Electronic and vibrational properties of gamma-AlH3

Aluminum hydride (alane) AlH3 is an important material in hydrogen storage applications. It is known that AlH3 exists in multiply forms of polymorphs, where alpha-AlH3 is found to be the most stable with a hexagonal structure. Recent experimental studies on gamma-AlH3 reported an orthorhombic structure with a unique double-bridge bond between certain Al and H atoms. This was not found in alpha-AlH3 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-AlH3 structure. The current calculation concludes that gamma-AlH3 is less stable than alpha-AlH3 by 1.2 KJ/mol, with the zero-point energy included. 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-AlH3 were attributed to the double-bridge and hexagonal-ring structures.