Theoretical prediction of different decomposition paths forCa(BH4)2andMg(BH4)2

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We have studied the decomposition pathways of both Ca- and Mg-borohydride using density-functional theory (DFT) calculations of the free energy (including vibrational contributions) in conjunction with a Monte Carlo-based crystal-structure prediction method, the prototype electrostatic ground-state (PEGS) search method. We find that a recently proposed ${\text{CaB}}_{2}{\text{H}}_{2}$ intermediate [M. D. Riktor, M. H. S\o{}rby, K. Ch\l{}opek, M. Fichtner, and B. C. Hauback, J. Mater. Chem. 19, 2754 (2009)] is energetically highly unfavorable and hence very unlikely to form. We systematically search for low-energy structures of ${\text{CaB}}_{2}{\text{H}}_{n}$ compounds with $n=2$, 4, and 6 using PEGS simulations, refining the resulting structures with accurate DFT calculations. We find that the lowest-energy ${\text{CaB}}_{2}{\text{H}}_{2}$ and ${\text{CaB}}_{2}{\text{H}}_{4}$ crystal structures do not lie on the thermodynamically stable decomposition path but rather are unstable with respect to a decomposition pathway involving the previously proposed ${\text{CaB}}_{12}{\text{H}}_{12}$ phase. We also predict a ${\text{CaB}}_{2}{\text{H}}_{6}$ compound which forms a low-energy intermediate in the calcium borohydride decomposition pathway. This new reaction pathway is practically degenerate with decomposition into the ${\text{CaB}}_{12}{\text{H}}_{12}$ phase. Similar calculations for magnesium borohydride show that a recently predicted ${\text{MgB}}_{2}{\text{H}}_{6}$ phase does not form a stable intermediate in the decomposition pathway of $\text{Mg}{({\text{BH}}_{4})}_{2}$.