Double-Bridge Bonding of Aluminium and Hydrogen in the Crystal Structure of γ-AlH3

Abstract

Aluminum trihydride (alane) is one of the most promising among the prospective solid hydrogen-storage materials, with a high gravimetric and volumetric density of hydrogen. In the present work, the alane, crystallizing in the γ-AlH₃ polymorphic modification, was synthesized and then structurally characterized by means of synchrotron X-ray powder diffraction. This study revealed that γ-AlH₃ crystallizes with an orthorhombic unit cell (space group Pnnm, a = 5.3806(1) Å, b = 7.3555(2) Å, c = 5.77509(5) Å). The crystal structure of γ-AlH₃ contains two types of AlH₆ octahedra as the building blocks. The Al−H bond distances in the structure vary in the range of 1.66−1.79 Å. A prominent feature of the crystal structure is the formation of the bifurcated double-bridge bonds, Al−2H−Al, in addition to the normal bridge bonds, Al−H−Al. This former feature has not been previously reported for Al-containing hydrides so far. The geometry of the double-bridge bond shows formation of short Al−Al (2.606 Å) and Al−H (1.68−1.70 Å) bonds compared to the Al−Al distances in Al metal (2.86 Å) and Al−H distances for Al atoms involved in the formation of normal bridge bonds (1.769−1.784 Å). The crystal structure of γ-AlH₃ contains large cavities between the AlH₆ octahedra. As a consequence, the density is 11% less than for α-AlH₃.