like an [[electrical conductor]]. This phase was predicted in 1935 on theoretical grounds by [[Eugene Wigner]] and [[Hillard Bell Huntington]].<ref name=Wigner1935>{{cite journal |last1=Wigner |first1=E. |last2=Huntington |first2=H. B. |year=1935 |title=On the possibility of a metallic modification of hydrogen |journal=[[Journal of Chemical Physics]] |volume=3 |issue=12 |page=764 |bibcode=1935JChPh...3..764W |doi=10.1063/1.1749590}}</ref>
'''Metallic hydrogen''' is a [[phase (matter)|phase]] of [[hydrogen]] in which it behaves like an [[electrical conductor]]. This phase was predicted in 1935 on theoretical grounds by [[Eugene Wigner]] and [[Hillard Bell Huntington]].<ref name=Wigner1935>{{cite journal |last1=Wigner |first1=E. |last2=Huntington |first2=H. B. |year=1935 |title=On the possibility of a metallic modification of hydrogen |journal=[[Journal of Chemical Physics]] |volume=3 |issue=12 |page=764 |bibcode=1935JChPh...3..764W |doi=10.1063/1.1749590}}</ref>


At [[high pressure]] and temperatures, metallic hydrogen can exist as a partial [[liquid]] rather than a [[solid]], and researchers think it might be present in large quantities in the hot and [[gravitationally compressed]] interiors of [[Jupiter]] and [[Saturn]], as well as in some [[exoplanets]].<ref name="guillot04">{{cite book |last1=Guillot |first1=T. |last2=Stevenson |first2=D. J. |last3=Hubbard |first3=W. B. |last4=Saumon |first4=D. |date=2004 |chapter=Chapter 3: The Interior of Jupiter |editor=Bagenal, Fran |editor2=Dowling, Timothy E. |editor3=McKinnon, William B. |title=Jupiter: The Planet, Satellites and Magnetosphere |publisher=[[Cambridge University Press]] |isbn=978-0-521-81808-7 }}</ref>
At [[high pressure]] and temperatures, metallic hydrogen can exist as a partial [[liquid]] rather than a [[solid]]. It is thought to appear in large quantities in the hot and [[gravitationally compressed]] interiors of [[Jupiter]] and [[Saturn]], as well as in some [[exoplanets]].<ref name="guillot04">{{cite book |last1=Guillot |first1=T. |last2=Stevenson |first2=D. J. |last3=Hubbard |first3=W. B. |last4=Saumon |first4=D. |date=2004 |chapter=Chapter 3: The Interior of Jupiter |editor=Bagenal, Fran |editor2=Dowling, Timothy E. |editor3=McKinnon, William B. |title=Jupiter: The Planet, Satellites and Magnetosphere |publisher=[[Cambridge University Press]] |isbn=978-0-521-81808-7 }}</ref>


==Theoretical predictions==
==Theoretical predictions==
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===Hydrogen under pressure===
===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).  
Though generally placed atop 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>
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>
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 |article-number=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 |article-number=105305 |doi=10.1103/PhysRevLett.114.105305 |pmid=25815944 |bibcode=2015PhRvL.114j5305M|doi-access=free }}</ref>


===Liquid metallic hydrogen===
===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>
[[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>
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 |article-number=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 |article-number=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>
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>
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===As a rocket propellant===
===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" />
[[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 |article-number=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|article-number=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===
===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>
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 |article-number=067001 |arxiv=cond-mat/0111192 |bibcode=2002PhRvL..89f7001B |doi=10.1103/PhysRevLett.89.067001 |pmid=12190602|s2cid=36484094 }}</ref>


===Lithium alloying reduces requisite pressure===
===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.
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 | article-number=30438 | bibcode=2018PhT..2018h0438G }}</ref> leading to speculation that other compounds may even be stable at mere MPa pressures with room-temperature superconductivity.


