Turning air into metal – using diamonds and lasers

More than three quarters of the air around you is nitrogen gas: specifically, N₂ molecules. Nitrogen is incredibly stable in this form – this is why it’s rarely noticed. It reacts with almost nothing.

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

Getting gaseous nitrogen to be something other than N₂ takes a lot of tricky chemistry to achieve – but when successful, it can lead to making fertilisers, or high-density fuels out of thin air.

An international team of chemists have just announced a new success in the struggle against nitrogen: after six years of work, they’ve managed to make pure nitrogen do something other than be a gaseous N₂ molecule. Instead, they’ve created a bigger, ring-shaped molecule with six nitrogen atoms.

The molecule, N₆^2-, is known as a hexazine ring. It can store and release tremendous amounts of chemical energy within its bonds, making it an interesting candidate for high energy-density materials.

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

The reason for N₂’s stability is its number of chemical bonds. Nitrogen can form three bonds with itself, and it tends to do this given any opportunity. While nitrogen atoms will happily form single bonds with other elements, they tend not to link to each other like this. Nitrogen atoms with just one bond between them are much more reactive.

“Low-order N-N bonds are hard to keep stable at low pressure,” says Dr Yu Wang, a researcher at the Hefei Institutes of Physical Science of the Chinese Academy of Sciences, and lead author on a paper describing the research, published in Nature Chemistry.

Wang and her colleagues had previously tricked N₂ gas molecules into forming a solid nitrogen crystal in a diamond anvil cell: a device which 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 within.

At a pressure of 110 gigapascals (a gigapascal is roughly 9869 times normal atmospheric pressure), and a temperature of 2500 Kelvin (2227°C), the gaseous nitrogen solidified and formed single, rather than double or triple, bonds. But the material wasn’t stable at lower temperatures or pressures.

This time around, the researchers began with something a little easier: potassium azide, or KN₃, at 45 gigapascals. After a lot of trial and error, the researchers were able to convert the KN₃ into N₂^6-, which they then stabilised with potassium again.

The resulting molecule, K₂N₆, contained the hallowed ring of single-bonded nitrogen.

The compound had a metallic lustre, and was still stable at the much lower pressure of 20 gigapascals – still around 200,000 times above your average room, but significantly lower than its precursors.

Wang says the researchers are “all very excited” with the result.