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 ≈ 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 α‐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.