Not all H atoms contribute to low activation barrier diffusion in Ba_1.75LiH_2.7O_0.9

However, statistical inspection of the tracer diffusion coefficient for each atom, which is very difficult with DFT-MD but is feasible with NNP-MD, revealed the existence of effectively immobile H atoms over the entire studied temperature range. Most H do not engage in concerted migration, according to a comparison with a universal curve for independent random walkers. These findings hold for all phases, which have different distributions of Li, O, and H atoms in the same structural framework.

Incorporating many vacancies, especially in ‘K’ sites of the K₂NiF₄ framework, could form a low activation barrier H diffusion network not available in conventional oxyhydrides with the same framework. Gaining access to such a percolating network above a certain temperature can explain the superionic transition at this temperature.</p> <p>Neural network potential molecular dynamics show that the superionic transition in H⁻-conductor Ba_1.75LiH_2.7O_0.9 that does not accompany a structural transition is caused by changes in non-migrating H with temperature.</p>