Nature of the chemical bond in complex hydrides, NaAlH4, LiAlH4, LiBH4 and LiNH2

The most stable crystal structures of complex hydrides, MXHn (NaAlH4, LiAlH4, LiBH4 and LiNH2) were simulated by the plane-wave pseudopotential method. The local chemical bonds between constituent ions were simulated using the DV-Xα molecular orbital method. As a result, it was found that the covalent interaction is operating between X and H ions to form a XHn ion in MXHn. In addition, the ionic interaction is operating between M and XHn ions through the charge transfer from M to XHn ions. On the basis of this understanding of the nature of the chemical bond between ions, a phase stability diagram of complex hydrides was proposed using two parameters. One is the bond energy of XH diatomic molecules and the other is electronegativity difference, ΔΦX M, between M and X ions. The calculated stability change by doping into NaAlH4 could by explained qualitatively following this diagram. This diagram will provide us a clue to the modification of hydrides to lower the hydrogen decomposition temperature.