Hydrogen Dynamics in Lightweight Tetrahydroborates

Abstract

Hydrogen . Dynamics . TetrahydroboratesThe high hydrogen content in complex hydrides such as M[AlH 4 ] x and M [BH 4 ] x (M = Li, Na, K, Mg, Ca) stimulated many research activities to utilize them as hydrogen storage materials. An understanding of the dynamical properties on the molecular level is important to understand and to improve the sorption kinetics. Hydrogen dynamics in complex hydrides comprise long range translational diffusion as well as localized motions like vibrations, librations or rotations. All the different motions are characterized by their specific length-and timescales. Within this review we give an introduction to the physical properties of lightweight complex hydrides and illustrate the huge variety of dynamical phenomena on selected examples.

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.…”

Di-hydrogen contact induced lattice instabilities and structural dynamics in complex hydride perovskites

“…B-H vibrations are readily studied by vibrational spectroscopy while the much lower energy transfers of stochastic reorientations are investigated by means of quasi-elastic neutron scattering. The structural dynamics of monometallic borohydrides have been extensively investigated by vibrational spectroscopy [28][29][30], solid state NMR spectroscopy [31][32][33][34][35], inelastic x-ray scattering IXS [36], inelastic neutron scattering INS [37], QENS [38][39][40] and recently by molecular dynamics assisted density functional theory (MD-DFT) [41,42]. The importance of the vibrational density of states for the stabilities of different polymorphs has also been discussed [43].…”

“…The alkali-borohydrides have been studied in detail by quasi-elastic neutron scattering and different jump models have successfully been applied to the low-and hightemperature phases [38,39,50]. A double-cation borohydride has so far not been studied with QENS.…”

Synthetic approaches to inorganic borohydrides

“…Parker 8 has extensively surveyed vibrational spectra and bonding of ternary metal hydride compounds, including borohydrides. Hydrogen dynamics in borohydrides have been reviewed by Remhof et al 9 Recently, Nakamori and Orimo 10 have addressed aspects related to the synthesis, crystal structures, electronic structure and thermodynamics of dehydrogenation of borohydrides.…”