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