Potential energy surface and hopping path for hydrogen inLaNi5

Abstract

Hydrogen hopping paths in LaNi 5 H solid solution were analyzed via first-principles calculations. Potential energy surfaces were determined for hydrogen on the plane with hydrogen sites 6m, 12o, and 4h and on the plane with hydrogen sites 12n, 3f, and 6i. From the zero-point vibration energy along the hopping path, it was found that hydrogen locations are grouped only at three regions; quasi-m site ͑o-m-o͒, h site, and quasi-f site ͑i-f-i͒. By applying the nudged elastic band method to hydrogen hopping paths between all the possible two sites, the most probable diffusion route was determined as quasi-f-quasi-m-quasi-f in the c direction and quasi-f-quasi-m-quasi-f-quasi-m-quasi-f in the a and b directions with the same saddle point energy of 0.37 eV, which is in good agreement with the measured activation energy, 0.3-0.5 eV, of the hydrogen diffusion in the solid solution phase of LaNi 5 -H system.

Hydrogen quantum effects in hydride LaNi5H7

“…2(a), the potential energy surface has a potential minimum point at the classical hydrogen position, while the potential energy surface at the 6i site for the solid solution phase has two minimum potential points. 3,4 In this case, the potential energy surface has a step structure in the Z direction as explained latter (see Fig. 3), and its step height is 0.25 eV, which is comparable to the zero point energy of the hydrogen atom.…”

“…These are powerful tools for exploring the quantum effects of light nuclei on neutron scatterings from dilute hydrogen in a solid solution of LaNi 5 H 0:5 . 3 We also showed that some zero point vibrational effects are present at all hydrogen sites in the solid solution phase of LaNi 5 H. 4 Some sites are combined into one site due to the zero point vibration. In this study, we reveal the hydrogenatom quantum effects in the hydride, and we clarify their role.…”

“…Here the potential energy surface in the solid solution phase has three minima. 4 The lowest one is at the 6m à site and another minimum is at the LaNi 3 tetrahedral site, which corresponds to the 12o site in LaNi 5 . Another 12o site is also located on the plane near the cross (X ' 0:4 and Z ' 0:65), but there is no potential energy minimum there.…”

“…In the solid solution phase, the potential energy surface has a minimum at each of the two 4h sites. 4 The structures of many hydrides of intermetallic compounds were investigated, and empirical criteria about the minimum hole size and the minimum H-H distance were obtained by Westlake. 29 According to the criteria, 2.1 Å is the required minimum H-H distance to occupy all the interstitial sites.…”

Quantum state of hydrogen inLaNi5

“…1͑b͒. According to the two-dimensional potential surface on the plane calculated by Tezuka et al, 11 the hydrogen should be extended around two 6i due to quantum fluctuation because the height of the potential barrier between neighboring i-i sites is too small to confine hydrogen in a single site. In this study, the three-dimensional potential-energy surface for the self-trapped hydrogen is calculated by applying first-principles electronic-structure calculations.…”