Not all H atoms contribute to low activation barrier diffusion in Ba 1.75 LiH 2.7 O 0.9

Superionic conductors have extremely high conductivity in some constituent ions even in the solid state, and many lose their superionicity below a critical temperature. The conductivity of K2NiF4 structure-based Ba1.75LiH2.7O0.9 jumps over three orders of magnitude at ~ 573 K. This jump is not at a first-order structural phase transition temperature. Molecular dynamics (MD) simulations using a density functional theory (DFT) trained universal neural network potential (NNP) were conducted in this study. The Arrhenius plot of the tracer diffusion coefficient is a straight line between 455 and 741 K, which is inconsistent with the conductivity jump. The 3D activation barrier is ~0.3 eV, which is much lower than typical K2NiF4 structure-based oxyhydrides. The features in the radial distribution function are almost temperature independent. 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 comparison with a universal curve for independent random walkers. These findings hold for all of β, γ, and δ 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.