Nuclear Magnetic Resonance Study of the Rotational Motion and the Phase Transition in LiBH₄

Abstract

To study the rotational motion of BH 4 tetrahedra in LiBH 4 , we have measured the 1 H and 11 B nuclear magnetic resonance spectra and spin-lattice relaxation rates in this compound over wide ranges of temperature (92-424 K) and resonance frequency (14-90 MHz for 1 H and 14-28 MHz for 11 B). In the low-temperature (orthorhombic) phase of LiBH 4 , our spin-lattice relaxation results are consistent with a coexistence of two types of the rotational motion of BH 4 tetrahedra with the activation energies of 0.182 ( 0.003 eV and 0.251 ( 0.004 eV. For both types of motions, the jump rates of the reorientations reach the values of the order of 10 11 s -1 near the upper limit of the temperature range of the orthorhombic phase stability (T 0 ≈ 381 K). In the high-temperature (hexagonal) phase, both the 1 H and 11 B spin-lattice relaxation rates are governed by an additional low-frequency fluctuation process (with the characteristic rate of the order of 10 7 s -1 just above T 0 ) due to the translational diffusion of Li ions.

Rotational disorder in lithium borohydride

“…Inelastic fixed window scans measured with highresolution back-scattering spectroscopy revealed the EPJ Web of Conferences presence of two kinds of rotational motion at low temperatures with activation energies of 162 meV and 232 meV [12], respectively, in accordance with NMR measurements [14,15]. The dominating motion could be identified as a three-fold jump rotation of the [BH 4 ] − anion around the c3 axis [12,13].…”

Structure and Dynamics for LiBH4−LiCl Solid Solutions

“…First, it is constant at 2440 Hz from RT to approximately 100°C, and then, it increases to 2720 Hz just before the phase transition at 116°C, and finally, it decreases to the same value of about 2450 Hz as observed below the temperature of 100°C. Although this behavior of the 1 H fwhm may seem somewhat puzzling, 46,47 it shows that a spinning frequency of ν r = 5 kHz (even at 116°C) is insufficient to completely average the residual homonuclear 1 H-1 H dipolar coupling which clearly makes a dominating contribution to the fwhm in the 1 H MAS spectra. Thermal Analysis.…”

Nanoconfined LiBH4 and Enhanced Mobility of Li+ and BH4– Studied by Solid-State NMR

“…5,6,25−32 Hwang et al 11 confirmed the formation of B 12 H 12 2− complexes in the decomposition of metal borohydrides by NMR. Conradi et al, 4,6,32,33 Skripov et al, 5,7 and Jimura et al 34 studied the structure and atomic mobilities of LiBH 4 .…”