Molecular hydrogen is normally a gas at room temperature, but when crushed with diamond anvils, it can convert into a totally different, previously unknown state of matter, according to a team of condensed-matter physicists.
The so-called "phase V," described in the journal Nature, poses a significant step toward finding what's been called the holy grail of high-pressure physics: solid metallic hydrogen.
Hydrogen is the most abundant element in the universe – stars are made almost entirely out of the stuff, with a little helium and traces of heavier chemicals for good measure. It's an essential ingredient in the building blocks of life, an atom necessary to make water and organic molecules. It's extremely lightweight, often found as a gas of molecular hydrogen (two hydrogen atoms bonded together). It's the most basic atom, made up of a single proton and electron, and it has served as an important model for scientists studying physics at smaller scales.
"The hydrogen system is very important to fundamental physics, and [has] paved the way to applied models in the early staged of quantum mechanics," study coauthor Philip Dalladay-Simpson, a high-pressure physicist at the University of Edinburgh, said in an email.
In spite of all this, relatively little is known about hydrogen's behavior in extreme conditions, Dalladay-Simpson said. After all, molecular hydrogen gas is pretty rare in Earth's atmosphere, and at Earth's temperatures and pressures it ventures into no other physical states (such as solid or liquid). That's not the case with other planets such as gas giant Jupiter, which holds enormous amounts of hydrogen under extreme pressures and temperatures.
So, if we want to fully understand the stars and planets around us, we have to have a better fundamental understanding of how hydrogen behaves in distinctly unearthly conditions, the thinking goes.
"Understanding it under these extended regimes can open up windows to large astrophysical bodies," Dalladay-Simpson said, "such as the interiors of the hydrogen-rich Jovian planets such as Jupiter."
We know a little bit about how hydrogen's physical state changes under different conditions. Hydrogen can be liquefied at extremely cold temperatures, and has long been used as liquid rocket fuel. At high temperatures like those found in the corona of the sun, the atom's electrons are stripped from the protons, forming an ionized gas known as plasma.
But for a long time, theorists have predicted that, under extreme pressures but at mild temperatures, hydrogen should actually form a solid – one where the covalent bonds holding hydrogen molecules together break apart and the atoms' electrons roam free, turning the normally clear gas into a shiny, grayish, metallic solid.
Finding this state through actual experiments, however, has proven far more difficult than expected when it was first predicted in 1935, Dalladay-Simpson said. Back then, scientists figured that this state would emerge if molecular hydrogen was put under 25 billion pascals, or 25 gigapascals, of pressure – "an unfathomable pressure for the time," he added.
"Since we've far exceeded 10 times this pressure and it remains experimentally elusive," Dalladay-Simpson said. "As such it has often been dubbed as the 'holy grail' of high-pressure physics."
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To get at this question, Dalladay-Simpson and colleagues took hydrogen molecules and crushed them between two anvils made of diamond, keeping the temperature a balmy 80 degrees Fahrenheit but raising the pressure to 325 gigapascals, equivalent to 3.21 million Earth atmospheres.
"These experiments are highly technically demanding – to reach the pressures desired, we have to use two brilliant-cut diamonds (the same as in your jewelry) but with the tips polished to a very fine point (8 microns, typically the width of a human hair)," Dalladay-Simpson said. "A small amount of hydrogen gas is then trapped between them and pressurized to greater pressures that are found at the center of the Earth, all on a volume of hydrogen comparable to that of a single human red blood cell!"
The scientists found that at these pressures, the structure of the material started to change in significant ways. Though it's hard to say what a chunk of hydrogen in this state would look like, it might resemble layers of molecular hydrogen interspersed with layers of atomic hydrogen. With that in mind, it could well be the precursor to the long-theorized solid metallic state, in which all molecular bonds are broken down.
The next step is to ratchet the pressure up by a few tens of gigapascals to see if they can actually reach the predicted metallic state – which shouldn't be too hard, "considering we reached 400 GPa," Dalladay-Simpson said.
Solid metallic hydrogen might exhibit such far-out properties as superfluidity and superconductivity – and so, if it were ever able to be mass produced, it could have game-changing technological implications, he added.
For example, "current superconducting devices, such as MRI machines, require [a] large amount of cryogenics. A room-temperature superconductor would mean you could reduce the size of these machines significantly and also increase the efficiency of all electronics," he said.
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