Structural studies of lithium zinc borohydride by neutron powder diffraction, Raman and NMR spectroscopy

Abstract

The crystal structure of LiZn 2(BH 4) 5 is studied in detail using a combination of powder neutron diffraction (PND), Raman spectroscopy, and 11B MAS NMR spectroscopy on LiZn 2(BH 4) 5 and LiZn 2( 11BD 4) 5. The aim is to obtain detailed structural knowledge of the first interpenetrated framework hydride compound, LiZn 2(BD 4) 5 which consists of doubly interpenetrated 3D frameworks built from dinuclear complex ions [Zn 2(BD 4) 5] − and lithium ions. The positions of the deuterium atoms are determined using Rietveld refinement of the PND data and the orientation of one of the four independent BD 4 − groups is revised. The current data reveal that the structure of [Zn 2(BD 4) 5] − is more regular than previously reported, as are also the coordinations around the Zn and Li atoms. Both Zn and Li atoms are found to coordinate to the BD 4 − units via the tetrahedral edges. Some distortion of the angles within the BD 4 units is observed, relative to the expected angles of 109.4 for the ideal tetrahedral coordination. Raman spectroscopy confirms bending and stretching modes from the expected terminal and bridging bidentate BH 4ˉ and BD 4ˉ units. The 11B MAS NMR spectrum of the satellite transitions resolves two distinct manifolds of spinning sidebands, which allows estimation of the 11B quadrupole coupling parameters and isotropic chemical shifts for the four distinct 11B sites of [Zn 2(BD 4) 5] −. These parameters agree favorably well with the isotropic triple-quantum shifts, observed in an 11B MQMAS NMR experiment. The present results may aid in the development and understanding of new borohydride materials for applications as fast ion conductors or as materials for hydrogen storage.