Potential energy surface and hopping path for hydrogen in LaNi5

Hydrogen hopping paths in LaNi5H 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-o), h site, and quasi-f site (i-f-i). 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 LaNi5-H system.