CRYSTAL STRUCTURE OF ALUMINUM HYDRIDE

X-Ray and neutron powder diffraction data for aluminum hydride, AlH3, and aluminum deuteride, AlD3, were used to solve this structure. Both compounds crystallize in the trigonal space group R3c with six molecules in a hexagonal unit cell of dimensions a = 4.449 Å and c = 11.804 Å for the hydride and a = 4.431 Å and c = 11.774 Å for the deuteride. Leastsquares refinement of this three-parameter problem using seven unique AlD3 neutron powder diffraction peaks gave a final Ri of 0.026. The refined parameters gave Ri = 0.040 for 29 AlH3 X-ray powder diffraction peaks. Columns of Al atoms and spirals of H atoms are parallel with the c axis and are packed so that the Al has octahedral coordination symmetry. The final Al…H distance of 1.72 Å, the participation of each Al in six bridges, and the equivalence of all Al…H distances suggest that 3c-2e bonding occurs. The closest Al-Al distance is 3.24 Å. The alternating planes of Al and H atoms, perpendicular to the c axis, result in a stable structure which is a three-dimensional network of Al…H…Al bridges and is consistent with the observed high density of the crystal. © 1969, American Chemical Society. All rights reserved.