Prediction of Growth Habit of β-Cyclotetramethylene-tetranitramine Crystals by the First-Principles Models

Experimentally, β-cyclotetramethylene-tetranitramine (β-HMX) crystals were found to dramatically elongate to the [100] direction when a relatively high supersaturation was imposed. A sudden growth of β-HMX to the [100] direction is closely associated with a mechanistic transition from spiral growth to two-dimensional (2D) nucleation for the (110) face. The onset supersaturation for the growth by 2D nucleation, σ2D, was found to play a key role in the growth of β-HMX. The present simulation results based on first-principles models such as the spiral growth model and the 2D nucleation model show that the values of σ2D on the (101) and (101̅) faces are smaller than those on the (020), (110), and (011) faces. This leads to the prediction of rapid growth rates for the (101) and (101̅) faces by 2D nucleation at low supersaturation and the appearance of a typical shape of β-HMX. On the other hand, the needle-like shape of β-HMX begins to prevail when the supersaturation exceeds the σ2D for the (110) face because its growth mechanism is transformed from the spiral growth mechanism to the 2D nucleation mechanism which accompanies rapid growth of the (110) face. As a result, the present predictions are in remarkable agreement with the experiments. Furthermore, the kinetic Monte Carlo (KMC) simulation also shows that the σ2D for the (110) face is lower than that for the (011) face because the (011) face provides the surface topology on which growth units are unfavorably incorporated into the lattice sites. It evidently shows that the relative positions of σ2D bring on the advent of needle-like growth of β-HMX.