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If you don’t go far enough in chemistry, it’s easy to get the impression that metallicity is an innate property of certain elements. But “metallic” is simply defined as substances with electrons that can move around easily. These electrons give metals properties like good conductivity and an opaque, shiny appearance. But these traits are not exclusive to specific elements; carbon nanotubes can be metallic, and elements like sulfur become metallic under sufficient pressure.
In 1935, scientists predicted that the simplest element, hydrogen, could also become metallic under pressure, and they calculated that it would take 25 GigaPascals to force this transition (each Gigapascal is about 10,000 atmospheres of pressure). That estimate, in the words of the people who have finally made metallic hydrogen, “was way off.” It took until last year for us to reach pressures where the normal form of hydrogen started breaking down into individual atoms—at 380 GigaPascals. Now, a pair of Harvard researchers have upped the pressure quite a bit more, and they have finally made hydrogen into a metal.
All of these high-pressure studies rely on what are called diamond anvils. This hardware places small samples between two diamonds, which are hard enough to stand up to extreme pressure. As the diamonds are forced together, the pressure keeps going up.
Current calculations suggested that metallic hydrogen might require just a slight boost in pressure from the earlier work, at pressures as low as 400 GigaPascals. But the researchers behind the new work, Ranga Dias and Isaac Silvera, discovered it needed quite a bit more than that. In making that discovery, they also came to a separate realization: normal diamonds weren’t up to the task. “Diamond failure,” they note, “is the principal limitation for achieving the required pressures to observe SMH,” where SMH means “solid metallic hydrogen” rather than “shaking my head.”
The team came up with some ideas about what might be causing the diamonds to fail and corrected them. One possibility was surface defects, so they etched all diamonds down by five microns to eliminate these. Another problem may be that hydrogen under pressure could be forced into the diamond itself, weakening it. So they cooled the hydrogen to slow diffusion and added material to the anvil that absorbed free hydrogen. Shining lasers through the diamond seemed to trigger failures, so they switched to other sources of light to probe the sample.
After loading the sample and cranking up the pressure (literally—they turned a handcrank), they witnessed hydrogen’s breakdown at high pressure, which converted it from a clear sample to a black substance, as had been described previously. But then, somewhere between 465 and 495 GigaPascals, the sample turned reflective, a key feature of metals.
The authors have no way of telling whether the metallic substance is a solid or liquid. They expect solid based on theoretical considerations, but all they know for sure is that it’s 15 times denser than hydrogen chilled to 15K, which is what they put into the diamond anvil.
One result they do have is that there was no change in appearance even as they allowed the sample to warm up to 83K. That’s intriguing, because some theoretical work has suggested that metallic hydrogen could be metastable, meaning it will remain metallic even as the pressure and temperature that forced it there is released. That will definitely be something worth checking into in more detail. Other calculations suggest it will be superconducting, but that hasn’t been looked at at all yet.
These sorts of details will probably have to wait until we’ve overcome what the authors term a “looming challenge”—producing metallic hydrogen in sufficient quantities to study it in detail. Still, we’ve waited 80 years just to see the stuff. We can probably afford to be patient for a bit more.
Science, 2017. DOI: 10.1126/science.aal1579 Â (About DOIs).
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