Effects of partial halide anion substitution on reorientational motion in NaBH4: A nuclear magnetic resonance study

Abstract

To study the effects of partial halide anion substitution on the reorientational motion of [BH 4 ]anions in NaBH 4 , we have measured the 1 H and 11 B NMR spectra and spin-lattice relaxation rates in the cubic solid solutions Na(BH 4 ) 0.5 Cl 0.5 and Na(BH 4 ) 0.5 I 0.5 over broad ranges of temperature (8 -324 K) and the resonance frequency (14 -90 MHz). For both solid solutions, the measured 1 H spin-lattice relaxation rates are governed by reorientations of BH 4 groups, and the experimental data can be satisfactorily described by the model with a Gaussian distribution of the activation energies. The average values of the activation energies derived from the 1 H spin-lattice relaxation data are 192 ± 7 meV for Na(BH 4 ) 0.5 Cl 0.5 and 120 ± 3 meV for Na(BH 4 ) 0.5 I 0.5 . At a given temperature, the reorientational jump rates are found to increase in the order of Na(BH 4 ) 0.5 Cl 0.5 -NaBH 4 -Na(BH 4 ) 0.5 I 0.5 , in agreement with the quasielastic neutron scattering (QENS) results. This trend correlates with the lattice expansion reflecting the increase in the size of the corresponding anions (in the order of Cl --[BH 4 ] --I -).

Low-Temperature Rotational Tunneling of Tetrahydroborate Anions in Lithium Benzimidazolate-Borohydride Li2(bIm)BH4

“…Besides invoking the structural-change arguments above, another possible reason may be related to the presence of a distribution of the reorientational jump rates. As discussed previously, in the presence of broad jump rate distributions, the standard analysis of QENS spectra is expected to underestimate the changes in the quasi-elastic line width with temperature.…”

Structural and Dynamical Properties of Potassium Dodecahydro-monocarba-closo-dodecaborate: KCB11H12

“…The barrier differences between QENS and NMR data for these types of systems can stem from factors involving the different sensitivities of NMR and QENS with respect to distributions in jump rates and/or to the presence of different types of concomitant jump motions. For instance, in circumstances where there is a distribution of reorientational jump rates, as one might expect for these orientationally disordered phases, a straightforward analysis of QENS data may underestimate the actual activation energies, as explained in ref . In particular, for QENS, unlike NMR, it is difficult to detect a jump rate distribution.…”

Reorientational Hydrogen Dynamics in Complex Hydrides with Enhanced Li+ Conduction

“…A single component with a smaller value of the activation energy, E A , could extend the IFWS signal over a broader temperature range, however, the resulting line is highly asymmetrical . Therefore, we included a Gaussian distribution of activation energies into our analysis, which is quite common for solid solutions with mixed anions: , where G ( E ; E A , Δ E A ) is the Gaussian function with the maximum at E = E A and the variance Δ E A . This approach leads to a much better agreement between the experimental data and the model IFWS curves (Figure ).…”

“…Moreover, QENS delivers information on both spatial and time characteristics of studied processes. This is an advantage of the method compared with other valuable techniques such as solid-state nuclear magnetic resonance spectroscopy (NMR). ,, As both species, [BH 4 ] − and [NH 2 ] − , contain hydrogen, the main challenge in the interpretation of QENS spectra would be to disentangle the two contributions. Nevertheless, the method provides a tool, namely deuterium labeling, to perform a sophisticated analysis.…”