==Experimental pursuit==
==Experimental pursuit==
===Shock-wave compression, 1996===
===Shock-wave compression, 1996===
In March 1996, a group of scientists at [[Lawrence Livermore National Laboratory]] reported that they had [[serendipity|serendipitously]] produced the first identifiably metallic hydrogen<ref>{{cite journal |last1=Weir |first1=S. T. |last2=Mitchell |first2=A. C. |last3=Nellis |first3=W. J. |year=1996 |title=Metallization of fluid molecular hydrogen at 140&nbsp;GPa (1.4&nbsp;Mbar) |journal=[[Physical Review Letters]] |volume=76 |issue=11 |pages=1860–1863 |bibcode=1996PhRvL..76.1860W |doi=10.1103/PhysRevLett.76.1860 |pmid=10060539 |quote=0.28–0.36&nbsp;mol/cm<sup>3</sup> and 2200–4400&nbsp;K}}</ref> for about a [[microsecond]] at [[temperature]]s of thousands of [[kelvin]]s, pressures of over {{convert|1000000|atm|GPa atm psi|order=out|abbr=on}}, and densities of approximately {{val|0.6|u=g/cm3}}.<ref name="Nellis">{{cite journal |last=Nellis |first=W. J. |year=2001 |title=Metastable Metallic Hydrogen Glass |url=https://e-reports-ext.llnl.gov/pdf/244531.pdf |journal=Lawrence Livermore Preprint UCRL-JC-142360 |osti=15005772 |osti-access=free |quote=minimum electrical conductivity of a metal at 140&nbsp;GPa, 0.6&nbsp;g/cm<sup>3</sup>, and 3000&nbsp;K |access-date=2018-02-24 |archive-url=https://web.archive.org/web/20161229095145/https://e-reports-ext.llnl.gov/pdf/244531.pdf |archive-date=2016-12-29 |url-status=dead }}</ref> 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 anvil]]s to pressures of up to {{convert|2500000|atm|GPa atm psi|order=out|abbr=on}}, did not confirm detectable metallization. The team had sought simply to measure the less extreme [[Electrical resistivity and conductivity|electrical conductivity]] changes they expected. The researchers used a [[1960 in science|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 {{convert|1400000|atm|GPa atm psi|order=out|abbr=on}}, 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 {{val|15|ul=eV}}, making it an [[electrical insulator|insulator]] but, as the pressure increases significantly, the band gap gradually fell to {{val|0.3|u=eV}}. Because the [[thermal energy]] of the fluid (the temperature became about {{convert|3000|K|C|disp=or|abbr=on}} due to compression of the sample) was above {{val|0.3|u=eV}}, the hydrogen might be considered metallic.
In March 1996, a group of scientists at [[Lawrence Livermore National Laboratory]] reported that they had [[serendipity|serendipitously]] produced the first identifiably metallic hydrogen<ref>{{cite journal |last1=Weir |first1=S. T. |last2=Mitchell |first2=A. C. |last3=Nellis |first3=W. J. |year=1996 |title=Metallization of fluid molecular hydrogen at 140&nbsp;GPa (1.4&nbsp;Mbar) |journal=[[Physical Review Letters]] |volume=76 |issue=11 |pages=1860–1863 |bibcode=1996PhRvL..76.1860W |doi=10.1103/PhysRevLett.76.1860 |pmid=10060539 |quote=0.28–0.36&nbsp;mol/cm<sup>3</sup> and 2200–4400&nbsp;K}}</ref> for about a [[microsecond]] at [[temperature]]s of thousands of [[kelvin]]s, pressures of over {{convert|1000000|atm|GPa atm psi|order=out|lk=on|abbr=on}}, and densities of approximately {{val|0.6|u=g/cm3}}.<ref name="Nellis">{{cite journal |last=Nellis |first=W. J. |year=2001 |title=Metastable Metallic Hydrogen Glass |url=https://e-reports-ext.llnl.gov/pdf/244531.pdf |journal=Lawrence Livermore Preprint UCRL-JC-142360 |osti=15005772 |osti-access=free |quote=minimum electrical conductivity of a metal at 140&nbsp;GPa, 0.6&nbsp;g/cm<sup>3</sup>, and 3000&nbsp;K |access-date=2018-02-24 |archive-url=https://web.archive.org/web/20161229095145/https://e-reports-ext.llnl.gov/pdf/244531.pdf |archive-date=2016-12-29 }}</ref> 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 anvil]]s to pressures of up to {{convert|2500000|atm|GPa atm psi|order=out|abbr=on}}, did not confirm detectable metallization. The team had sought simply to measure the less extreme [[Electrical resistivity and conductivity|electrical conductivity]] changes they expected. The researchers used a [[1960 in science|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 {{convert|1400000|atm|GPa atm psi|order=out|abbr=on}}, 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 {{val|15|ul=eV}}, making it an [[electrical insulator|insulator]] but, as the pressure increases significantly, the band gap gradually fell to {{val|0.3|u=eV}}. Because the [[thermal energy]] of the fluid (the temperature became about {{convert|3000|K|C|disp=or|abbr=on}} due to compression of the sample) was above {{val|0.3|u=eV}}, the hydrogen might be considered metallic.


