Icosahedral twins

An icosahedral twin is an atomic structure found in atomic clusters and also nanoparticles with some thousands of atoms. Their atomic structure is slightly different from what is found for bulk materials, and contains five-fold symmetries. They have been analyzed in many areas of science including crystal growth, crystallography, chemical physics, surface science and materials science, and are sometimes considered as beautiful due to their high symmetry.

The simplest form of these clusters is twenty interlinked tetrahedral crystals joined along triangular (e.g. cubic-(111)) faces, although more complex variants of the outer surface also occur. A related structure has five units similarly arranged with twinning, which were known as "fivelings" in the 19th century, and more recently as "decahedral multiply twinned particles", "pentagonal particles" or "star particles".^: 9–13 A variety of different methods (e.g. condensing metal nanoparticles in argon, deposition on a substrate, wet chemical synthesis) lead to the icosahedral form, and they also occur in virus capsids.

These forms occur at small sizes where they have lower total surface energy than other configurations. This is balanced by an elastic deformation (strain) energy, which dominates at larger sizes. This leads to a competition between different forms as a function of size, and often there is a population of different shapes.