Probing the unusual anion mobility of LiBH_4 confined in highly ordered nanoporous carbon frameworks via solid state NMR and quasielastic neutron scattering

Particle size and particle–framework interactions have profound effects on the kinetics, reaction pathways, and even thermodynamics of complex hydrides incorporated in frameworks possessing nanoscale features. Tuning these properties may hold the key to the utilization of complex hydrides in practical applications for hydrogen storage. Using carefully synthesized, highly-ordered, nanoporous carbons (NPCs), we have previously shown quantitative differences in the kinetics and reaction pathways of LiBH_4 when incorporated into the frameworks. In this paper, we probe the anion mobility of LiBH_4 confined in NPC frameworks by a combination of solid state NMR and quasielastic neutron scattering (QENS) and present some new insights into the nanoconfinement effect. NMR and QENS spectra of LiBH_4 confined in a 4 nm pore NPC suggest that the BH_4− anions nearer the LiBH_4–carbon pore interface exhibit much more rapid translational and reorientational motions compared to those in the LiBH_4 interior. Moreover, an overly broadened BH_4− torsional vibration band reveals a disorder-induced array of BH_4− rotational potentials. XRD results are consistent with a lack of LiBH_4 long-range order in the pores. Consistent with differential scanning calorimetry measurements, neither NMR nor QENS detects a clear solid–solid phase transition as observed in the bulk, indicating that borohydride–framework interactions and/or nanosize effects have large roles in confined LiBH_4

The Nature of BH₄^– Reorientations in Hexagonal LiBH₄

Lithium borohydride (LiBH₄) has lately been the subject of intense inquiry within the hydrogen storage community. Quasi-elastic neutron scattering spectra were measured for LiBH₄ in the high-temperature hexagonal crystal phase. The elastic incoherent structure factor associated with the rapid BH₄^– anion reorientations was determined at 400, 410, and 420 K for momentum transfers as high as 4.2 Å^–1. The results strongly suggest a BH₄^– reorientational mechanism approaching quasi-free, trigonal-axis rotation of three borohydride H atoms, combined with reorientational jump exchanges between these delocalized “orbiting” H atoms and the remaining axial borohydride H atom. This mechanism is consistent with previously reported diffraction and spectroscopy studies

Evolution of the Reorientational Motions of the Tetrahydroborate Anions in Hexagonal LiBH₄–LiI Solid Solution by High‑<i>Q</i> Quasielastic Neutron Scattering

The reorientational dynamics of tetrahydroborate (BH₄^–) anions in the hexagonal 1:1 LiBH₄–LiI solid solution were characterized by quasielastic neutron scattering (QENS) with results extended to high momentum transfers (<i>Q</i>). Measurements are compared in detail to results for LiBH₄ and to a range of models describing the various possible reorientational mechanisms. The high reorientational mobility compared to that for BH₄^– in other solid-state environments reflects a favorable combination of the underlying hexagonal close-packed lattice and the unusually large BH₄^– crystallographic site stabilized by the presence of the I^– anions throughout the structure. QENS data up to momentum transfers of 4.2 Å^–1 at 125 K reveal a dominant uniaxial reorientation mechanism consisting of rapid BH₄^– diffusive-like rotational motions of three H atoms in a ring around the <i>c</i>-directed trigonal B–H axis, with the fourth axial H atom remaining stationary. By 200 K, this diffusive ring of three H atoms undergoes noticeable jump exchanges with the axial H atom, identical to what has been observed for BH₄^– reorientations in hexagonal LiBH₄ at much higher temperature. The two separate mechanisms are consistent with the two reorientational motions revealed recently by NMR measurements. An average rotational activation energy of 36 meV ± 1 meV is derived over a wide temperature range

Anion Reorientations in the Superionic Conducting Phase of Na₂B₁₂H₁₂

Quasielastic neutron scattering (QENS) methods were used to characterize the reorientational dynamics of the dodecahydro-<i>closo</i>-dodecaborate (B₁₂H₁₂^2–) anions in the high-temperature, superionic conducting phase of Na₂B₁₂H₁₂. The icosahedral anions in this disordered cubic phase were found to undergo rapid reorientational motions, on the order of 10¹¹ jumps s^–1 above 530 K, consistent with previous NMR measurements and neutron elastic-scattering fixed-window scans. QENS measurements as a function of the neutron momentum transfer suggest a reorientational mechanism dominated by small-angle jumps around a single axis. The results show a relatively low activation energy for reorientation of 259 meV (25 kJ mol^–1)

Vibrational Spectroscopic Study of Subtle Phase Transitions in Alkali Borohydrides: Comparison with First-Principles Calculations

Neutron vibrational spectra have been measured for the alkali borohydrides KBH₄, RbBH₄, and CsBH₄. The BH₄^– torsional band for each compound changes noticeably across the corresponding low-temperature phase transition previously identified using thermodynamic (NaBH₄, KBH₄, RbBH₄, and CsBH₄) and crystallographic (NaBH₄ and KBH₄) techniques. Previous neutron diffraction measurements show that the transitions for both NaBH₄ and KBH₄ are order–disorder transitions involving the relative orientations of the BH₄^– anions. However, diffraction measurements for both RbBH₄ and CsBH₄ fail to unequivocally identify long-range-ordered phases below the transitions. The present measurements of BH₄^– torsional as well as translational optic bands across the transitions, corroborated by first-principles phonon calculations, suggest that the subtle RbBH₄ and CsBH₄ transitions are indeed analogous to those observed for NaBH₄ and KBH₄ but of shorter range