Atomic structures and energetics of ${\mathrm{LaNi}}_5-\mathrm{H}$ solid solution and hydrides

First-principles calculations on a primary solid solution of ${\mathrm{LaNi}}_5-\mathrm{H}$ and a hypothetically ordered full hydride, ${\mathrm{LaNi}}_5{\mathrm{H}}_7$ have been made employing ultrasoft pseudopotentials and plane-wave basis. Some intermediate hydrides were calculated as well. A full geometry optimization has been made to investigate their heat of formation in detail. Atomic positions of ${\mathrm{LaNi}}_5{\mathrm{H}}_7$ have been described in various ways through the Rietveld analyses of neutron-diffraction profile. The lowest energy structure by the present calculation is close to that of the model analyzed with the space group of ${P6\mathrm{}}_3\mathrm{mc}$ by Lartigue et al. However, we found that the structure model with a single unit cell, i.e., $Z=1,$ cannot be ruled out for ${\mathrm{LaNi}}_5{\mathrm{H}}_7.$ Regarding the primary solid solution, the $12n$ site of ${\mathrm{LaNi}}_5$ is most stable among five possible interstices proposed in literature. The stability of the interstices can be explained by the number of near-neighbor Ni atoms, which is substantially different from the widely accepted view that the geometric radius by the rigid sphere model determines the stability. The theoretical heat of solution in the primary solid solution is $-33{\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}-\mathrm{H}}_2,$ which roughly agrees with the experimental value. On the other hand, the heat of formation of ${\mathrm{LaNi}}_5{\mathrm{H}}_7$ is $-45{\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}-\mathrm{H}}_2,$ which is 30–40 % more negative than the experimental value. This discrepancy may be ascribed to the fact that all hydride samples are generally highly defective. The theoretical heat of formation of intermediate phases indicates that the ${\mathrm{LaNi}}_5-\mathrm{H}$ system dissociates to the primary solid solution and the full hydride. Expansion of the cell volume associated with hydrogenation is well reproduced by the calculation.