===Other experimental research, 1996–2004===
===Other experimental research, 1996–2004===
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,<ref>{{cite journal |last1=Ruoff |first1=A. L. |display-authors=etal |year=1998 |title=Solid hydrogen at 342&nbsp;GPa: No evidence for an alkali metal |journal=[[Nature (journal)|Nature]] |volume=393 |issue=6680 |pages=46–49 |bibcode=1998Natur.393...46N |doi=10.1038/29949|s2cid=4416578 }}</ref> 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 [[planetary core|center of the Earth]] ({{convert|3200000|-|3400000|atm|GPa|order=flip|disp=or|abbr=on}}) and temperatures of {{convert|100|-|300|K|C|abbr=on}}, 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 ongoing on [[deuterium]].<ref>{{cite journal |last1=Baer |first1=B.J. |last2=Evans |first2=W.J. |last3=Yoo |first3=C.-S. |year=2007 |title=Coherent anti-Stokes Raman spectroscopy of highly compressed solid deuterium at 300&nbsp;K: Evidence for a new phase and implications for the band gap |journal=[[Physical Review Letters]] |volume=98 |issue=23 |page=235503 |bibcode=2007PhRvL..98w5503B |doi=10.1103/PhysRevLett.98.235503|pmid=17677917 |url=https://zenodo.org/record/1233957 }}</ref> 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,<ref>{{cite journal |last1=Badiei |first1=S. |last2=Holmlid |first2=L. |year=2004 |title=Experimental observation of an atomic hydrogen material with H–H bond distance of 150&nbsp;pm suggesting metallic hydrogen |journal=[[Journal of Physics: Condensed Matter]] |volume=16 |issue=39 |pages=7017–7023 |bibcode=2004JPCM...16.7017B |doi=10.1088/0953-8984/16/39/034|s2cid=250885119 }}</ref> however these results are disputed.<ref>{{cite conference | title=Comment on 'Ultradense protium p(0) and deuterium D(0) and their relation to ordinary Rydberg matter: a review' 2019 Physica Scripta 94, 075005 | author=Klavs Hansen | year=2022| arxiv=2207.08133 }}</ref>
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,<ref>{{cite journal |last1=Ruoff |first1=A. L. |display-authors=etal |year=1998 |title=Solid hydrogen at 342&nbsp;GPa: No evidence for an alkali metal |journal=[[Nature (journal)|Nature]] |volume=393 |issue=6680 |pages=46–49 |bibcode=1998Natur.393...46N |doi=10.1038/29949|s2cid=4416578 }}</ref> 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 [[planetary core|center of the Earth]] ({{convert|3200000|-|3400000|atm|GPa|order=flip|disp=or|abbr=on}}) and temperatures of {{convert|100|-|300|K|C|abbr=on}}, 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 ongoing on [[deuterium]].<ref>{{cite journal |last1=Baer |first1=B.J. |last2=Evans |first2=W.J. |last3=Yoo |first3=C.-S. |year=2007 |title=Coherent anti-Stokes Raman spectroscopy of highly compressed solid deuterium at 300&nbsp;K: Evidence for a new phase and implications for the band gap |journal=[[Physical Review Letters]] |volume=98 |issue=23 |article-number=235503 |bibcode=2007PhRvL..98w5503B |doi=10.1103/PhysRevLett.98.235503|pmid=17677917 |url=https://zenodo.org/record/1233957 }}</ref> 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,<ref>{{cite journal |last1=Badiei |first1=S. |last2=Holmlid |first2=L. |year=2004 |title=Experimental observation of an atomic hydrogen material with H–H bond distance of 150&nbsp;pm suggesting metallic hydrogen |journal=[[Journal of Physics: Condensed Matter]] |volume=16 |issue=39 |pages=7017–7023 |bibcode=2004JPCM...16.7017B |doi=10.1088/0953-8984/16/39/034|s2cid=250885119 }}</ref> however these results are disputed.<ref>{{cite conference | title=Comment on 'Ultradense protium p(0) and deuterium D(0) and their relation to ordinary Rydberg matter: a review' 2019 Physica Scripta 94, 075005 | author=Klavs Hansen | year=2022| arxiv=2207.08133 }}</ref>


