Double-Bridge Bonding of Aluminium and Hydrogen in the Crystal Structure of γ-AlH 3
2006; American Chemical Society; Volume: 46; Issue: 4 Linguagem: Inglês
10.1021/ic0617487
ISSN1520-510X
AutoresV.A. Yartys, R.V. Denys, Jan Petter Mæhlen, Christoph Frommen, Maximilian Fichtner, Boris M. Bulychev, Hermann Emerich,
Tópico(s)Superconductivity in MgB2 and Alloys
ResumoAluminum 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.
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