Metallic hydrogen

A diagram showing the inside of Jupiter
Gas giants such as Jupiter (pictured above) and Saturn might contain large amounts of metallic hydrogen (depicted in grey) and metallic helium.[1]

Metallic hydrogen is a kind of degenerate matter, 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.[2]

At high pressure and temperatures, metallic hydrogen might exist as a liquid rather than a solid, and researchers think it is present in large amounts in the hot and gravitationally compressed interiors of Jupiter, Saturn, and in some extrasolar planets.[3]

In October 2016, there were claims that metallic hydrogen had been observed in the laboratory at a pressure of around 495 gigapascals (4,950,000 bar; 4,890,000 atm; 71,800,000 psi).[4] In January 2017, scientists at Harvard University reported the first creation of metallic hydrogen in a laboratory, using a diamond anvil cell.[5] However, several researchers in the field doubt this claim.[6] Some observations consistent with metallic behavior had been reported earlier, such as the observation of new phases of solid hydrogen under static conditions[7][8] and, in dense liquid deuterium, electrical insulator-to-conductor transitions associated with an increase in optical reflectivity.[9]

Theoretical predictions

Metallization of 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 H2 molecules, analogous to halogens and non-metals in the second row 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 degrees and 14 degrees above absolute zero, respectively). However, Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 25 GPa (250000 atm; 3600000 psi) hydrogen would display metallic properties: instead of discrete 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 throughout.[2] Since then, producing metallic hydrogen in the laboratory has been described as "...the holy grail of high-pressure physics."[10]

The initial prediction about the amount of pressure needed was eventually shown to be too low.[11] Since the first work by Wigner and Huntington, the more modern theoretical calculations were pointing toward higher but nonetheless potentially accessible metallization pressures of 100 GPa and higher.

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, 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 (3,900,000 atm), where hydrogen would be a liquid metal, even at low temperatures.[12][13]

Superconductivity

In 1968, Neil Ashcroft suggested that metallic hydrogen might be a superconductor, up to room temperature (290 K or 17 °C), far higher than any other known candidate material. This hypothesis is based on an expected strong coupling between conduction electrons and lattice vibrations.[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 magnetic field, hydrogen might exhibit phase transitions from superconductivity to superfluidity and vice versa.[15][16][17]

Lithium alloying reduces requisite pressure

In 2009, Zurek et al. predicted that the alloy 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 LiHn and possibly other related alloys of type Lin.[18]

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[19] for about a microsecond at temperatures of thousands of kelvins, pressures of over 1000000 atm (100 GPa), and densities of approximately 0.6 g/cm3.[20] 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 2500000 atm (250 GPa), 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 1400000 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 3000 K or 2730 °C due to compression of the sample) was above 0.3 eV, 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,[21] 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 (3200000–3400000 atm or 320–340 GPa) and temperatures of 100–300 K (−173–27 °C), 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.[22] 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.[23]

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.[24] Hydrogen-rich molecular silane (SiH4) was claimed to be metallized and become superconducting by M.I. Eremets et al..[25] This claim is disputed, and their results have not been repeated.[26][27]

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 2600000–3000000 atm (260–300 GPa).[7] This claim was questioned by other researchers in 2012.[28][29]

Z machine, 2015

In 2015, scientists at the Z Pulsed Power Facility announced the creation of metallic deuterium.[30]

Claimed observation of solid metallic hydrogen, 2016

On October 5, 2016, Ranga Dias and Isaac F. Silvera of Harvard University released claims of experimental evidence that solid metallic hydrogen had been synthesised in the laboratory. This manuscript was available in October 2016,[31] and a revised version was subsequently published in the journal Science in January 2017.[4][5]

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

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 6.7×1023 particles/cm3, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.
— Dias & Silvera (2016) [31]

Silvera stated that they did not repeat their experiment, since more tests could damage or destroy their existing sample, but assured the scientific community that more tests are coming.[32][6]

Shortly after the claim was published in Science, Nature's news division published an article stating that some other physicists regarded the result with skepticism. Recently, prominent members of the high pressure research community have criticised the claimed results, [33] [34] [35] questioning the claimed pressures or the presence of metallic hydrogen at the pressures claimed.

