Electronic and Vibrational Properties of gamma-AlH3

TL;DR

This study uses density functional theory to analyze the structural, electronic, and vibrational properties of the gamma polymorph of aluminum hydride, revealing its relative stability and unique bonding features compared to other forms.

Contribution

It provides the first detailed theoretical investigation of gamma-AlH3's properties, highlighting its unique double-bridge bond and vibrational spectrum features.

Findings

Gamma-AlH3 is less stable than alpha-AlH3 by 2.1 KJ/mol.

Unique double-bridge bonds influence electronic and vibrational properties.

Distinct vibrational features are linked to gamma-AlH3's structural motifs.

Abstract

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.