===Pulsed laser heating experiment, 2008===
===Pulsed laser heating experiment, 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.<ref>{{cite journal |last1=Deemyad |first1=S. |last2=Silvera |first2=I. F |year=2008 |title=The melting line of hydrogen at high pressures |journal=[[Physical Review Letters]] |volume=100 |issue=15 |pages=155701 |arxiv=0803.2321 |bibcode=2008PhRvL.100o5701D |doi=10.1103/PhysRevLett.100.155701 |pmid=18518124|s2cid=37075773 }}</ref> Hydrogen-rich molecular [[silane]] ({{chem2|SiH4}}) was claimed to be metallized and become [[superconductivity|superconducting]] by [[Mikhail Eremets|M.I. Eremets]] ''et al.''.<ref>{{cite journal |last1=Eremets |first1=M. I. |year=2008 |display-authors=etal |title=Superconductivity in hydrogen dominant materials: Silane |journal=[[Science (journal)|Science]] |volume=319 |issue=5869 |pages=1506–1509 |bibcode=2008Sci...319.1506E |doi=10.1126/science.1153282 |pmid=18339933|s2cid=19968896 }}</ref> This claim is disputed, and their results have not been repeated.<ref>{{cite journal |last1=Degtyareva |first1=O. |display-authors=etal |year=2009 |title=Formation of transition metal hydrides at high pressures |journal=[[Solid State Communications]] |volume=149 |issue=39–40 |pages=1583–1586 |arxiv=0907.2128 |bibcode=2009SSCom.149.1583D |doi=10.1016/j.ssc.2009.07.022|s2cid=18870699 }}</ref><ref name="HanflandProctor2011">{{cite journal |last1=Hanfland |first1=M. |last2=Proctor |first2=J. E. |last3=Guillaume |first3=C. L. |last4=Degtyareva |first4=O. |last5=Gregoryanz |first5=E. |year=2011 |title=High-Pressure Synthesis, Amorphization, and Decomposition of Silane |journal=[[Physical Review Letters]] |volume=106 |issue=9 |page=095503 |bibcode=2011PhRvL.106i5503H |doi=10.1103/PhysRevLett.106.095503 |pmid=21405634}}</ref>
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.<ref>{{cite journal |last1=Deemyad |first1=S. |last2=Silvera |first2=I. F |year=2008 |title=The melting line of hydrogen at high pressures |journal=[[Physical Review Letters]] |volume=100 |issue=15 |article-number=155701 |arxiv=0803.2321 |bibcode=2008PhRvL.100o5701D |doi=10.1103/PhysRevLett.100.155701 |pmid=18518124|s2cid=37075773 }}</ref> Hydrogen-rich molecular [[silane]] ({{chem2|SiH4}}) was claimed to be metallized and become [[superconductivity|superconducting]] by [[Mikhail Eremets|M.I. Eremets]] ''et al.''.<ref>{{cite journal |last1=Eremets |first1=M. I. |year=2008 |display-authors=etal |title=Superconductivity in hydrogen dominant materials: Silane |journal=[[Science (journal)|Science]] |volume=319 |issue=5869 |pages=1506–1509 |bibcode=2008Sci...319.1506E |doi=10.1126/science.1153282 |pmid=18339933|s2cid=19968896 }}</ref> This claim is disputed, and their results have not been repeated.<ref>{{cite journal |last1=Degtyareva |first1=O. |display-authors=etal |year=2009 |title=Formation of transition metal hydrides at high pressures |journal=[[Solid State Communications]] |volume=149 |issue=39–40 |pages=1583–1586 |arxiv=0907.2128 |bibcode=2009SSCom.149.1583D |doi=10.1016/j.ssc.2009.07.022|s2cid=18870699 }}</ref><ref name="HanflandProctor2011">{{cite journal |last1=Hanfland |first1=M. |last2=Proctor |first2=J. E. |last3=Guillaume |first3=C. L. |last4=Degtyareva |first4=O. |last5=Gregoryanz |first5=E. |year=2011 |title=High-Pressure Synthesis, Amorphization, and Decomposition of Silane |journal=[[Physical Review Letters]] |volume=106 |issue=9 |article-number=095503 |bibcode=2011PhRvL.106i5503H |doi=10.1103/PhysRevLett.106.095503 |pmid=21405634}}</ref>


