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 α-AlH_3 is found to be the most stable with a hexagonal structure. Recent experimental studies on γ-AlH_3 reported an orthorhombic structure with a unique double-bridge bond between certain Al and H atoms. This was not found in α-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 γ-AlH_3 structure. The current calculation concludes that γ-AlH_3 is less stable than α-AlH_3 by 2.1 KJ/mol. Interesting binding features associated with the unique geometry of γ-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 γ-AlH_3 were attributed to the double-bridge and hexagonal-ring structures.
@article{arxiv.0707.4604,
title = {Electronic and Vibrational Properties of gamma-AlH3},
author = {Yan Wang and Jia-An Yan and M. Y. Chou},
journal= {arXiv preprint arXiv:0707.4604},
year = {2009}
}