Dynamical Perturbations of Tetrahydroborate Anions in LiBH4 due to Nanoconfinement in Controlled-Pore Carbon Scaffolds

Abstract

Neutron vibrational spectroscopy and quasielastic neutron scattering (QENS) were used to probe the dynamical properties of BH4– anions in both bulk LiBH4 and LiBH4 confined in nanoporous carbons (NPCs) having ≤4-nm-diameter, hexagonally arranged, cylindrical pores. The BH4– torsional band of the confined LiBH4 is significantly broadened relative to that of bulk LiBH4, reflecting a disruption of the bulk crystal lattice and thus a broader distribution of BH4– rotational potentials. QENS measurements of bulk orthorhombic LiBH4 indicate a single quasielastic component yielding an activation energy for localized BH4– jump reorientation of 19.2 ± 0.8 kJ/mol, consistent with previous QENS and NMR results. At room temperature, the measurements are in good agreement with BH4– reorientational jumps about a single C2 or C3 tetrahedral symmetry axis, with evidence for multiaxis rotations emerging as the temperature increases. In contrast, the QENS spectra of the NPC-confined LiBH4 exhibit two quasielastic components, one an order of magnitude broader than the other. The narrower component is presumably associated with more slowly reorienting BH4– anions in the interior of the pores and the broader component with much more rapidly reorienting BH4– anions in the vicinity of the pore surfaces. For 4-nm pores, these components yield two corresponding activation energies for reorientation: 16 ± 1 and 10.6 ± 0.7 kJ/mol. The data suggest that both components undergo single C2- or C3-axis reorientational jumps below 330 K, albeit with one an order of magnitude faster than the other. By 400 K (which is above the bulk phase transition temperature), both reorient more diffusively around multiple axes. These results were found to be qualitatively consistent with comparative dynamical measurements of LiBH4 confined in a 13-nm-average-pore-size carbon aerogel, which exhibited a much broader pore size distribution.