===Observation of liquid metallic hydrogen, 2011===
===Observation of liquid metallic hydrogen, 2011===
Line 53: Line 53:


===Claimed observation of solid metallic hydrogen, 2016===
===Claimed observation of solid metallic hydrogen, 2016===
On 5 October 2016, [[Ranga P. Dias|Ranga Dias]] and Isaac F. Silvera of [[Harvard University]] released claims in a pre-print manuscript of experimental evidence that solid metallic hydrogen had been synthesized in the laboratory at a pressure of around {{convert|495|GPa|atm psi}} using a [[diamond anvil cell]].  A revised version was published in ''[[Science (journal)|Science]]'' in 2017.<ref name="DiasArxiv"><!-- deliberate link to arxiv version - don't update to the Science paper -->{{Cite arXiv<!--Deny Citation Bot-->|last1=Dias|first1=R.|last2=Silvera|first2=I. F.|date=2016|title=Observation of the Wigner-Huntington Transition to Solid Metallic Hydrogen|arxiv=1610.01634|class=cond-mat.mtrl-sci<!-- do not add |doi= -->}}</ref><ref name=":1">{{Cite journal |last1=Dias |first1=R. P. |last2=Silvera |first2=I. F. |date=2017 |title=Observation of the Wigner-Huntington transition to metallic hydrogen |journal=[[Science (journal)|Science]] |volume=355 |issue= 6326|pages=715–718 |arxiv=1610.01634 |bibcode=2017Sci...355..715D |doi=10.1126/science.aal1579 |pmid=28126728 |s2cid=52851498}}</ref><ref name=":0">{{Cite news |last=Crane |first=L. |date=26 January 2017 |title=Metallic hydrogen finally made in lab at mind-boggling pressure |newspaper=[[New Scientist]] |url=https://www.newscientist.com/article/2119442-metallic-hydrogen-finally-made-in-lab-at-mind-boggling-pressure/ |access-date=26 January 2017}}</ref>
On 5 October 2016, [[Ranga P. Dias|Ranga Dias]] and Isaac F. Silvera of [[Harvard University]] released claims in a pre-print manuscript of experimental evidence that solid metallic hydrogen had been synthesized in the laboratory at a pressure of around {{convert|495|GPa|atm psi|abbr=on}} using a [[diamond anvil cell]].  A revised version was published in ''[[Science (journal)|Science]]'' in 2017.<ref name="DiasArxiv"><!-- deliberate link to arxiv version - don't update to the Science paper -->{{Cite arXiv<!--Deny Citation Bot-->|last1=Dias|first1=R.|last2=Silvera|first2=I. F.|date=2016|title=Observation of the Wigner-Huntington Transition to Solid Metallic Hydrogen|arxiv=1610.01634|class=cond-mat.mtrl-sci<!-- do not add |doi= -->}}</ref><ref name=":1">{{Cite journal |last1=Dias |first1=R. P. |last2=Silvera |first2=I. F. |date=2017 |title=Observation of the Wigner-Huntington transition to metallic hydrogen |journal=[[Science (journal)|Science]] |volume=355 |issue= 6326|pages=715–718 |arxiv=1610.01634 |bibcode=2017Sci...355..715D |doi=10.1126/science.aal1579 |pmid=28126728 |s2cid=52851498}}</ref><ref name=":0">{{Cite news |last=Crane |first=L. |date=26 January 2017 |title=Metallic hydrogen finally made in lab at mind-boggling pressure |newspaper=[[New Scientist]] |url=https://www.newscientist.com/article/2119442-metallic-hydrogen-finally-made-in-lab-at-mind-boggling-pressure/ |access-date=26 January 2017}}</ref>


