A new phase in the decomposition of Mg(BH4)2: first-principles simulated annealing

Abstract

The experimentally found decomposition temperature of Mg(BH₄)₂ and the equilibrium temperature that follows from thermodynamics calculated for this reaction using density functional theory methods differ significantly. This is an indication of the fact that a strong kinetic barrier exists, preventing the formation of the phases considered in these DFT calculations. On the other hand, the decomposition temperature shows a clear dependence on the hydrogen back-pressure, indicating a thermodynamically determined decomposition temperature. The conclusion is that a phase must exist that was not considered before. In this paper, we use first-principles molecular dynamics to identify a candidate for this phase. We chose a number of stoichiometries by starting from a Mg(BH₄)₂ structure containing artificial hydrogen vacancies. By performing simulated annealing we obtain stable crystal structures for these stoichiometries. For the stoichiometries studied, the most stable structures turn out to contain Mg²⁺ and (B₂H₆)²⁻ ions. An important factor for the stability of these structures is that all hydrogen atoms also bond to magnesium atoms at a distance of about 2.05 Å. From the phonon frequencies, calculated in the most stable structure for this phase, we calculate the temperature-dependent free energy, which shows that it becomes thermodynamically available just at the temperature at which the desorption starts, confirming our hypothesis.