Binary colloidal crystals: disorder and stacking faults

Crystals formed by two different species of particles show - as a consequence of the huge number of variations of system parameters - an incredibly rich plethora of ordered structures, each of them being characterized by different physical properties. This holds in particular for crystals formed by soft matter particles (such as colloids) as the properties of each of the two species can be tuned in a seemingly unlimited manner via suitable synthesis processes. Thus theoretical tools that allow in a predictive manner to explore the properties of the potentially emerging crystals are urgently required both from the academic as well as in technological point of view. The focus of this project is laid on the evaluation of elastic, thermodynamic, and transport properties of such (colloidal) crystals, whose properties are, similar as in the related one component systems, distinctively influenced by the inevitable crystalline defects; in striking contrast to the single component case, binary systems do show a considerably larger spectrum of crystalline defects (such as antisite defects or concentration variations). Similar as in the preceding project (which was uniquely dedicated to one component systems) we plan to dedicate the planned research work to colloidal crystals, formed by binary mixtures. In the project at hand we will base our cooperation in a well-proven manner on three complementary approaches, namely density functional theory, projection operator formalism, and molecular dynamics simulation. This approach will allow us to predict the above mentioned properties via a bottom-up route, starting from the microscopic level. In an effort to cope with the huge number of binary systems we will focus our investigations on two archetypical model systems: binary mixtures of hard spheres (which are inherently defect-poor) with a particular focus on specific ranges of size ratio, and binary mixtures of ultrasoft particles (which self-assemble into cluster crystals) which are obviously defect rich. We are convinced that our combined efforts will provide a more profound microscopic understanding of how defects influence elastic, thermodynamic, and transport properties of binary crystals in model systems spanning a wide range in the strength of the local disorder. Our joint efforts to investigate the above mentioned phenomena via a bottom-up route is highly original. Furthermore our methodologically complementary approaches provide a solid basis to study the above mentioned properties of the crystals in depth; thus we will be able to provide a profound insight into the mechanisms that are responsible for these material properties. The project will be carried out within the weave funding scheme, involving Prof. Matthias Fuchs (Universität Konstanz), Prof. Martin Oettel (Universität Tübingen) and Prof. Gerhard Kahl (TU Wien) and their respective research groups.