Reorientational Dynamics in Y(BH 4 ) 3 · x NH 3 ( x = 0, 3, and 7): The Impact of NH 3 on BH 4 - Dynamics.

The reorientational dynamics of Y(BH 4 ) 3 · x NH 3 ( x = 0, 3, and 7) was studied using quasielastic neutron scattering (QENS) and neutron spin echo (NSE). The results showed that changing the number of NH 3 ligands drastically alters the reorientational mobility of the BH 4 - anion. From the QENS experiments, it was determined that the BH 4 - anion performs 2-fold reorientations around the C 2 axis in Y(BH 4 ) 3 , 3-fold reorientations around the C 3 axis in Y(BH 4 ) 3 ·3NH 3 , and either 2-fold reorientations around the C 2 axis or 3-fold reorientations around the C 3 axis in Y(BH 4 ) 3 ·7NH 3 . The relaxation time of the BH 4 - anion at 300 K decreases from 2 × 10 -7 s for x = 0 to 1 × 10 -12 s for x = 3 and to 7 × 10 -13 s for x = 7. In addition to the reorientational dynamics of the BH 4 - anion, it was shown that the NH 3 ligands exhibit 3-fold reorientations around the C 3 axis in Y(BH 4 ) 3 ·3NH 3 and Y(BH 4 ) 3 ·7NH 3 as well as 3-fold quantum mechanical rotational tunneling around the same axis at 5 K. The new insights constitute a significant step toward understanding the relationship between the addition of ligands and the enhanced ionic conductivity observed in systems such as LiBH 4 · x NH 3 and Mg(BH 4 ) 2 · x CH 3 NH 2 .