A new way to grow large, defect-free quasicrystals has been developed by researchers in the US. Through a combination of experiments and simulations, Ashwin Shahani and colleagues at the University of Michigan showed how clusters of growing quasicrystals can coalesce to create larger structures, provided they are mostly aligned with each other. The results could pave the way for a new wave of interest in the exotic materials.
A quasicrystal is an arrangement of atoms that has long range order but does not have the translational symmetry possessed by conventional crystals. A mathematical example is Penrose tiling, in which tiles with two different shapes can be arranged to form intricate patterns.
The first quasicrystal material was discovered in the 1980s and as more quasicrystals were found it became clear that the materials have a range of interesting mechanical, thermal and electrical properties that could potentially be used in a wide range of practical applications.
Insurmountable barrier
However, by the 2000s the development of quasicrystal technologies had stalled because of a seemingly insurmountable barrier: it seemed that making suitably large quasicrystals was extremely difficult. When conventional methods such as bulk crystal growth and thin-film deposition are used, large quasicrystal samples have defects, such as grain boundaries, in their atomic structures that invite corrosion. This limits the useful size of a quasicrystal to just a few centimetres.
In their study, Shahani’s team used 3D X-ray tomography to observe the real-time formation of quasicrystals within a molten mixture of aluminium, cobalt and nickel. Within a setup at Argonne National Laboratory, they initially observed the growth of multiple, defect-free quasicrystals in the mixture. As the mixture cooled, the researchers saw that these small quasicrystals collided with each other, then seamlessly coalesced to form a larger, defect free quasicrystals with tenfold rotational symmetry.
Molecular dynamics
Then the team did molecular dynamics simulations to try to understand how defects present when the quasicrystals initially joined together were able to “self heal”during coalescence. By varying the conditions in each run of their simulation, they identified the conditions required for smaller quasicrystals to coalesce, on the same time and length scales they observed in real life.
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When the virtual quasicrystals were slightly misaligned with each other, they rotated into alignment under the influence of quasiparticles called “phasons” – which are associated with atomic rearrangements within quasicrystals. This behaviour did not occur when the quasicrystals were outside a certain range of alignment, and unwanted grain boundaries could form.
Shahani’s team hopes that its new insights into growing quasicrystals could provide valuable guidance in the development of industrial processes that are capable of producing large, high-quality quasicrystals. This may, in turn, lead to a resurgence of commercial interest in the exotic materials, potentially opening new avenues of research into their practical applications.
The research is described in Nature Communications.
