Interplay of NH4+ and BH4− reorientational dynamics in NH4BH

Abstract

The reorientational dynamics of ammonium borohydride (NH 4 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 NH 4 BH 4. In the low-temperature (< 50 K) ordered polymorph of NH 4 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 K) disordered polymorph, the analysis establishes the onset of NH + 4 and BH − 4 dynamics at around 50 and 125 K, respectively. The relaxation time at 150 K for NH + 4 is approximately 1 ps, while around 100 ps for BH − 4. The NH + 4 dynamics at temperatures below 125 K is associated with preferential tetrahedral tumbling motions, where each of the hydrogen atoms in the 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 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 BH − 4 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 BH − 4 anion imposes a noncubic environment on the NH + 4 cation.

Neutron Scattering Investigations of the Global and Local Structures of Ammine Yttrium Borohydrides

“…The monoclinic distortion of βto α-Y( 11 BD 4 ) 3 •3ND 3 does not appear to be caused by an orientational ordering as has been reported for other borohydride-based compounds at low temperatures, for example, MBH 4 (M = NH 4 + , Na + , K + , Rb + , and Cs + ), 21,22,44 as the refinements of the β-Y( 11 BD 4 ) 3 •3ND 3 polymorph suggests that the structure is ordered at 300 K. The slight distortion may rather be a consequence of other interactions. An analysis of short heterogeneous dihydrogen interactions, N−H δ+...−δ H−B (<2.4 Å), reveals that there are more interactions in α-Y( 11 BD 4 ) 3 •3ND 3 (4.83 dihydrogen bonds per NH 3 ) compared to β-Y( 11 BD 4 ) 3 •3ND 3 (3.67 dihydrogen bonds per NH 3 ).…”

“…Metal borohydrides and derivatives thereof are a continuously expanding class of materials, and numerous new compositions have been reported in the past decade. − Besides high hydrogen densities, many of these materials exhibit other interesting properties such as magnetism, , luminescence, − or ionic conductivity. − Ammine metal borohydrides have been particularly investigated due to dihydrogen interactions, N–H δ+ ··· –δ H–B, which can facilitate the release of hydrogen at low temperatures. , Recently, these dihydrogen interactions have received new interest due to their influence on dynamics and crystal structures, ,, and it has been suggested that these interactions can facilitate fast ionic conductivity, forming the basis for a new type of solid-state electrolyte. ,, …”

“…12−19 Ammine metal borohydrides have been particularly investigated due to dihydrogen interactions, N−H δ+ ••• −δ H−B, which can facilitate the release of hydrogen at low temperatures. 1,20 Recently, these dihydrogen interactions have received new interest due to their influence on dynamics and crystal structures, 4,21,22 and it has been suggested that these interactions can facilitate fast ionic conductivity, forming the basis for a new type of solid-state electrolyte. 15,16,23 Ammonia derivatives are known for the majority of the metal borohydrides, 1 and the largest series of compositions and polymorphs among ammine metal borohydrides are observed for Y(BH 4 ) 3 •xNH 3 , with x = 7, 6, 5, 3, 2 (α-and βpolymorph), and 1.…”

Interplay between the Reorientational Dynamics of the B3H8– Anion and the Structure in KB3H8

“…Upon further heating, the α′-polymorph exhibits a first-order polymorphic transition to the cubic β-polymorph at 303 K. The study also showed that the [B 3 H 8 ] − anion is in an ordered state in the lower-temperature α- and α′-polymorphs, while transforming to an orientationally disordered state in the high-temperature β-polymorph (see Figure ). The anion disorder suggests that it might be undergoing rapid reorientations similar to what has been observed in other metal borohydrides such as MBH 4 (M = Li, Na, K, and NH 4 ) and the closo-polyborates M 2 B 10 H 10 and M 2 B 12 H 12 (M = Li and Na). − While the reorientations are primarily a local property, it can drastically affect some global properties such as ionic conductivity and the crystal structure of the material. − Therefore, understanding the reorientational dynamics is of vital importance for fully understanding the properties of these technologically important materials.…”

Pressure dependence of atomic dynamics in barocaloric ammonium sulfate: I. Rotations

“…We will support this claim using the pressure data in the next section. Reorientational dynamics according to model C have been observed in other ammonium salts including ammonium perchlorate, 38 ammonium borohydride 39 and ammonium cyanate. 40…”