Alloying effects of transition metals on chemical bonding in magnesium hydride MgH2

By use of the first-principles density functional theory for a cluster model, the alloying effect of transition element M on the electronic structure of magnesium hydride, MgH2, is investigated. It is found that the chemical interaction between Mg and H may play a dominant role in the pure MgH2 system. Such a strong interaction remains in the alloyed Mg(M)H-2 when the central Mg is replaced by an alloying element M. By a satisfactory fit of the calculated M (or Mg)-H interactions with the enthalpies of formation for some pure dihydrides, we have found that 70% chemical (ionic plus covalent) interactions would appear to be ionic but the covalent interaction of an adjacent Mg to M with a H atom around it would be sensitive to the substitution of M. It is also found that in Mg(M)H-2 some weaker hydride forming elements than Mg would form a little stronger bonds with hydrogen than Mg-H bond in MgH2. The hydride formation enthalpy difference for Mg(M)H-2 alloyed by the transition element M has been calculated, by which we can know which element could be used to improve the hydriding kinetics of MgH2. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.