Hydrogen dynamics in the low temperature phase of LiBH4 probed by quasielastic neutron scattering

- Inelastic fixed window sans offer new possibilities in neutron backscattering spectrometers.
- Two different kind of reorientational motion were identified in the low temperature phase of LiBH4.
- Thermally activated jump rotation.

LiBH4 contains 18.5 wt% hydrogen and undergoes a structural phase transition (orthorhombic → hexagonal) at 381 K which is associated with a large increase in hydrogen and lithium solid-state mobility. We investigated the hydrogen dynamics in the low temperature phase of LiBH4 by quasielastic neutron scattering, including a new kind of inelastic fixed window scan (IFWS). In the temperature range from 175 to 380 K the H-dynamics is dominated by thermally activated rotational jumps of the [BH4]− anion around the c3 axis with an activation energy of about 162 meV. In agreement with earlier NMR data, a second type of thermally activated motion with an activation energy of about 232 meV could be identified using the IFWS. The present study of hydrogen dynamics in LiBH4 illustrates the feasibility of using IFWS on neutron backscattering spectrometers as a probe of localised motion.