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

Manganese borohydride (Mn(BH 4 ) 2 ) is considered to be a high-capacity (∼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(BH 4 ) 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(BH 4 ) 2 is identified. The calculation of Gibbs energy at finite temperatures suggests that the stable phase is of I 4̅ m 2 symmetry. The formation energy of Mn(BH 4 ) 2 for the I 4̅ m 2 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(BH 4 ) 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 BH 4 complexes has an ionic character, while the internal bonding of BH 4 is essentially covalent.