Non-classical nucleation pathways in stacking-disordered crystals

Abstract

The nucleation of crystals from the liquid melt is often characterized by a competition between different crystalline structures or polymorphs, and can result in nuclei with heterogeneous compositions. These mixed-phase nuclei can display nontrivial spatial arrangements, such as layered and onion-like structures, whose composition varies according to the radial distance, and which so far have been explained on the basis of bulk and surface free-energy differences between the competing phases. Here we extend the generality of these non-classical nucleation processes, showing that layered and onion-like structures can emerge solely based on structural fluctuations even in absence of free-energy differences. We consider two examples of competing crystalline structures, hcp and fcc forming in hard spheres, relevant for repulsive colloids and dense liquids, and the cubic and hexagonal diamond forming in water, relevant also for other group 14 elements such as carbon and silicon. We introduce a novel structural order parameter that combined with a neural network classification scheme allows us to study the properties of the growing nucleus from the early stages of nucleation. We find that small nuclei have distinct size fluctuations and compositions from the nuclei that emerge from the growth stage. The transition between these two regimes is characterized by the formation of onion-like structures, in which the composition changes with the distance from the center of the nucleus, similarly to what seen in two-step nucleation process.

A simple solution to the problem of self-assembling cubic diamond crystals

“…[11][12][13][14][15][16][17][18] The second common problem with the assembly process is that during the nucleation process different crystalline phases (called polymorphs) can nucleate. 19,20 Systems that can assemble a CD lattice are often found to be able to assemble into the hexagonal diamond (HD) lattice, resulting in imperfect crystals with defects and stacking faults. Avoiding or minimizing the formation of polymorphs requires specific solutions: to avoid the formation of stacking faults in CD formations usually requires either torsional interactions 21,22 or hierarchical assembly.…”

A simple solution to the problem of self-assembling cubic diamond crystals

“…The second common problem with the assembly process is that during the nucleation process different crystalline phases (called polymorphs) can nucleate [19,20]. Systems that can assemble a CD lattice are often found to be able to assemble into the hexagonal diamond (HD) lattice, resulting in imperfect crystals with defects and stacking faults.…”

Crystal polymorph selection mechanism of hard spheres hidden in the fluid

“…One therefore might expect to find an approximately 50% occurrence of fcc-and hcp-like particles in the crystal nucleus of hard spheres. However this prediction is not realised either in experiments [1,3,6,19,22] nor in simulations [5,[23][24][25][26], which both show a hitherto unexplained predominance of fcc particles in the final crystal phase. In this Letter, we investigate, using Molecular Dynamics (MD) simulations and particle-resolved studies of colloids, the early stages of nucleation of hard spheres in order to shed light on the selection mechanism of the crystal polymorph.…”