In the preprint version of the paper, Dias and Silvera write:
In the preprint version of the paper, Dias and Silvera write:
Line 59: Line 59:
{{Blockquote| text=With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa... the reflectance using a [[Drude Model|Drude free electron model]] to determine the plasma frequency of 30.1&nbsp;eV at ''T''&nbsp;=&nbsp;5.5&nbsp;K, with a corresponding electron carrier density of {{val|6.7|e=23}} particles/cm<sup>3</sup>, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory. | | Dias & Silvera (2016)<ref name=DiasArxiv/>}}
{{Blockquote| text=With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa... the reflectance using a [[Drude Model|Drude free electron model]] to determine the plasma frequency of 30.1&nbsp;eV at ''T''&nbsp;=&nbsp;5.5&nbsp;K, with a corresponding electron carrier density of {{val|6.7|e=23}} particles/cm<sup>3</sup>, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory. | | Dias & Silvera (2016)<ref name=DiasArxiv/>}}


In June 2019 a team at the [[Commissariat à l'énergie atomique et aux énergies alternatives]] (French Alternative Energies & Atomic Energy Commission) claimed to have created metallic hydrogen at around 425GPa.<ref>{{cite journal |last1=Loubeyre |first1=Paul |last2=Occelli |first2=Florent |last3=Dumas |first3=Paul |title=Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen |journal=Nature |date=30 January 2020 |volume=577 |issue=7792 |pages=631–635 |doi=10.1038/s41586-019-1927-3|pmid=31996819 |bibcode=2020Natur.577..631L }}</ref>
In June 2019 a team at the [[Commissariat à l'énergie atomique et aux énergies alternatives]] (French Alternative Energies & Atomic Energy Commission) claimed to have created metallic hydrogen at around {{convert|425|GPa|atm|abbr=on}}.<ref>{{cite journal |last1=Loubeyre |first1=Paul |last2=Occelli |first2=Florent |last3=Dumas |first3=Paul |title=Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen |journal=Nature |date=30 January 2020 |volume=577 |issue=7792 |pages=631–635 |doi=10.1038/s41586-019-1927-3|pmid=31996819 |bibcode=2020Natur.577..631L }}</ref>