In February 2017, it was reported that the sample of claimed metallic hydrogen was lost, after the diamond anvils it was contained between broke.[36]

See also

References

  1. ↑ Stevenson, D. J. (2008). "Metallic helium in massive planets". Proceedings of the National Academy of Sciences. 105 (32): 11035–11036. Bibcode:2008PNAS..10511035S. PMC 2516209 Freely accessible. doi:10.1073/pnas.0806430105 Freely accessible.
  2. 1 2 Wigner, E.; Huntington, H. B. (1935). "On the possibility of a metallic modification of hydrogen". Journal of Chemical Physics. 3 (12): 764. Bibcode:1935JChPh...3..764W. doi:10.1063/1.1749590.
  3. ↑ Guillot, T.; Stevenson, D. J.; Hubbard, W. B.; Saumon, D. (2004). "Chapter 3: The Interior of Jupiter". In Bagenal, F.; Dowling, T. E.; McKinnon, W. B. Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press. ISBN 0-521-81808-7.
  4. 1 2 Crane, L. (26 January 2017). "Metallic hydrogen finally made in lab at mind-boggling pressure". New Scientist. Retrieved 2017-01-26.
  5. 1 2 Dias, R. P.; Silvera, I. F. (2017). "Observation of the Wigner-Huntington transition to metallic hydrogen". Science. 355: 715–718. arXiv:1610.01634 Freely accessible. doi:10.1126/science.aal1579.
  6. 1 2 Castelvecchi, D. (2017). "Physicists doubt bold report of metallic hydrogen". Nature. 542: 17. doi:10.1038/nature.2017.21379 Freely accessible.
  7. 1 2 Eremets, M. I.; Troyan, I. A. (2011). "Conductive dense hydrogen". Nature Materials. 10 (12): 927–931. Bibcode:2011NatMa..10..927E. doi:10.1038/nmat3175.
  8. ↑ Dalladay-Simpson, P.; Howie, R.; Gregoryanz, E. (2016). "Evidence for a new phase of dense hydrogen above 325 gigapascals". Nature. 529 (7584): 63–67. Bibcode:2016Natur.529...63D. PMID 26738591. doi:10.1038/nature16164.
  9. ↑ Knudson, M.; Desjarlais, M.; Becker, A. (2015). "Direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium". Science. 348 (6242): 1455–1460. Bibcode:2015Sci...348.1455K. doi:10.1126/science.aaa7471.
  10. ↑ "High-pressure scientists 'journey' to the center of the Earth, but can't find elusive metallic hydrogen" (Press release). ScienceDaily. 6 May 1998. Retrieved 2017-01-28.
  11. ↑ Loubeyre, P.; et al. (1996). "X-ray diffraction and equation of state of hydrogen at megabar pressures". Nature. 383 (6602): 702–704. Bibcode:1996Natur.383..702L. doi:10.1038/383702a0.
  12. ↑ Ashcroft, N. W. (2000). "The hydrogen liquids". Journal of Physics: Condensed Matter. 12 (8A): 129. Bibcode:2000JPCM...12..129A. doi:10.1088/0953-8984/12/8A/314.
  13. ↑ Bonev, S. A.; et al. (2004). "A quantum fluid of metallic hydrogen suggested by first-principles calculations". Nature. 431 (7009): 669–672. Bibcode:2004Natur.431..669B. PMID 15470423. arXiv:cond-mat/0410425 Freely accessible. doi:10.1038/nature02968.
  14. ↑ Ashcroft, N. W. (1968). "Metallic Hydrogen: A High-Temperature Superconductor?". Physical Review Letters. 21 (26): 1748–1749. Bibcode:1968PhRvL..21.1748A. doi:10.1103/PhysRevLett.21.1748.
  15. ↑ Babaev, E.; Ashcroft, N. W. (2007). "Violation of the London law and Onsager–Feynman quantization in multicomponent superconductors". Nature Physics. 3 (8): 530–533. Bibcode:2007NatPh...3..530B. arXiv:0706.2411 Freely accessible. doi:10.1038/nphys646.
  16. ↑ Babaev, E.; Sudbø, A.; Ashcroft, N. W. (2004). "A superconductor to superfluid phase transition in liquid metallic hydrogen". Nature. 431 (7009): 666–668. Bibcode:2004Natur.431..666B. PMID 15470422. arXiv:cond-mat/0410408 Freely accessible. doi:10.1038/nature02910.
  17. ↑ Babaev, E. (2002). "Vortices with fractional flux in two-gap superconductors and in extended Faddeev model". Physical Review Letters. 89 (6): 067001. Bibcode:2002PhRvL..89f7001B. PMID 12190602. arXiv:cond-mat/0111192 Freely accessible. doi:10.1103/PhysRevLett.89.067001.
