A first principles study of the thermal stability of A_m(MH₄)_n light complex hydrides

From the physical point of view, the cohesive energy of a reactant is preferable to its formation energy for characterizing its influence on the reaction processes from the reactants to the products. In fact it has been found that there is a certain correlation between the experimental hydrogen desorption temperature and the cohesive energy calculated by a first principles method for a series of A_m(MH₄)_n (A = Li, Na, Mg; M = Be, B, Al) light complex hydrides (including Na₂BeH₄, Li₂BeH₄, NaAlH₄, LiAlH₄, Mg(AlH₄)₂, LiBH₄ and NaBH₄), which suggests that cohesive energy may be a useful physical quantity for evaluating the hydrogen desorption ability of complex hydrides, especially in cases when dehydrogenation products have unknown crystal structures, or may even be unknown. To understand this correlation more deeply, the ionic interaction between A and the MH₄ complex and the covalent interaction between M and H were calculated and their contributions to the cohesive energy evaluated quantitatively. The calculated results show that the covalent M–H interaction in the MH₄ complex is the dominant part of the cohesive energy E_coh (up to more than 75%) and hardly changes during high-pressure structural transitions of A_m(MH₄)_n. It was also found that low electronegativity of M or high electronegativity of A is responsible for the weak covalent M–H interaction and finally leads to the low thermodynamic stability of A_m(MH₄)_n, suggesting that complex hydrides A_m(MH₄)_n can be destabilized by partial substitution of M (A) with an element with electronegativity lower (higher) than Ms (As). This conclusion has been confirmed by lots of experimental results and may be a useful guideline for the future design of new complex hydrides of the type A_m(MH₄)_n.