Metallic hydrogen made in the lab?

[lpetrich]

When ordinary matter is compressed enough, it becomes what is called "degenerate matter". Instead of being bound to individual nuclei, its electrons wander through the material, making a sort of "electron fluid". If the pressure is not as high, then some of the electrons can be bound to each individual nucleus, while the rest form an electron fluid. In fact, there are many materials that are partially degenerate matter at zero pressure. Metals.

As I was thinking about what to say in this post, I looked around and noticed many metal objects surrounding me in my home - doorknobs, kitchen sink fixtures, scissors, spoons, nail clippers, ... here is some partially degenerate matter all around me, something like the interiors of white-dwarf stars, but at essentially zero pressure compared to its internal-structure pressures. The electrons in metals are loose enough to make them conduct heat and electricity much better than nonmetals.

Many other materials are nonmetals -- they are completely non-degenerate. But if they are degenerate at high enough pressure, then one must ask what pressure is high enough to make them degenerate, to make them metallic.

The most discussion of this question, at least outside of those who research high-pressure matter, has been of metallic hydrogen. Has it ever been made metallic? What pressure is necessary for making it do so?

 

[lpetrich]

French scientists claim to have created metallic hydrogen

As they indicate in their study, it is indisputable that "metal hydrogen should exist," thanks to the rules of quantum confinement. Specifically, they indicate that if the electrons of any material are restricted enough in their motion, what is known as the "band gap closure" will eventually take place. In short, any insulator material (like oxygen) should become a conductive metal if it is pressurized enough.

They also explain how two advances made their experiment possible. The first has to do with the diamond anvil setup they used, which had toroidal (donut-shaped) diamond tips instead of flat tips. This allowed the team to push past the previous pressure limit established by other diamond anvils (400 GPa) and get as high as 600 Gpa.

The second innovation involved a new type of infrared spectrometer the research team designed themselves at the Synchrotron SOLEIL facility, which allowed them to measure the sample. Once their hydrogen sample had reached pressures of 425 GPa and temperatures of 80 K (-193 °C; -316 °F), they reported that it began absorbing all the infrared radiation, thereby indicating that they had "closed the band gap."

Journal preprint: [1906.05634] Observation of a first order phase transition to metal hydrogen near 425 GPa notes the problem that "the various claims of its observation over the past 30 years have remained controversial".

Another one: [1907.03198] Comment on: Observation of a first order phase transition to metal hydrogen near 425 GPa -- its authors are skeptical of that claim.

 

[lpetrich]

Researchers find evidence for metallic hydrogen at 425 gigapascals - more recently
noting
Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen | Nature

In the past several years, however, theorists have come to a consensus—their math showed that hydrogen should transition at approximately 425 gigapascals—but a way to generate that much pressure did not exist.

...
The researchers found that hydrogen samples compressed to 425 gigapascals blocked all infrared and visible light and showed optical reflectivity, as well. They suggest their results indicate that hydrogen does become a solid at 425 gigapascals—but they are already planning another test to bolster their findings. They want to repeat the experiment to determine if the sample begins conducting electricity at 425 gigapascals.

Something typical of metals.


Back in 2017, a different team made a similar claim: Claim made for hydrogen 'wonder material' - BBC News Harvard researchers Ranga Dias and Isaac Silvera claim that they have successfully turned hydrogen into a metal by applying a very high pressure to it: 495 gigapascals (GPa) or 4.95 megabar, nearly 5 million times our planet's sea-level atmospheric pressure.

They did it by crushing a tiny sample of hydrogen with a diamond anvil, a common technique for creating high pressures. They then observed how the sample, less than a millimeter across, changed from transparent to opaque to reflective.

The sample was also chilled down to -270 C or 3 K.

Observation of the Wigner-Huntington transition to metallic hydrogen | Science -- the sample was metallic at 495 GPa (4.95 Mbar), close to the prediction of 400 - 500 GPa by quantum-chemistry methods.

 

[lpetrich]

I've seen some claims about metallic hydrogen that seem to me very farfetched. Like that it would be metastable at low pressures. That would only be possible if it had to climb a big energy "hump" to get into a molecular state. I don't know if anyone has ever demonstrated the existence of such a hump. But similar effects do occur, like diamond being metastable at low pressures.


