Rotational Diffusion in NaBH₄

Abstract

Hydrogen dynamics in complex hydrides comprise long-range translational diffusion as well as localized motions like vibrations, librations, or rotations. All different motions are characterized by their specific length scale and time scales. Here we present a combined experimental and theoretical study on the rotational diffusion of the (BH4)− ions in crystalline NaBH4. The motion is thermally activated and characterized by an activation energy of 117 meV and a prefactor of 11 fs. Thereby the motion is dominated by 90° reorientations of the (BH4)− ion around the 4-fold symmetry axis of the cubic crystal. The experimental results are discussed on the basis of DFT calculations, revealing the potential energy landscape of a (BH4)− subunit in the crystalline matrix.

Nuclear magnetic resonance studies of atomic motion in borohydrides

“…This unique feature of the spin-lattice relaxation measurements leads to high precision of the E a values derived from the R 1 (T) data. For comparison, the ranges of −1 values traced by recent QENS experiments [19][20][21][22] for NaBH 4 and KBH 4 do not exceed two [19,20,22] or three [21] orders of magnitude. The main mechanism of the 11 B spin-lattice relaxation in alkali borohydrides is the 11 B-1 H dipole-dipole interaction modulated by reorientational motion of BH 4 groups, and the results of the 11 B relaxation measurements at different resonance frequencies [14,16] yield practically the same motional parameters as the 1 H relaxation measurements.…”

“…The spatial aspects of the reorientational motion can be effectively studied by QENS measurements of the elastic incoherent structure factor (EISF) [23]. In particular, the behavior of the EISF in NaBH 4 and KBH 4 is found to be consistent with combined reorientations around 2-fold and 3-fold axes [19][20][21][22]. Lithium borohydride LiBH 4 has an orthorhombic structure at low temperatures and undergoes a first-order phase transition to a hexagonal structure at T 0 ≈ 384 K. The behavior of the proton spinlattice relaxation rates in the low-T [14] and high-T [24] phases of LiBH 4 is shown in Fig.…”

Rotational Motion in LiBH4/LiI Solid Solutions

“…19,20 QENS is a powerful tool to observe hydrogen dynamics 21,22 and has recently been used to study the hydrogen mobility in solid and liquid borohydride. 6,17,23,24 For these experiments, we used neutrons with an incident wavelength of λ i = 4 Å, corresponding to an energy of 5.11 meV and resolution of ΔE ≈ 0.2 meV at the elastic line. The samples were loaded in lead sealed, double walled-hollow cylindrical containers.…”

Hydrogen Dynamics in Lightweight Tetrahydroborates

“…In a combined experimental and theoretical study the rotational motion of the BH 4 tetrahedra in NaBH 4 by means of incoherent quasielasic neutron scattering (QENS) and density functional theory (DTF) calculations were investigated [48]. QENS measurements were carried out using the time-of-flight neutron spectrometer FOCUS for cold neutrons located at the continuous spallation source SINQ at the Paul Scherrer Institute in Villigen, Switzerland [49,50].…”