Bonding and stability of the hydrogen storage material Mg2NiH4

Structural stability and bonding properties of the hydrogen storage material Mg2NiH4 (monoclinic, C2/c, Z = 8) were investigated and compared to those of Ba2PdH4 (orthorhombic, Pnma, Z = 8) using ab initio density functional calculations. Both compounds belong to the family of complex transition metal hydrides. Their crystal structures contain discrete tetrahedral 18 electron complexes (TH44-)-H-0 (T = Ni, Pd). However, the bonding situation in the two systems was found to be quite different. For Ba2PdH4, the electronic density of states mirrors perfectly the molecular states of the complex PdH44-, whereas for Mg2NiH4 a clear relation between molecular states of TH44- and the density of states of the solid-state compound is missing. Differences in bonding of Ba2PdH4 and Mg2NiH4 originate in the different strength of the T-H interactions (Pd-H interactions are considerably stronger than Ni-H ones) and in the different strength of the interaction between the alkaline-earth metal component and H (Ba-H interactions are substantially weaker than Mg-H ones). To lower the hydrogen desorption temperature of Mg2NH4, it is suggested to destabilize this compound by introducing defects in the counterion matrix surrounding the tetrahedral (NiH44-)-H-0 complexes. This might be achieved by substituting Mg for Al.