The Nature of BH₄^– Reorientations in Hexagonal LiBH₄

Abstract

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