Manganese Borohydride As a Hydrogen-Storage Candidate: First-Principles Crystal Structure and Thermodynamic Properties

Manganese borchydride (Mn(BH4)(2)) is considered to be a high-capacity (similar to 10 wt %) solid-state hydrogen storage candidate, but so far has not been shown to exhibit reversible hydrogenation. This study presents the calculated crystal structure and electronic structure of Mn(BH4)(2) from density-functional theory within the generalized gradient approximation and thermodynamic properties from the direct method lattice dynamics. A thermodynamically stable phase of Mn(BH4)(2) is identified. The calculation of Gibbs energy at finite temperatures suggests that the stable phase is of I (4) over bar m2 symmetry. The formation energy of Mn(BH4)(2) for the I (4) over bar m2 symmetry solid is -28.93 kJ/f.u. at 0 K including zero-point energy corrections, and the standard state enthalpy of formation is predicted to be -58.89 kJ/f.u. The most feasible dehydrogenation reaction is found to be Mn(BH4)(2) = Mn + 2B+ 4H(2), which is an endothermic reaction at decomposition temperature. The spin-polarized electronic density of states shows that manganese borohydride has a half-metallic nature due to the presence of half-filled 3d electrons from Mn. The electronic structure calculations and analysis show that the interaction between Mn atoms and BH4 Complexes has an ionic character, while the internal bonding of BH4 is essentially covalent.