Abstract
The mechanism of the dehydrogenation−rehydrogenation of Ti-doped NaAlH4 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.
Published Version
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