Hydrogen mobility in LaNi 5 hydride and its aluminium-and manganese-substituted hydrides

Abstract

Quasi-elastic and inelastic neutron scattering measurements were performed at different resolutions on LaNi 5 and LaNi 4M (M  Al, Mn) hydrides. The existence of two different types of hydrogen motion in both LaNi 5 and LaNi 4Mn hydrides is demonstrated: a rapid and fairly localized mode is superimposed on the long-range diffusion mode. Aluminium substitution, which is known to decrease the long-range diffusion coefficient, also induces a strong reduction in the intensity of the local diffusion mode. This rather fast motion can be described using a jump model for the very short distance (0.6 Å) between the neighbouring 3f ( z = 0) and 12n ( z ≈ 0.1) sites. This model leads to an incoherent neutron scattering law in good agreement with the measured spectra and to a mean residence time on the 12n site of (1.4 ± 0.2) × 10 −11 s in LaNi 5H 5.8 and (1.9 ± 0.3) × 10 −11 s in LaNi 4MnH 5.7. An inelastic peak centred around 60 meV in LaNi 5H 5.7 has previously been interpreted as a hydrogen vibrational mode on an octahedral site. We show here that this very broad peak becomes even broader and is strongly reduced in LaNi 4AlH 4.3. We suggest that the very close 3f (octahedral) and 12n hydrogen sites cannot be separated in discussions of this optical mode. These quasi-elastic and inelastic studies reinforce the validity of our previous crystallographic analysis.