Structures and bonding natures of tin- and zirconium-aluminum hydrides

Abstract

With hydrogen contents 6.69 and 7.49 wt%, Sn(AlH4)4 and Zr(AlH4)4 were experimentally synthesized, but their structures, bonding natures and hydrogen storage properties were not further investigated. This paper determines the low energy structures of Sn(AlH4)4 and Zr(AlH4)4 crystals by using global optimization algorithm combined with density functional theories, and analyze their electronic structures and bonding properties. The most stable structures of Sn(AlH4)4 and Zr(AlH4)4 contain isolated H2 molecules. The formation enthalpies of Zr(AlH4)4 reaches 1.0 eV and the value of Sn(AlH4)4 is a little smaller. In Zr(AlH4)4, the AlH5, AlH6 and Al2H6 units are connected tightly by Zr atoms through H bridges. In Sn(AlH4)4, the AlH6 units form a net structure, and the Sn atom exists in SnH3 with one shared H atom. The bonding characteristics are analyzed by calculating charge populations and crystal orbital hamilton population (COHP). The metal components (Al, Sn, Zr) and H atoms form polarized covalent bonds. The covalent contributions are close for Sn–H and Al–H bonds but the charge values on the Sn atoms are much smaller. For the Zr–H bonds, both the charges on the Zr atoms and the covalent contributions are smaller than the values of the Al–H bonds. Combining the bonding natures and the structure features, it suggests that proper electronegativity of the metal atoms (Sn and Zr here) is the key factor for the stabilities and reversible hydrogen storage properties of metal aluminum hydrides.