Reorientational Dynamics in Y(BH₄)₃?xNH₃ (x=0, 3, and 7): The Impact of NH₃ on BH₄^- Dynamics

The reorientational dynamics of Y(BH4)(3)center dot xNH(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 NH3 ligands drastically alters the reorientational mobility of the BH4- anion. From the QENS experiments, it was determined that the BH4- anion performs 2-fold reorientations around the C-2 axis in Y(BH4)(3), 3-fold reorientations around the C-3 axis in Y(BH4)(3)center dot 3NH(3), and either 2-fold reorientations around the C-2 axis or 3-fold reorientations around the C-3 axis in Y(BH4)(3)center dot 7NH(3). The relaxation time of the BH4- anion at 300 K decreases from 2 x 10(-7) s for x = 0 to 1 x 10(-12) s for x = 3 and to 7 x 10(-13) s for x = 7. In addition to the reorientational dynamics of the BH4- anion, it was shown that the NH3 ligands exhibit 3-fold reorientations around the C-3 axis in Y(BH4)(3)center dot 3NH(3) and Y(BH4)(3)center dot 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 LiBH4 center dot xNH(3) and Mg(BH4)(2)center dot xCH(3)NH(2).