Towards Direct Synthesis of Alane: A Predicted Defect-Mediated Pathway Confirmed Experimentally

Alane (AlH3) is a unique energetic material that has not found a broad practical use for over 70 years because it is difficult to synthesize directly from its elements. Using density functional theory, we examine the defect-mediated formation of alane monomers on Al(111) in a two-step process: (1)dissociative adsorption of H-2 and (2)alane formation, which are both endothermic on a clean surface. Only with Ti dopant to facilitate H-2 dissociation and vacancies to provide Al adatoms, both processes become exothermic. In agreement, in situ scanning tunneling microscopy showed that during H-2 exposure, alane monomers and clusters form primarily in the vicinity of Al vacancies and Ti atoms. Moreover, ball milling of the Al samples with Ti (providing necessary defects) showed a 10% conversion of Al into AlH3 or closely related species at 344bar H-2, indicating that the predicted pathway may lead to the direct synthesis of alane from elements at pressures much lower than the 10(4)bar expected from bulk thermodynamics.