Hydrogen site occupancy and hydrogen diffusion inLaNi4BH1.5

The ${\mathrm{CeCo}}_4$B-type ternary compound ${\mathrm{LaNi}}_4$B reacts with hydrogen gas at room temperature to form the hydride ${\mathrm{LaNi}}_4$ ${\mathrm{BH}}_{1.5}$ in the pressure range 1-100 atm. Rigid-lattice proton second moments indicate that hydrogen only occupies sites within the La-Ni basal planes in this hydride. The La-B planes, which do not accommodate hydrogen, appear to act as barriers to long-range hydrogen diffusion along the [001] direction. We have studied the diffusion behavior of hydrogen in ${\mathrm{LaNi}}_4$ ${\mathrm{BH}}_{1.4}$ by NMR measurements of the proton relaxation times $T_1$ and $T_{1\rho }$ and find evidence for at least two activated-jump processes. The long-range diffusion process is characterized by lower mobilities and larger activation energies than observed in ${\mathrm{LaNi}}_5$ ${\mathrm{H}}_6$; comparison of the results with those previously obtained for ${\mathrm{LaNi}}_4$ ${\mathrm{AlH}}_{4.3}$ suggests that the activated step for diffusion in these hydrides is passage of hydrogen through the $12o$ sites, or through saddle points joining these sites to other sites occupied by hydrogen. Evidence for a low-activation-energy process is also observed; this process is interpreted as a localized hopping within clusters of $3f-12n$ sites that does not result in hydrogen transport. These observations are consistent with the five-site model [$\frac{P6\mathrm{}}{\mathrm{mmm}} \left(D_{6h}^1\right)$ space group] for hydrogen site occupancy in ${\mathrm{LaNi}}_5$ ${\mathrm{H}}_6$, but they are less easily related to site occupancies for the $P31m \left(C_{3v}^2\right)$ model. Additionally, the presence of several anomalies in the ${\mathrm{LaNi}}_4$B x-ray powder pattern prompted us to conduct single-crystal investigations which indicate that the ${\mathrm{CeCo}}_4$B structure represents only a subcell in a larger hexagonal antiphase domain structure extending along the directions of the basal-plane axes.