Reversible vacancy formation and recovery during dehydrogenation- hydrogenation cycling of Ti-doped NaAlH₄

The mechanism of the dehydrogenation-rehydrogenation of Ti-doped NaAlH ₄ was investigated through X-ray diffraction (XRD) studies and Doppler broadening measurements of positron annihilation. Phase transformations during dehydrogenation and hydrogenation half-cycles were studied by the Rietveld analysis of XRD profiles. Changes in lattice defects were monitored by determination of the S parameter of the Doppler broadening spectra, which shows the ratio of positron annihilation to valence electrons. No significant changes in lattice parameters and the S parameter were observed directly following Ti doping. However, the S parameter increased upon dehydrogenation and decreased following rehydrogenation. These results indicate that vacancies are not introduced into NaAlH₄ upon Ti doping, but rather, they arise during dehydrogenation and disappear during rehydrogenation.