The reorientational dynamics can be linked to the occurrence of many interesting properties, e.g. high ionic conductivities in M2BnHn and MCBn-1Hn (M = Li, Na; n = 10, 12). While most studies on reorientational dynamics have focused on compounds that contain one active species, new systems with interesting properties which exhibit dynamics on similar timescales with two or more dynamically active species are emerging. Due to that many of these systems are complex there is a need of studies of dynamically interacting model systems like NH4BH4. The beauty of this system is that the two species, which are both tetrahedral ions have the same type of environment due to rock salt type crystal structure (Fm-3m). This means that the NH4+ cation/BH4- anion is at the center of a cube coordinated by six BH4- anions/NH4+ cations at the center of each of the sides of the cube and thus effects on the dynamics can be attributed to the interactions between these complex ions.
NH4BH4 was investigated by quasi elastic neutron scattering (QENS) using the OSIRS instrument at the ISIS Neutron and Muon Source in the UK. The results suggest that at low temperatures the BH4- anions, due to their relatively slow reorientational speed, imposes a non-cubic environment on the NH4+ cations resulting in a preferred axis of reorientation around one of their C2 or C3 axis. Furthermore, the experiment also suggests that in addition to the reorientations around the preferred axis the NH4+ cations perform slower reorientations around the remaining C2 or C3 axes, see Fig. 1. While the above could be concluded from the parameters extracted from scattering data, we expanded the current methods for calculating the S(q,ω) for QENS to include rigid bodies (complex ions/molecules) with unequal sites (preferential axis) to more rigorously show this. At higher temperatures the scattering data implied that the reorientations of the NH4+ cations evolve to isotropic rotational diffusion on a sphere and thus impose a cubic environment on the BH4- anions, which becomes dynamically active on QENS timescales at the same temperatures. The cubic environment of the BH4- anions is reflected by their cubic tumbling reorientational motion.
In summary the study shows that the reorientational dynamics of the two active species greatly affect one another leading to drastically different reorientational motions.
Fig. 1 a) Elastic incoherent structure factor (EISF) of NH4BH4 at 100 K at which only the NH4+ dynamics are visible on the instrument timescale. b) Threefold reorientation around a single C3 axis. c) Tetrahedral tumbling, where the hydrogen atoms can visit any of the four hydrogen positions of the molecule. This type of reorientation can be built up by a combination of rotations around the four C3 axis or the three C2 axes.
The project was led by Dr. Mikael S. Andersson at the department of Chemistry and Chemical Engineering at Chalmers University, in collaboration with researchers from Denmark, UK, USA and Sweden. The results are available in the journal Physical Review Materials.
M. S. Andersson, J. B. Grinderslev, T. R. Jensen, V. García Sakai, U. Häussermann, T. J Udovic and M. Karlsson, Interplay of NH4+ and BH4- reorientational dynamics in NH4BH4, Phys. Rev Mater. 2020, 4, 085002
We are currently investigating systems such as Mg(BH4)2-xNH3 (x = 0-7), which has shown promise as a potential solid state ionic conductor. The ionic conductivity in this system is dependent on the NH3 content and it is therefore of great interest to study how this is related to the reorientational dynamics and how the two dynamical species affect one another.