  18. ↑ Zurek, E.; et al. (2009). "A little bit of lithium does a lot for hydrogen". Proceedings of the National Academy of Sciences. 106 (42): 17640–3. Bibcode:2009PNAS..10617640Z. PMC 2764941 Freely accessible. PMID 19805046. doi:10.1073/pnas.0908262106 Freely accessible.
  19. ↑ 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): 1860–1863. Bibcode:1996PhRvL..76.1860W. doi:10.1103/PhysRevLett.76.1860. 0.28–0.36 mol/cm3 and 2200–4400 K
  20. ↑ Nellis, W. J. (2001). "Metastable Metallic Hydrogen Glass" (PDF). Lawrence Livermore Preprint UCRL-JC-142360. OSTI 15005772 Freely accessible. minimum electrical conductivity of a metal at 140 GPa, 0.6 g/cm3, and 3000 K
  21. ↑ Ruoff, A. L.; et al. (1998). "Solid hydrogen at 342 GPa: No evidence for an alkali metal". Nature. 393 (6680): 46–49. Bibcode:1998Natur.393...46N. doi:10.1038/29949.
  22. ↑ Baer, B.J.; Evans, W.J.; Yoo, C.-S. (2007). "Coherent anti-Stokes Raman spectroscopy of highly compressed solid deuterium at 300 K: Evidence for a new phase and implications for the band gap". Physical Review Letters. 98 (23): 235503. Bibcode:2007PhRvL..98w5503B. doi:10.1103/PhysRevLett.98.235503.
  23. ↑ 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): 7017–7023. Bibcode:2004JPCM...16.7017B. doi:10.1088/0953-8984/16/39/034.
  24. ↑ Deemyad, S.; Silvera, I. F (2008). "The melting line of hydrogen at high pressures". Physical Review Letters. 100 (15): 155701. Bibcode:2008PhRvL.100o5701D. PMID 18518124. arXiv:0803.2321 Freely accessible. doi:10.1103/PhysRevLett.100.155701.
  25. ↑ Eremets, M. I.; et al. (2008). "Superconductivity in hydrogen dominant materials: Silane". Science. 319 (5869): 1506–9. Bibcode:2008Sci...319.1506E. PMID 18339933. doi:10.1126/science.1153282.
  26. ↑ Degtyareva, O.; et al. (2009). "Formation of transition metal hydrides at high pressures". Solid State Communications. 149 (39–40): 1583–1586. Bibcode:2009SSCom.149.1583D. arXiv:0907.2128 Freely accessible. doi:10.1016/j.ssc.2009.07.022.
  27. ↑ Hanfland, M.; Proctor, J. E.; Guillaume, C. L.; Degtyareva, O.; Gregoryanz, E. (2011). "High-Pressure Synthesis, Amorphization, and Decomposition of Silane". Physical Review Letters. 106 (9): 095503. Bibcode:2011PhRvL.106i5503H. PMID 21405634. doi:10.1103/PhysRevLett.106.095503.
  28. ↑ Nellis, W. J.; Ruoff, A. L.; Silvera, I. S. (2012). "Has Metallic Hydrogen Been Made in a Diamond Anvil Cell?". arXiv:1201.0407 Freely accessible [cond-mat.other]. no evidence for MH
  29. ↑ Amato, I. (2012). "Metallic hydrogen: Hard pressed". Nature. 486 (7402): 174–176. Bibcode:2012Natur.486..174A. PMID 22699591. doi:10.1038/486174a Freely accessible.
  30. ↑ "Z machine puts the squeeze on metallic deuterium". Chemistry World. Retrieved 2017-01-27.
  31. 1 2 Dias, R.; Silvera, I. F. (2016). "Observation of the Wigner-Huntington Transition to Solid Metallic Hydrogen". arXiv:1610.01634 Freely accessible [cond-mat].
  32. ↑ Lemmonick, S. (27 January 2017). "There's Reason To Be Skeptical About Metallic Hydrogen". Forbes. Retrieved 2017-01-28.
  33. ↑ Goncharov, A.F.; Struzhkin, V. V. (2017). "Comment on Observation of the Wigner-Huntington Transition to Solid Metallic Hydrogen". arXiv:1702.04246 Freely accessible [cond-mat].
  34. ↑ Eremets, M.I.; Drozdov, A. P. (2017). "Comments on the claimed observation of the Wigner-Huntington Transition to Metallic Hydrogen". arXiv:1702.05125 Freely accessible [cond-mat].
  35. ↑ Loubeyre, P.; Occelli, F.; Dumas, P. (2017). "Comment on: Observation of the Wigner-Huntington Transition to Metallic Hydrogen". arXiv:1702.07192 Freely accessible [cond-mat].
  36. ↑ Johnston, Ian (13 February 2017). "World's only piece of a metal that could revolutionise technology has disappeared, scientists reveal". Independent.
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