Theoretical prediction of different decomposition paths for Ca(BH ₄)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 CaB₂ H ₂ intermediate is energetically highly unfavorable and hence very unlikely to form. We systematically search for low-energy structures of CaB ₂ 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 CaB₂ H₂ and CaB₂ 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 CaB₁₂ H₁₂ phase. We also predict a CaB₂ H₆ compound which forms a low-energy intermediate in the calcium borohydride decomposition pathway. This new reaction pathway is practically degenerate with decomposition into the CaB₁₂ H₁₂ phase. Similar calculations for magnesium borohydride show that a recently predicted MgB₂ H₆ phase does not form a stable intermediate in the decomposition pathway of Mg(BH ₄)₂.