Crystallization Processes

We unravel crystallization processes across the scales.

Multistep nucleation pathways

The transformation from a fluid to a crystal can be complicated and involve one or more precursors. Such multistep crystallization pathways occur in many chemical and physical systems, and the driving force for the appearance of the precursors has been explained by unique characteristics of the interactions among the system’s building blocks. We investigated multistep crystallization pathways in hard particles, where interaction is dictated solely by building-block shape and thus entropy. We categorized multistep crystallization pathways based on the dimension of the prenucleation motifs and discuss possible comparisons to other crystallization processes.

Multistep Nucleation
Classification of nucleation pathways by dimen-sionality of the prenucleation motifs including realizations of the processes in systems of polyhedra.

Dynamics and stabilization of quasicrystals

A quasiperiodic crystal, or quasicrystal, is a structure that is ordered but not periodic. Shortly after the discovery of quasicrystals in 1982, it was proposed that the atoms are not fully trapped in a single local equilibrium position but can chose dynamically between several minima. Elementary excitations are called phason flips and the associated collective motions are phason modes. We established the existence of phason flips in simulation and proved that the entropy contribution of phason modes is sufficient to stabilize random tiling quasicrystals over similar approximant crystals.

Quasicrystal Dynamics
(Left) Decagonal quasicrystal with Tübingen triangle tiling. Particles are colored according to their energy. (Middle1) Cluster quasicrystal resembling soft matter systems. (Middle2) Particle trajectory involving phason flips. (Right) Diffraction diagrams indicating the continuous transition from a crystal to a quasicrystal involving modulated superstructures.

Dislocations in complex metallic alloys

Complex metallic alloys, characterized by large unit cells, are interesting because they show unusual physical behaviors and properties. One such property is the appearance of metadislocations. These line defects involve a reorganization of the material in an extended region around the dislocation core. We developed a geometric theory and derived an optimized crystal structure with ab initio methods for the description of metadislocations in Al-Pd-Mn alloys.

Metadislocation
(Left) Crystal structure of an ξ’-Al-Pd-Mn intermetallic structure from side and top. (Right) Tiling model around a metadislocation in the center.