Diamond sandwich makes metallic hydrogen

Hydrogen, the most abundant element in the universe, is commonly found as a clear gas. But squish some hydrogen with an enormous amount of pressure and it will turn into a metal, according to researchers at the Max Planck Institute for Chemistry in Mainz, Germany.

Chemists Mikhail Eremets and Ivan Troyan sandwiched hydrogen between two diamonds and compressed it while carefully monitoring the atoms with a set of lasers and electrodes. To apply the pressure, they used a diamond anvil, which is similar to the machines that crush coal with so much force that it turns into artificial diamonds.

Hydrogen as we know it is clear and it isn't a very good conductor of electricity, but at extremely high pressures it may turn into an opaque metallic material with low electrical resistance.

Scientists first theorized that hydrogen could transform into a metallic substance back in 1935, and they have been trying to make the exotic material ever since. In theory, it could be used as a rocket fuel. Astronomers believe that this substance can be found in bulk beneath the surfaces of large planets like Saturn and Jupiter.

The hydrogen atoms become electrically conductive when they are under about 220 gigapascals of pressure, explained Mikhail Eremets and Ivan Troyan in a report to the journal Nature Materials. That's very similar to the pressure that you would experience within the inner layers of Saturn or the mantle of Jupiter, and ten times higher pressure than what's found at the bottom of the Mariana trench.

Eremets and Troyan are not the first researchers to see evidence that hydrogen can become a metallic substance. Back in 1991, researchers at the Carnegie Institution of Washington noticed that the element becomes opaque at high pressures. And later, researchers at Lawrence Livermore National Laboratory used a shockwave to compress hydrogen and saw that it became electrically conductive.

Other chemists tried cooling hydrogen down to really low temperatures and then squeezing it very hard, and so did the German team. But initially, those experiments didn’t work. When the German researchers switched up their method a bit, things started to click. When they tried filling the diamond anvil with hydrogen at room temperature, instead of cold hydrogen, their experiments started working.

As the pressure between the diamonds increased, Eremets and Troyan watched the hydrogen with a technique called Raman spectroscopy. This gave them a sense of what was going on inside. When the pressure reached 220 gigapascals, the spectrum of the gas changed drastically, meaning that the atoms had rearranged themselves. At the same time, the hydrogen atoms' electrical conductivity increased dramatically.

If the researchers can squeeze hydrogen a little harder, its electrical resistance may disappear completely. In other words, it may be a superconductor. But during their tests, the diamonds broke before they could increase the pressure any further.

The observations that Eremets and Troyan made line up well with what several other researchers have seen, but there is some reason for skepticism. On several occasions, other scientists have crunched hydrogen to even higher pressures and it hasn't become metallic, and also the electrical behavior of the squished hydrogen wasn't perfectly in line with what theorists have predicted.

The German duo acknowledged those discrepancies in their report, and explained that their method for pressurizing the diamond anvil at room temperature may account for the differences. It's quite likely that rival researchers will soon try to imitate that procedure, but until that happens, their findings will remain controversial.