Hydrogen is commonplace — it forms around 75% of all the matter we know about.
Science had concentrated on hydrogen atoms for a very long time, because it is the simplest chemical element known to us. Hydrogen reveals to us a lot of the earliest moments of the universe we live in, before everything started getting complicated and seperate.
One of the things we’ve learned from studying hydrogen is that when in solid form,it behaves like an alkali metal — having one free valence electron,it is expected to behave like other metals with a +1 charge. But we can’t make it behave like a metal. We’ve done a lot of things to hydrogen; by now, we can cool it into a liquid pretty easily. We humans have teased hydrogen atoms into a Bose-Einstein condensate. But until now we still haven’t managed to make it behave like a metal, no matter how we crush or chill it.
What we have managed, as of last week, is to create a new phase of hydrogen — not quite a liquid, not quite a gas.
One moon circles — and has phases
Thank condensed-matter physics for the phase diagram: a chart like this one, which lays out the behavior of hydrogen at different pressures, volumes, and temperatures. Everything else has a phase diagram, too.
For example, water is chemically interesting: the behavior of the hydrogen in water means that there are a dozen different kinds of ice, and not just in a thirty-words-for-snow kind of sense. The bonds that give water molecules the properties they have can change depending on pressure and temperature — and that in turn changes the properties of the water itself.
We just don’t have the requisite pressures on our planet to see metallic hydrogen. To find the stuff, you have to look to the cores of gas giants like Jupiter, or back in time to the first few hundred thousand years after the Big Bang. These are phases of hydrogen that we don’t see in our natural world. Using nearly incompressible diamonds to crush hydrogen atoms together with monumental force, though, is one way we can start to replicate the insane pressures found inside Jupiter. Jupiter is a relatively pedestrian place, here in our own temperate and clement solar system. Comfortable in the crook of one of our unremarkable galaxy’s spiral arms, we live in a rich and calm eddy, far from the extremes we see elsewhere in our still-expanding universe. But not so far away at Jupiter’s core, the conditions are pretty crazy.
The crushing pressure inside Jupiter would fuse it into a star if it was much bigger. And it’s still pretty hot, even if it didn’t start stellar fusion. At the “surface” of Jupiter, inasmuch as Jupiter has a well-distinguished surface that isn’t just a boundary between slightly different densities of fluffy noble-gas slush, the temperature is a balmy ~340K, about 150F. Toward the core, though, there’s a thick mantle of metallic hydrogen responsible for Jupiter’s staggering magnetic field, and the high pressure in the mantle means that the hydrogen atoms have to pack together in a highly ordered state. It gets hotter and the behavior gets stranger, the further down you go.
Heat and high pressure is one way that you can create the conditions for metallic hydrogen. But there’s another way. Low temperature and high pressures are more practical here on Earth, if only because it’s difficult to contain a few very hot atoms of hydrogen when the stuff can diffuse through basically anything you try to bottle it in.
Three researchers from the University of Edinburgh used a diamond anvil cell to apply 388 gigaPascals of pressure to some hydrogen atoms. This is ludicrous pressure — greater than the pressures inside Jupiter’s metallic hydrogen mantle. But the scientists chose to do so at a much cooler temperature — 300 Kelvins, which is pretty close to room temperature here on Earth.
As they dialed up the pressure, the researchers saw spectroscopic evidence of the bonds holding electrons and hydrogen nuclei together begin to disappear. The high pressure forces atoms to crowd in and interact with each other, and when they’re packed firmly enough together the properties of the stuff start to change. The boundaries between phase states aren’t all-or-nothing; under extreme conditions we can observe partial phase changes, in-between states where hydrogen isn’t all the way liquid, but also isn’t all the way gaseous, depending on what the individual particles are actually doing.
Since some signs of bonding were still present under Raman spectroscopy, the team asserts that they’ve found a new phase of hydrogen that exists “between” liquid and gas. The electrons haven’t all been freed from their nuclei, so this may represent an intermediate step in the gas-to-liquid phase transition of hydrogen (condensation from gas to liquid) at high pressure and low temperature. They’re calling it hydrogen V.
One potential use for metallic hydrogen might be as a room-temperature superconductor. Such superconductors have proven elusive in the real world, and the pressure required to create metallic hydrogen might put it beyond our grasp. Current thinking is that it would require approximately 400 gPa of pressure to create metallic hydrogen at room temperature. That’s the equivalent of four million atmospheres of pressure. The research team intends to keep repeating the experiment at higher pressures until either they form metallic hydrogen or the diamond anvil shatters.
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