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

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.