Is hydrogen a metal?
What's shiny and conducts electricity? The answer is usually a metal.
Then, there's hydrogen, a colorless and odorless gas. At least at first glance, it's not an element that suggests metallic properties. Yet, since the late 1800s, physicists have theorized that metallic hydrogen could be created under certain conditions.
These theories gained traction, in part, because metallic hydrogen is found throughout the solar system. The interior of Jupiter, for example, is believed to be metallic hydrogen — the intense pressure inside the gas giant squeezes the gas into a superconductive metal that creates the planet's strong magnetic field. But on Earth, experimental difficulties have made metallic hydrogen elusive for almost a century.
In 1935, physicists Eugene Wigner, a leader in solid state physics, and Hillard Bell Huntington published a paper in The Journal of Chemical Physics proposing that hydrogen could be metallic under high pressures. They hypothesized that this would occur at 25 gigapascals (GPa) — 250,000 times the atmospheric pressure at sea level.
"In reality, it's way, way higher," Eugene Gregoryanz, a professor of physics at the University of Edinburgh who studies extreme conditions, told Live Science. Wigner and Huntington's predictions serve as a lower limit of the high pressure needed to achieve a metallic state, he said.
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Over the years, several research groups have claimed to have created metallic hydrogen, only to have their results explained by poor measurement. A 2017 Harvard University study published in the journal Science claimed to have made metallic hydrogen at 495 GPa, but it triggered skepticism and debate because of concerns about how they calibrated pressure measurements, the models they compared their observations to and a lack of reproducibility. "The only measurements which were presented were four photos made from [an] iPhone," Gregoryanz said.
A 2019 study published in the journal Nature Physics reported semimetallic hydrogen at 350 GPa.
"We squeezed it enormously, almost 20 times in volume," study co-author Mikhail Eremets, an experimental scientist in high-pressure physics, chemistry and materials science at the Max Planck Institute for Chemistry in Germany, told Live Science. These high-pressure experiments only became possible with the creation of the diamond anvil cell.
With this technique, dense hydrogen gas, or liquid gas, is loaded into a small canister and squeezed between two diamonds, the hardest known material. Although hydrogen has just one electron, it naturally forms H2, when two hydrogen atoms are held together by two unpaired electrons, forming a covalent bond. As hydrogen molecules are compressed, the force between the two atoms, like a spring, starts to vibrate. The frequency of these vibrations goes up, meaning the atoms are getting closer to each other.
At this point, the distance between the atoms is so slight that the molecules transition into solid hydrogen. A solid state of hydrogen was achieved in 1979 at a pressure of 5.5 GPa and slightly above room temperature.
But if scientists ratchet up the pressure, a strange thing happens once it goes above 33 GPa: The frequency starts to decrease, meaning the atoms are moving away from each other.
A 1980 study published in the journal Physical Review Letters observed this effect. The researchers calculated that if the pressure continued to increase, the bond between the hydrogen atoms would eventually break, creating a pure alkali metal with a single valence, or outermost electron. Alkali metals are bonded into solids and share their valence electrons to conduct electricity. Hydrogen atoms, on the other hand, naturally form the H2 molecule, which has one of the strongest bonds in chemistry, Gregoryanz told Live Science in an email. Only high pressure — like going over 33 GPa — or low temperatures can break this bond to create an alkali metal. Alkali metals, like lithium and sodium, are located in group one of the periodic table, right below hydrogen. They react with water to form strong bases, or alkalis.
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Over 40 years later, the work is slow going. "It's really very difficult to experiment," Eremets said, because the diamonds sometimes break, the highest pressures can't be reached, or measurements can't be taken on such tiny samples, which are only a few micrometers.
Even still, solid metallic hydrogen is probably not far off, Gregoryanz said. His research group and others have observed a darkening of the hydrogen sample, which suggests the "band gap" is closing. The band gap is the space between the conduction zone and the valence band. In the conduction zone, electrons move freely and create electric current, according to the Energy Education encyclopedia from the University of Calgary.
In metals, the space between the conduction zone and the valence band overlaps and creates electrical conductivity.
"Properties of this metallic state [are] what ... I think is even more interesting than the metal itself," Gregoryanz said. One idea predicts hydrogen will be liquid in its metallic form and could be a superconductor.
New superconductors are important because current options are fragile and only work at extremely low temperatures, Eremets said. Superconductors are essential for modern technologies such as computer chips and MRI machines.
But Gregoryanz doesn't think hydrogen superconductors will be used in industry anytime soon because pure metallic hydrogen is beyond current technological capabilities.
Instead, many scientists including Gregoryanz are focusing their efforts on hydrides, which are made of a metal plus hydrogen. These samples are still tiny, but hydrides actually form superconductors that work under pressures much lower than those required by pure hydrogen. However, these pressures are still too high to use in industry, said Gregoryanz. "But as a physical phenomenon, it's absolutely fascinating," he said.
