Temperature Variation of the Local Structure and Dihydrogen Bonds in Ammonia Borane

Because ammonia borane (AB) exhibits a high hydrogen storage density derived from dihydrogen bonds, it has the potential to be a safe, lightweight, and compact hydrogen storage material. In the tetragonal phase at around room temperature, the hydrogen occupancy is low, and the atomic arrangement and electron density distribution have not been clarified. Therefore, the hydrogen storage properties, including the decomposition reaction mechanism, which proceeds from ∼373 K, also remain unclear. In this study, neutron/X-ray total scattering measurements were performed on AB above and below its phase-transition temperature of 225 K, and the disordered atomic arrangement of hydrogen in the tetragonal phase was clarified by reverse Monte Carlo modeling of the obtained pair distribution function. In addition, the charge of each atom was quantitatively investigated by first-principles calculations and Bader charge analysis. Hydrogen in AB was orderly arranged, forming N-H^δ+···H^δ--B dihydrogen bonds in the low-temperature orthorhombic phase; however, in the room-temperature tetragonal phase, where the hydrogen atoms were disorderly arranged, the distribution of negatively charged hydrido-like H^δ- atoms coordinated to boron extended toward neutral. This situation shows that the dihydrogen bonds are unstable, which may lead to the decomposition of AB.