Phys. Rev. Lett. 115, 075501 (2015)

Credit: APS

Quasicrystals defy translational periodicity — you can't construct one by adding identical unit cells along three crystallographic directions. So how do they grow?

Keisuke Nagao and colleagues addressed this puzzle by looking at the structural evolution of a well-known quasicrystal former (a decagonal Al–Ni–Co alloy) when heated, in a transmission electron microscope. The authors recorded and analysed a series of high-resolution images over a period of 15 seconds (pictured).

Prior to heating, the sample consisted of many quasicrystalline grains with different orientations. In their in situ, high-temperature imaging experiments, the authors observed one grain 'eating' a neighbouring grain. They found that growth into a defect-free single quasicrystal occurs through error-and-repair events. A so-called phason defect arises because an atom occupies a position prescribed by local interactions; the defects are then corrected through phason relaxation, which can be achieved by a rearrangement of the quasicrystalline tile structure.

Importantly, these observations show that quasicrystals can form via a mechanism independent of nonlocal interactions. The alternative implies that atoms are 'aware' of a global structure-defining force, which is indeed difficult to motivate from a physics point of view.