  Aluminum hydride (alane) AlH_3 is an important material in hydrogen storage
applications. It is known that AlH_3 exists in multiply forms of polymorphs,
where $\alpha$-AlH_3 is found to be the most stable with a hexagonal structure.
Recent experimental studies on $\gamma$-AlH_3 reported an orthorhombic
structure with a unique double-bridge bond between certain Al and H atoms. This
was not found in $\alpha$-AlH_3 or other polymorphs. Using density functional
theory, we have investigated the energetics, and the structural, electronic,
and phonon vibrational properties for the newly reported $\gamma$-AlH_3
structure. The current calculation concludes that $\gamma$-AlH_3 is less stable
than $\alpha$-AlH_3 by 2.1 KJ/mol. Interesting binding features associated with
the unique geometry of $\gamma$-AlH3 are discussed from the calculated
electronic properties and phonon vibrational modes. The binding of H-s with
higher energy Al-p,d orbitals is enhanced within the double-bridge arrangement,
giving rise to a higher electronic energy for the system. Distinguishable new
features in the vibrational spectrum of $\gamma$-AlH_3 were attributed to the
double-bridge and hexagonal-ring structures.
