We finally understand why quasicrystals can exist

Quasicrystals are rare and odd, but researchers have now shown that they can be the most stable configuration for some atoms – and why they can exist at all.

In crystals, atoms form predictable grids, which make them very stable. In glass – both the ordinary kind that makes up drinking glasses and more exotic glasses like obsidian formed in volcanoes – atoms do not follow any order. Glasses are metastable, so a change in their environment like heating, or small impurities from a few stray atoms of the wrong element, can make them become a different kind of matter. Given enough time, anything atomically amorphous enough to be categorised as glass will also eventually crystallise.

But quasicrystals straddle the middle – their atoms are arranged into patterns, but those patterns never repeat – and just how they remain stable has long been a question mark.

Wenhao Sun at the University of Michigan and his colleagues have now used advanced computer simulations to find the answer. They focused on two known quasicrystals, one made from scandium and zinc and the other from ytterbium and cadmium, and simulated a series of larger and larger quasicrystal nanoparticles. At each step, they calculated the quasicrystals’ energy and compared it to the energies the atoms would have in more conventional crystal-like arrangements.

The laws of physics dictate that most stable objects are made of atoms whose collective energy is as low as possible, and that is exactly what the researchers found – the odd quasicrystal was favoured over more common atomic structures because the energy required to maintain it was low.

Sun says this was somewhat unexpected because the comparison with glass often leads physicists to intuit that quasicrystals should be metastable. They were previously difficult to understand because state-of-the-art simulation methods tend to assume perfectly periodic arrangements of atoms, says team member Vikram Gavini at the University of Michigan. The researchers used an innovative computational approach, and their simulations showed that growing quasicrystals in the lab would require very specific conditions, which is not unexpected because they are rarely found in nature.

“Quasicrystals have extraordinary vibrational properties, which link to heat conductivity and thermoelectric effects. With the new method we might be able to study those,” says Peter Brommer at the University of Warwick in the UK. “Maybe the next supermaterial will be discovered not in a lab but on a computer.”