Turning air into metal with nitrogen, diamonds and lasers

More than three quarters of the air around you is nitrogen gas: specifically, N_two molecules. Nitrogen is incredibly stable in this form, which is why you rarely notice it. It reacts with almost nothing.

Chemists, goblin-like alchemists that they are, see nitrogen stability as a challenge.

Make nitrogen gas something other than N_two It takes a lot of complicated chemistry to pull off, but when it’s successful, it can lead to making high-density fertilizers or fuels out of thin air.

An international team of chemists has just announced a new success in the fight against nitrogen: after six years of work, they have managed to make pure nitrogen do more than just gaseous N_two molecule. Instead, they have created a larger ring-shaped molecule with six nitrogen atoms.

The molecule, N₆^two-, is known as a hexazine ring. It can store and release huge amounts of chemical energy within its bonds, making it an interesting candidate for energy-dense materials.

And all it took to make it was a pair of diamond anvils, laser heating, and air pressure more than 400,000 times the pressure we feel at sea level. Oh, and a little potassium.

The reason for N_twostability is its number of chemical bonds. Nitrogen can form three bonds with itself, and tends to do so given the opportunity. While nitrogen atoms will happily form single bonds with other elements, they tend not to bond with each other in this way. Nitrogen atoms with only one bond between them are much more reactive.

“Low-order NN bonds are difficult to keep stable at low pressure,” says Dr. Yu Wang, a researcher at the Chinese Academy of Sciences’ Hefei Institutes of Physical Sciences and lead author of a paper describing the research, published today. in Chemistry of nature.

Wang and his colleagues had previously misled N_two gas molecules to form a solid nitrogen crystal in a diamond anvil cell: a device that creates extremely high pressure. It uses two anvils made of high quality diamonds, because very little else is hard enough to create the desired pressure. A laser is used to heat the materials inside.

At a pressure of 110 gigapascals (a gigapascal is about 9,869 times normal atmospheric pressure) and a temperature of 2,500 Kelvin (2,227°C), nitrogen gas solidified and formed single bonds, rather than double or triple bonds. But the material was not stable at lower temperatures or pressures.

This time, the researchers started with something a little easier: potassium azide, or KN.₃, at 45 gigapascals. After much trial and error, the researchers were able to convert the KN₃ in N_two^6-which were then stabilized with potassium again.

The resulting molecule, K_twonorth₆it contained the sacred single-bond nitrogen ring.

The compound had a metallic sheen and remained stable at the much lower pressure of 20 gigapascals, still some 200,000 times above room average, but significantly lower than its predecessors.

Wang says the researchers are “very excited” about the result.