SOFT QUASICRYSTALS

by Prof. Stephan Förster (Forschungszentrum Jülich)

Monday 4 Jun 2018, 14:30 → 15:30 Europe/Berlin

HS 3 (Physics Department)

Quasicrystals have a unique aperiodic order, which leads to diffraction patterns of 8-, 10-, or 12-fold rotational symmetry. They have been discovered in 1982 by Dan Shechtman. Since then they have been found in more than one hundred binary and ternary metal alloys. Beginning in 2004 there have been reports on quasicrystalline structures formed by soft materials such as dendrimers, polymers and surfactants. This indicates that there might be general principles that favor the formation of quasicrystals, and if this would be the case, quasicrystals could be rationally designed with characteristic length scales up to the submicron range for photonic applications.

We have discovered quasicrystalline phases with 12- and 18-fold rotational symmetry for polymer micellar in water and organic solvents. Their stability range and phase transitions into FCC, BCC, and Frank-Kasper phases as a function of polymer concentration, temperature and molecular weight can be well determined by X-ray and neutron diffraction experiments. From our studies it emerges that a condition for the formation of soft quasicrystals is a soft repulsive interaction potential that facilitates the formation of polytetrahedral micellar arrangements. Using MD-simulations we are able to map the stability regions of a variety of crystalline and quasi-crystalline phases in terms of the micellar volume fraction and core/shell size ratio. The phase diagram for the first time links previous reports of quasicrystals in nanoparticles, polymers and metal alloys as well as MD-simulations to provide a fundamental understanding of the structure and stability of soft quasicrystals.