Researchers Study Suppression of Crystallization by Five-Fold Symmetry

Sir Charles Frank and the icosahedron: five-fold symmetry suppresses crystallization. (Credit: University of Bristol, HH Wills Physics Laboratory)

Using a revolutionary computer simulation, the University of Bristol researchers have tested a theory that was devised in the 1950s. The theory explains how atoms suppress crystallization when they arrange themselves into 3D pentagons.

This theory was devised by Sir Charles Frank, a famous Bristol physicist, and has been setting a firm foundation for metallic glass development right from high-tech aerospace materials to mobile phone covers. However, the mechanism with which 3D pentagons stop the formation of crystal nuclei is still not known.

Since metallic glasses comprise of various advantageous properties of standard metals and are hard and tough at the same time, they possess the potential to revolutionize various commercial applications. This is mainly because the atoms are frozen into a tangled, complex structure, and as a result are disordered.

This is unlike conventional metals that can naturally form well-arranged ordered structures known as crystals. The presence of faults in crystals is the reason for a material to break when stress is applied. Metallic glasses can be stronger because they have no faults between crystal grains.

In order to manufacture these amorphous materials we need to find a way to stop them from forming crystals. This is challenging - decades of research have resulted in a largest sample just 7cm in size. The key question - what is the most effective way of stopping crystallization, remains unsolved.

Dr Patrick Royall, School of Chemistry, University of Bristol

Recently, Drs Taffs and Royall used computer simulation to uncover the mechanism by which fivefold symmetry (3D pentagons) in liquids stops crystallization.

Dr Taffs said: "When a crystal is in contact with its liquid, the atoms at the surface of each phase cannot satisfy their bonding constraints: they are "neither liquid nor solid".

"This means the material must pay energy due to the lack of satisfied bonds at the interface between crystal and liquid, and this surface energy is much higher in the case of liquids with fivefold symmetry."

Liquids crystallize through the spontaneous creation of small crystals, and this process is extremely dependent on the size of the surface energy of the crystals. Because the surface energy is higher when the liquid has fivefold symmetry, nuclei form at a much lower rate. Identifying the mechanism by which crystallization may be suppressed is an important step in the development of metallic glasses, and may open the door to using metallic glass in applications from vehicles to spacecraft.

Dr Patrick Royall, School of Chemistry, University of Bristol

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