X-ray diffraction studies of titanium and zirconium doped NaAlH4:: elucidation of doping induced structural changes and their relationship to enhanced hydrogen storage properties

X-ray diffraction patterns of NaAlH4 doped with up to 10 mol.% of either titanium or zirconium do not contain Bragg peaks for the bulk metals or their aluminum alloys. Instead the hydride lattice parameters a and c undergo significant contraction upon 2 mol.% doping and then expand as the doping level increases from 2 to 5 mol.%. These results are explained by a model that entails substitution of sodium cations by variable valence transition metal cations and the creation of Na+ vacancies in the bulk hydride lattice. (C) 2002 Elsevier Science B.V. All rights reserved.