W. Ferreira et al. (including Dias and Silvera) repeated their experiments multiple times after the Science article was published, finally publishing in 2023 and finding metallisation of hydrogen between {{convert|477 and 491|GPa|atm}}. This time, the pressure was released to assess the question of metastability. Metallic hydrogen was not found to be metastable to zero pressure.<ref>{{cite journal |last1=Ferreira |first1=W. C. |last2=Møller |first2=M. |last3=Linsuain |first3=K. |last4=Song |first4=J. |last5=Salamat |first5=A. |last6=Dias |first6=R. |last7=Silvera |first7=I. F. |title=Metallic hydrogen: Study of metastability |journal=APL Materials |date=1 December 2023 |volume=11 |issue=12 |doi=10.1063/5.0178261|bibcode=2023APLM...11l1116F |doi-access=free }}</ref>
W. Ferreira et al. (including Dias and Silvera) repeated their experiments multiple times after the Science article was published, finally publishing in 2023 and finding metallisation of hydrogen between {{convert|477 and 491|GPa|atm|abbr=on}}. This time, the pressure was released to assess the question of metastability. Metallic hydrogen was not found to be metastable to zero pressure.<ref>{{cite journal |last1=Ferreira |first1=W. C. |last2=Møller |first2=M. |last3=Linsuain |first3=K. |last4=Song |first4=J. |last5=Salamat |first5=A. |last6=Dias |first6=R. |last7=Silvera |first7=I. F. |title=Metallic hydrogen: Study of metastability |journal=APL Materials |date=1 December 2023 |volume=11 |issue=12 |article-number=121116 |doi=10.1063/5.0178261|bibcode=2023APLM...11l1116F |doi-access=free }}</ref>


===Experiments on fluid deuterium at the National Ignition Facility, 2018===
===Experiments on fluid deuterium at the National Ignition Facility, 2018===

Latest revision as of 07:21, 22 October 2025

Template:Short description

Metallic hydrogen is a phase of hydrogen in which it behaves like an electrical conductor. This phase was predicted in 1935 on theoretical grounds by Eugene Wigner and Hillard Bell Huntington.[1]

At high pressure and temperatures, metallic hydrogen can exist as a partial liquid rather than a solid. It is thought to appear in large quantities in the hot and gravitationally compressed interiors of Jupiter and Saturn, as well as in some exoplanets.[2]

Theoretical predictions

File:Jupiter diagram.svg
A diagram of Jupiter showing a model of the planet's interior, with a rocky 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 generally placed atop 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 Template:Chem2 molecules, similar to halogens and some nonmetals in the second period of the periodic table, such as nitrogen and oxygen. Diatomic hydrogen is a gas that, at atmospheric pressure, liquefies and solidifies only at very low temperature (20 K and 14 K respectively).

In 1935, physicists Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around Template:Convert, hydrogen would display metallic properties: instead of discrete Template:Chem2 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 throughout.[1] Since then, producing metallic hydrogen in the laboratory has been described as "the holy grail of high-pressure physics".[3]

The initial prediction about the amount of pressure needed was eventually shown to be too low.[4] Since the first work by Wigner and Huntington, the more modern theoretical calculations point toward higher but potentially achievable metallization pressures of around Template:Convert.[5][6]

Liquid metallic hydrogen

Helium-4 is a liquid at 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,[7] 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.[8][9]

Geng predicted that the ZPE of protons indeed lowers the melting temperature of hydrogen to a minimum of Template:Convert at pressures of Template:Convert.[10][11]

Within this flat region there might be an elemental mesophase intermediate between the liquid and solid state, which could be metastably stabilized down to low temperature and enter a supersolid state.[12]

Superconductivity

Script error: No such module "Labelled list hatnote". Script error: No such module "labelled list hatnote". In 1968, Neil Ashcroft suggested that metallic hydrogen might be a superconductor, up to room temperature (Template:Convert). This hypothesis is based on an expected strong coupling between conduction electrons and lattice vibrations.[13]

