Reorientational Motions and Ionic Conductivity in (NH₄)₂B₁₀H₁₀ and (NH₄)₂B₁₂H₁₂

Abstract

We investigated molecular dynamics in two ammonium borane systems from the group of promising ion conductors. The investigation was performed by means of 1 H and 11 B NMR spectroscopy and spin−lattice relaxation techniques. We identified two reorientational processes, the rotations of NH 4 units that are present already at low temperatures and rotations of large boron cages, B 10 H 10 or B 12 H 12 , which are thermally activated and become prominent above 250 K. Activation energies for these processes were determined. In addition, solid-state ion conductivity measurements were conducted to determine poor NH 4 + conductivity of both systems.

Molecular Dynamics in Ag2B12H12 Studied by Nuclear Magnetic Resonance

“…This is consistent with the fact that even if they are close to spherical in shape, the B 12 H 12 cages still have preferential reorientational axes, such as C 2 , C 3 , and C 5 . 30 The shape of the 11 B spectra, with boron being a quadrupole nucleus with a spin of 3/2, is also affected by the interactions of the electric quadrupole moment of boron nuclei with the electric field gradient (EFG) tensor generated by the surrounding atoms. As seen in Figure 2, the boron spectra at low temperatures consist of a strong central line and weak unresolved satellite transitions.…”

“…On the other hand, there are two major mechanisms that influence the relaxation of 11 B, a quadrupole nucleus with a spin of 3/2. Similar to the proton relaxation, we take into account the fluctuations in dipolar interactions, both homonuclear and heteronuclear (although the B−B and B−Ag interactions are orders of magnitude smaller than the B−H interactions).…”

“…In (NH 4 ) 2 B n H n (n = 10, 12), rotational tunneling of the NH 4 cations was observed at the lowest temperatures. 11,12,21,22 In parallel, several recent studies using ab initio calculations also showed strong indications that the reorientations of large boron cages can facilitate the mobility of cations. 10,23−26 Our previous studies of Ag 2 B n H n (n = 10, 12) 1 presented strong crystallographic evidence of the so-called paddle-wheel cation conductivity mechanism.…”

“…The sample was sealed in a quartz tube under an Ar atmosphere to prevent the contact with water vapor or oxygen and stored in dark to prevent light exposure due to photosensitivity. 1 1 H and 11 B NMR spectra and spin−lattice relaxation times were measured as a function of temperaturein a heating run from 80 to 420 K (proton) and to 380 K (boron). 1 H measurements were conducted at a superconducting magnet with the field of 2.35 T (corresponding to the Larmor frequency ν L ( 1 H) = 100 MHz) while the 11 B measurements were conducted in a setup with a field of 4.7 T, corresponding to ν L ( 11 B) = 64.2 MHz.…”

“…2,6,7 Some of the closo-borates with excellent ionic conductivity include Li 2 B n H n , Na 2 B n H n , 4,5 Ag 2 B n H n 1 (n = 10, 12), mixed-anion compounds, such as Ag (2+x) I x B n H n 1 or Na 3 BH 4 B 12 H 12 , 8 and the carborates NaCB 11 H 12 or NaCB 9 H 10 , 9,10 while (NH 4 ) 2 B n H n were found to exhibit much lower conductivities, likely because of the larger mobile ion (NH 4 + ). 11,12 Recently, a system with the cation complex N 2 H 7 + was stabilized in the solid state. 13 Research interest has also focused on systems where hydrogen in the anion cages is replaced with a halogen (such as B 12 H 12 or B 12 Cl 12 ).…”