Hydrogen-related defects in sodium alanate

We present first-principles simulations to elucidate the role of hydrogen-related point defects in the hydrogen absorption/desorption processes in sodium alanate. In particular, we investigate the energetics and structural properties of crystal imperfections caused by addition or removal of hydrogen atoms. We specifically take the various charge states of the defects into account. We find that charged hydrogen-related defects are energetically more favorable than neutral defects; these systems have a negative effective correlation energy U. Significant local lattice relaxations are involved in stabilizing the hydrogen-related defects. (C) 2007 Elsevier B.V. All rights reserved.