Interplay of NH4+ and BH4− reorientational dynamics in NH4BH4

Abstract

The reorientational dynamics of ammonium borohydride (${\mathrm{NH}}_{4}{\mathrm{BH}}_{4}$) was studied using quasielastic neutron scattering in the temperature interval from 10 to 240 K, which covers both the dynamically ordered and disordered polymorphs of ${\mathrm{NH}}_{4}{\mathrm{BH}}_{4}$. In the low-temperature $(<50\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ ordered polymorph of ${\mathrm{NH}}_{4}{\mathrm{BH}}_{4}$, analysis of the quasielastic neutron scattering data reveals that no reorientational dynamics is present within the probed timescale region of 0.1 to 100 ps. In the high-temperature $(>50\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ disordered polymorph, the analysis establishes the onset of ${\mathrm{NH}}_{4}^{+}$ and ${\mathrm{BH}}_{4}^{\ensuremath{-}}$ dynamics at around 50 and 125 K, respectively. The relaxation time at 150 K for ${\mathrm{NH}}_{4}^{+}$ is approximately 1 ps, while around 100 ps for ${\mathrm{BH}}_{4}^{\ensuremath{-}}$. The ${\mathrm{NH}}_{4}^{+}$ dynamics at temperatures below 125 K is associated with preferential tetrahedral tumbling motions, where each of the hydrogen atoms in the ${\mathrm{NH}}_{4}^{+}$ tetrahedron can visit any of the four hydrogen sites, however, reorientations around a specific axis are more frequently occurring (${C}_{2}$ or ${C}_{3}$). At higher temperatures, the analysis does not exclude a possible evolution of the ${\mathrm{NH}}_{4}^{+}$ dynamics from tetrahedral tumbling to either cubic tumbling, where the hydrogen atoms can visit any of the eight positions corresponding to the corners of a cube, or isotropic rotational diffusion, where the hydrogen atoms can visit any location on the surface of a sphere. The ${\mathrm{BH}}_{4}^{\ensuremath{-}}$ dynamics can be described as cubic tumbling. The difference in reorientational dynamics between the two ions is related to the difference of the local environment where the dynamically much slower ${\mathrm{BH}}_{4}^{\ensuremath{-}}$ anion imposes a noncubic environment on the ${\mathrm{NH}}_{4}^{+}$ cation.