Li₅(BH₄)₃NH: Lithium-Rich Mixed Anion Complex Hydride

The Li₅(BH₄)₃NH complex hydride, obtained by ball milling LiBH₄ and Li₂NH in various molar ratios, has been investigated. Using X-ray powder diffraction analysis the crystalline phase has been indexed with an orthorhombic unit cell with lattice parameters a = 10.2031(3), b = 11.5005(2), and c = 7.0474(2) Å at 77 °C. The crystal structure of Li₅(BH₄)₃NH has been solved in space group Pnma, and refined coupling density functional theory (DFT) and synchrotron radiation X-ray powder diffraction data have been obtained for a 3LiBH₄:2Li₂NH ball-milled and annealed sample. Solid-state nuclear magnetic resonance measurements confirmed the chemical shifts calculated by DFT from the solved structure. The DFT calculations confirmed the ionic character of this lithium-rich compound. Each Li⁺ cation is coordinated by three BH₄^- and one NH^2- anion in a tetrahedral configuration. The room-temperature ionic conductivity of the new orthorhombic compound is close to10^-6 S/cm at room temperature, with activation energy of 0.73 eV.