Aluminum hydride (alane) $\mathrm{Al}{\mathrm{H}}_3$ is an important material in hydrogen storage applications. It is known that $\mathrm{Al}{\mathrm{H}}_3$ exists in multiply forms of polymorphs, where $\alpha \text{−}\mathrm{Al}{\mathrm{H}}_3$ is found to be the most stable with a hexagonal structure. Recent experimental studies on $\gamma \text{−}\mathrm{Al}{\mathrm{H}}_3$ reported an orthorhombic structure with a unique double-bridge bond between certain Al and H atoms. This was not found in $\alpha \text{−}\mathrm{Al}{\mathrm{H}}_3$ or other polymorphs. Using density functional theory, we have investigated the energetics, and the structural, electronic, and phonon vibrational properties for the newly reported $\gamma \text{−}\mathrm{Al}{\mathrm{H}}_3$ structure. The current calculation concludes that $\gamma \text{−}\mathrm{Al}{\mathrm{H}}_3$ is less stable than $\alpha \text{−}\mathrm{Al}{\mathrm{H}}_3$ by $1.2\mathrm{KJ}∕\mathrm{mol}$, with the zero-point energy included. Interesting binding features associated with the unique geometry of $\gamma \text{−}\mathrm{Al}{\mathrm{H}}_3$ are discussed from the calculated electronic properties and phonon vibrational modes. The binding of $\mathrm{H}\text{−}s$ with higher energy $\mathrm{Al}\text{−}p,d$ 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 $\gamma \text{−}\mathrm{Al}{\mathrm{H}}_3$ were attributed to the double-bridge and hexagonal-ring structures.

• Received 26 June 2007

DOI:https://doi.org/10.1103/PhysRevB.77.014101