title: Tailoring the properties of ammine metal borohydrides for solid-state hydrogen storage.
abstract: A series of halide-free ammine manganese borohydrides, Mn(BH4 )2 ⋅nNH3 , n=1, 2, 3, and 6, a new bimetallic compound Li2 Mn(BH4 )4 ⋅6NH3 , and the first ammine metal borohydride solid solution Mg1-x Mnx (BH4 )2 ⋅6NH3 are presented. Four new crystal structures have been determined by synchrotron radiation powder X-ray diffraction and the thermal decomposition is systematically investigated for all the new compounds. The solid-gas reaction between Mn(BH4 )2 and NH3 provides Mn(BH4 )2 ⋅6NH3 . The number of NH3 per Mn has been varied by mechanochemical treatment of Mn(BH4 )2 ⋅6NH3 -Mn(BH4 )2 mixtures giving rise to increased hydrogen purity for n/m≤1 for M(BH4 )m ⋅nNH3 . The structures of Mg(BH4 )2 ⋅3NH3 and Li2 Mg(BH4 )4 ⋅6NH3 have been revisited and new structural models are presented. Finally, we demonstrate that ammonia destabilizes metal borohydrides with low electronegativity of the metal (χp <∼1.6), while metal borohydrides with high electronegativity (χp >∼1.6) are generally stabilized.
authors.affiliation: Center for Materials Crystallography, Interdisciplinary Nanoscience Center (iNANO); Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C (Denmark).
authors.forename: Lars H
authors.initials: LH
authors.lastname: Jepsen
authors.forename: Morten B
authors.initials: MB
authors.lastname: Ley
authors.forename: Yaroslav
authors.initials: Y
authors.lastname: Filinchuk
authors.forename: Flemming
authors.initials: F
authors.lastname: Besenbacher
authors.forename: Torben R
authors.initials: TR
authors.lastname: Jensen
mesh_terms: D000641:Ammonia / Q000737:chemistry*; D001894:Borohydrides / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006859:Hydrogen / Q000737:chemistry*; D055595:Mechanical Phenomena; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D013696:Temperature
chemical_list: D001894:Borohydrides; D009942:Organometallic Compounds; D000641:Ammonia; D006859:Hydrogen
keywords: hydrides; hydrogen storage; mechanochemistry; synchrotron radiation; x-ray diffraction