Formation and migration of native defects in NaAlH$_4$

Details Formation and migration of native defects in NaAlH$_4$ Formation and migration of native defects in NaAlH$_4$

2009-11-14

arxiv.org/abs/0911.2813v1

Phys. Rev. B 80, 224102 (2009)

Physics

Condensed Matter

Materials Science

26 pages, 8 figures, 1 table; to appear in Phys. Rev. B 80 (2009)

Scientific paper

We present a first-principles study of native defects in NaAlH$_4$. Our analysis indicates that the structure and energetics of these defects can be interpreted in terms of elementary building blocks, which include $V_{\rm{AlH_4}}^+$, $V_{\rm{Na}}^-$, $V_{\rm{H}}^+$, H$_i^-$, and (H$_2$)$_i$. We also calculate migration barriers for several key defects, in order to compare enthalpies of diffusion to experimentally measured activation energies of desorption. From this, we estimate activation energies for diffusion of defects and defect pairs. We suggest that $V_{\rm{AlH_4}}^+$ and H$_i^-$, or $V_{\rm{Na}}^-$ and $V_{\rm{H}}^+$, may be responsible for diffusion necessary for desorption. We discuss the possible role of $V_{\rm{H}}^+$-H$_i^-$ complex formation. The values we find are in the range of activation energies reported for catalyzed desorption.

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