I'll now estimate the pressure corresponding to the energy density of molecular-hydrogen chemical bonds. From Bond Energies - Chemistry LibreTexts, the energy is 432 kJ/mol and from  Hydrogen that the density of solid hydrogen is 0.0763 g/cm3 and that its atomic weight is 1.008. The energy density is equal to 11.4 gigapascals in pressure units.

The molar volume is 13.2 (per atom) and 26.4 (per H2 molecule) cm3.

I've found a paper on the equation of state of lab-compressed hydrogen:
VanStraaten1988.pdf

From the van Straaten paper, at 13 GPa pressure, the H2 molar volume goes down to about 6 cm3, nearly a factor of 4 of compression. The maximum pressure that the experimenters used was 37 GPa. At that pressure, that volume should be 4.3 cm3, over a factor of 6 in compression. Its authors predicted that the metallic transition would occur at 280 GPa or 2.8 megabar.

 

[lpetrich]

I have found it difficult to find stuff on nonmetal-to-metal transitions in general, but I have found some discussions of such transitions for various materials. I'd mentioned  Degenerate matter in my OP -- it's essentially a metallic state. So everything will become a metal if it's compressed enough.

The van Straaten paper mentioned something called the "Herzfeld criterion" for the insulator-metal transition, as it is called, and I found The Metal-Hydrogen System: Basic Bulk Properties - Yuh Fukai - Google Books It states that the transition should occur where the dielectric constant becomes infinite. That constant measures a material's polarizability, and the more its electrons can move, the higher its polarizability will be.

That reference also stated that an insulator-metal transition has been observed in several materials, like I2, HI, BaTe, and CSI.

The Physics of Phase Transitions: Concepts and Applications - Pierre Papon, Jacques Leblond, Paul H.E. Meijer - Google Books has some numbers:

Xe: 1.4 Mbar (140 GPa), CsI: 1.1 Mbar (110 GPa), I2: 170 kbar (17 GPa), monatomic at 210 kbar (21 GPa).

This source mentions another name for the transition: Mott transition.

Phys. Rev. B 37, 2491 (1988) - Metal-insulator transition of elemental iodine under pressure (paywalled, number in abstract), also Metal-insulator transition of elemental iodine under pressure (ResearchGate)

Phys. Rev. B 45, 1896 (1992) - Thermodynamic model of the insulator-metal transition in nickel iodide (paywalled, no numbers)

Phys. Rev. B 21, 1658 (1980) - Condensed xenon at high pressure (predicts metallic transition at 1.3 MBar (130 GPa))

Calculation of the pressure-induced insulator-metal transition of nitrogen - IOPscience (predicted at 1940 kbar (194 GPa))

The pressure-induced insulator-metal transition of solid oxygen - band-structure calculations - IOPscience (paywalled, no numbers in abstract)

First-principles study of solid iodine and bromine under high pressure - IOPscience (paywalled, no numbers in abstract -- discusses metallic bromine)

High-pressure phase transformations in CaH2 - IOPscience (predicted: 138 GPa (1.38 Mbar))

Considering water ice (solid H2O), it has numerous phases ( Ice has a nice diagram), Scientists predict an out-of-this-world kind of ice | Cornell Chronicle t is stable over 1 - 5 terapascals (10 - 50 Mbar), with a metallic transition beyond 4.8 TPa, contrary to earlier predictions of a metallic transition beyond 1.555 TPa.

looked for carbon, but I found Phys. Rev. B 84, 144104 (2011) - Insulator-metal transition of highly compressed carbon disulfide (predicted: 50 GPa (0.5 Mbar)).

Pressure-Induced Insulator-Metal Transition - Springer (review article, paywalled)

Metallic helium in massive planets (Jupiter's center pressure: around 40 MBar (4 TPa)).
noting Fluid helium at conditions of giant planetary interiors

Hydrogen becomes metallic at 1.4 Mbar (140 GPa) at temperatures > 10³ K, but > 4 MBar for low temperature (< 300 K).

The same thing happens with helium, it seems.

At 0 K, the transition happens at 13 g/cm³ and 100 Mbar (10 TPa)

At 20,000 K, the transition happens at 6.6 g/cm³ and 30 Mbar (3 TPa)

At boiling point at 1-bar pressure, helium has a density of 0.125 g/cm³. So both hydrogen and helium are very compressed in Jupiter's interior.