High Hydrogen Mobility in an Amide-Borohydride Compound Studied by Quasielastic Neutron Scattering

The hydrogen storage performance of reactive hydride composite Mg ( NH 2 ) 2 + 2 LiH can be significantly improved by the addition of LiBH 4 and the subsequent formation of an amide-borohydride compound Li 4 ( BH 4 ) ( NH 2 ) 3 during hydrogen release. Herein, an investigation into the structure and anion motions of Li 4 ( BH 4 ) ( NH 2 ) 3 using synchrotron radiation powder X-ray diffraction (SR-PXD; 295-573 K) and quasielastic neutron scattering (QENS; 297-514 K) is described. The highest temperature studied with QENS (514 K) is above the melting point of Li 4 ( BH 4 ) ( NH 2 ) 3 . The neutron measurements confirm a long-range diffusive motion of hydrogen-containing species with the diffusion coefficient D approximate to 10 - 6 cm 2 s - 1 . Interestingly, this value is comparable to that of Li + diffusion inferred from conductivity measurements. SR-PXD confirms the recrystallization of Li 4 ( BH 4 ) ( NH 2 ) 3 from the melt into the alpha-phase upon cooling. At temperatures below 514 K, localized rotational motions are observed that are attributed to ( BH 4 ) - tetrahedra units mainly undergoing rotations around the C 3 axes. The activation energy for this thermally activated process is found to be E a = 15.5 +/- 0.9 and 17.4 +/- 0.9 kJ mol - 1 respectively for the two instrumental resolutions utilized in the QENS measurements, corresponding to observation times of 55 and 14 ps.