Determination of the crystal structure of Mg(AlH4)2by combined X-ray and neutron diffraction

The structure of magnesium alanate, Mg(AlH4)(2), has been examined in detail by combined powder synchrotron X-ray and neutron diffraction. The space group is confirmed to be P (3) over barm 1 with unit cell dimensions of a = 5.2084(3) Angstrom and c = 5.8392(5) Angstrom at 8 K, a = 5.20309(12) Angstrom and c = 5.8400(2) Angstrom at 111 K and a = 5.1949(2) Angstrom and c = 5.8537(2) Angstrom at 295 K. The structure consists of isolated and slightly distorted AlH4- tetrahedra that are connected via six-coordinated Mg atoms in a distorted octahedral geometry, resulting in a sheet-like structure along the crystallographic c-axis. The distortion of the AlH4- tetrahedra decreases with increasing temperature, whereas, the opposite is the case for the MgH6 octahedra. The Al-H distances are 1.606(10)-1.634(4) Angstrom, 1.602(10)-1.682(3) Angstrom and 1.561(12)-1.672(4) Angstrom at 8, 111 and 295 K, respectively. (C) 2004 Elsevier B.V. All rights reserved.