NMR Study of Molecular Dynamics in Complex Metal Borohydride LiZn₂(BH₄)₅

Abstract

In searching for hydrogen storage materials with an improved storage capacity and low hydrogen decomposition temperature, a lithium zinc borohydride LiZn2(BH4)5 (LZBH) was investigated. LZBH shows the structure of two identical interpenetrated three-dimensional frameworks with no bonds between them, not observed before in metal hydrides. The structural peculiarity and uniqueness among metal hydrides prompts the investigation of molecular dynamics responsible for the thermodynamic and kinetic properties of LZBH. Molecular dynamics was investigated experimentally by 1H and 7Li NMR spectrum and spin–lattice relaxation techniques. Different thermally activated reorientational processes of BH4 tetrahedra about their 2-fold and 3-fold symmetry axes were identified from the temperature-dependent proton and lithium spin–lattice relaxation rates and were quantified by their activation energies, in relation to the LiZn2(BH4)5 structural details. The five BH4 tetrahedra of a given [Zn2(BH4)5]− complex anion were classified into two groups with different dynamic properties, the first group containing terminal tetrahedra with crystallographically inequivalent B1, B2, and B4 boron atoms and the second group containing the bridging tetrahedron with the B3 boron atom. Our study presents physical insight into the dynamic properties of LZBH on a microscopic level of atomic groups, providing link between the microscopic and the bulk properties of this phase.