As a rocket propellant

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 Template:Math less than 500 s[14]), although a metastable form suitable for mass-production and conventional high-volume storage may not exist.[15][16] 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.[14]

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 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 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 a magnetic field, hydrogen might exhibit phase transitions from superconductivity to superfluidity and vice versa.[17][18][19]

Lithium alloying reduces requisite pressure

In 2009, Zurek et al. predicted that the alloy Template:Chem2 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 LiHn and possibly "other alkali high-hydride systems", i.e. alloys of type XHn, where X is an alkali metal.[20] This was later verified in AcH8 and LaH10 with Tc approaching 270 K[21] leading to speculation that other compounds may even be stable at mere MPa pressures with room-temperature superconductivity.

Experimental pursuit

Shock-wave compression, 1996

In March 1996, a group of scientists at Lawrence Livermore National Laboratory reported that they had serendipitously produced the first identifiably metallic hydrogen[22] for about a microsecond at temperatures of thousands of kelvins, pressures of over Template:Convert, and densities of approximately Template:Val.[23] 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 Template:Convert, did not confirm detectable metallization. The team had sought simply to measure the less extreme electrical conductivity changes they expected. 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 Template:Convert, 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 Template:Val, making it an insulator but, as the pressure increases significantly, the band gap gradually fell to Template:Val. Because the thermal energy of the fluid (the temperature became about Template:Convert due to compression of the sample) was above Template:Val, the hydrogen might be considered metallic.

Other experimental research, 1996–2004

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,[24] 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 (Template:Convert) and temperatures of Template:Convert, 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 ongoing on deuterium.[25] 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,[26] however these results are disputed.[27]

Pulsed laser heating experiment, 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.[28] Hydrogen-rich molecular silane (Template:Chem2) was claimed to be metallized and become superconducting by M.I. Eremets et al..[29] This claim is disputed, and their results have not been repeated.[30][31]

Observation of liquid metallic hydrogen, 2011

In 2011 Eremets and Troyan reported observing the liquid metallic state of hydrogen and deuterium at static pressures of Template:Convert.[32][33] This claim was questioned by other researchers in 2012.[34][35]

Z machine, 2015

In 2015, scientists at the Z Pulsed Power Facility announced the creation of metallic deuterium using dense liquid deuterium, an electrical insulator-to-conductor transition associated with an increase in optical reflectivity.[36][37]

Claimed observation of solid metallic hydrogen, 2016

On 5 October 2016, Ranga Dias and Isaac F. Silvera of Harvard University released claims in a pre-print manuscript of experimental evidence that solid metallic hydrogen had been synthesized in the laboratory at a pressure of around Template:Convert using a diamond anvil cell. A revised version was published in Science in 2017.[38][39][40]

In the preprint version of the paper, Dias and Silvera write:

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With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa... the reflectance using a Drude free electron model to determine the plasma frequency of 30.1 eV at T = 5.5 K, with a corresponding electron carrier density of Template:Val particles/cm3, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.

— Template:Comma separated entries

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In June 2019 a team at the Commissariat à l'énergie atomique et aux énergies alternatives (French Alternative Energies & Atomic Energy Commission) claimed to have created metallic hydrogen at around Template:Convert.[41]

W. Ferreira et al. (including Dias and Silvera) repeated their experiments multiple times after the Science article was published, finally publishing in 2023 and finding metallisation of hydrogen between Template:Convert. This time, the pressure was released to assess the question of metastability. Metallic hydrogen was not found to be metastable to zero pressure.[42]

Experiments on fluid deuterium at the National Ignition Facility, 2018

In August 2018, scientists announced new observations[43] regarding the rapid transformation of fluid deuterium from an insulating to a metallic form below 2000 K. Remarkable agreement is found between the experimental data and the predictions based on quantum Monte Carlo simulations, which is expected to be the most accurate method to date. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[44][45]

See also

References

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