Theoretical prediction of different decomposition paths for Ca(BH4)2 and Mg(BH4)2

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 CaB2H2 intermediate [M. D. Riktor, M. H. Sorby, K. Chlopek, 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 CaB2Hn compounds with n=2, 4, and 6 using PEGS simulations, refining the resulting structures with accurate DFT calculations. We find that the lowest-energy CaB2H2 and CaB2H4 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 CaB12H12 phase. We also predict a CaB2H6 compound which forms a low-energy intermediate in the calcium borohydride decomposition pathway. This new reaction pathway is practically degenerate with decomposition into the CaB12H12 phase. Similar calculations for magnesium borohydride show that a recently predicted MgB2H6 phase does not form a stable intermediate in the decomposition pathway of Mg(BH4)(2).