title: Double-bridge bonding of aluminium and hydrogen in the crystal structure of gamma-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 gamma-AlH3 polymorphic modification, was synthesized and then structurally characterized by means of synchrotron X-ray powder diffraction. This study revealed that gamma-AlH3 crystallizes with an orthorhombic unit cell (space group Pnnm, a = 5.3806(1) A, b = 7.3555(2) A, c = 5.77509(5) A). The crystal structure of gamma-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 A. 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 A) and Al-H (1.68-1.70 A) bonds compared to the Al-Al distances in Al metal (2.86 A) and Al-H distances for Al atoms involved in the formation of normal bridge bonds (1.769-1.784 A). The crystal structure of gamma-AlH3 contains large cavities between the AlH6 octahedra. As a consequence, the density is 11% less than for alpha-AlH3.
authors.affiliation: Institute for Energy Technology, Kjeller NO 2027, Norway. Volodymyr.Yartys@ife.no
authors.forename: Volodymyr A
authors.initials: VA
authors.lastname: Yartys
authors.forename: Roman V
authors.initials: RV
authors.lastname: Denys
authors.forename: Jan Petter
authors.initials: JP
authors.lastname: Maehlen
authors.forename: Christoph
authors.initials: C
authors.lastname: Frommen
authors.forename: Maximilian
authors.initials: M
authors.lastname: Fichtner
authors.forename: Boris M
authors.initials: BM
authors.lastname: Bulychev
authors.forename: Hermann
authors.initials: H
authors.lastname: Emerich