Molecular Modeling on Supersaturation-Dependent Growth Habit of 1,1-Diamino-2,2-dinitroethylene

Abstract

In the cooling crystallization of 1,1-diamino-2,2-dinitroethylene (DADNE), it was found that the aspect ratio of crystals decreases as the cooling rate of the solution increases. To reveal the effect of supersaturation on the growth shape of DADNE, molecular modeling was carried out by the step energy calculation and kinetic Monte Carlo (KMC) simulation. The rodlike shape of DADNE with basal {111̅} faces was accurately predicted by the step energy calculation, resulting in remarkable agreement with the experiments. The reason behind the slowest growth rate of the {111̅} faces was found to originate from the high energy barrier in the formation of a 2D nucleus on the crystal face. Furthermore, it was shown that the aspect ratio of DADNE decreases by the lowered free energy of 2D nucleation at high supersaturation, in which the distinctive characteristics on the anisotropic growth behavior of DADNE are blurred. The KMC simulation results also provided an understanding of the growth kinetics of growth units on each crystal face: the {111̅} face shows a lower sticking fraction, which means that the {111̅} face offers the surface topology where growth units are difficult to incorporate into the lattice sites. However, as the supersaturation increases, the crystal faces start to be strongly roughened, and the aspect ratio becomes reduced.