Atomic Simulations of Alane Phase Transformations and Dehydrogenation Mechanisms

Density functional theory atomic ground state, molecular dynamics, and direct method lattice dynamic simulations were used to mechanistically probe phase transformations between the various crystallographically refined $\alpha $, $\alpha $'$_{, }\beta $, and $\gamma $ AlH$_{3}$ phases. Lattice dynamic predictions of the AlH$_{3}$ structures provided an ideal test case for systematically accessing the accuracy of the vibrational thermodynamic property contributions with the harmonic approximation. The predicted transformation pathways involved coordinated tilting and rotation mechanisms, similar to that observed in perovskite structures. Further simulations were conducted to elucidate the mechanism for $\alpha $ AlH$_{3}$ phase decomposition to the Al and H$_{2}$ products and to identify probable barriers to reversible rehydrogenation.