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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, 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 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.

Rights:

• Creative Commons BY Attribution 4.0 International
  

Keyword: Superionic conductivity, tracer diffusion coefficient, molecular dynamics, neural network potential, Ba1.75LiH2.7O0.9

Date published: 2026-12-31

Publisher: Taylor & Francis

Journal:

• Science and Technology of Advanced Materials (ISSN: 27660400) vol. 6 2676562

Funding:

Manuscript type: Author's version (Accepted manuscript)

MDR DOI: https://doi.org/10.48505/nims.8635

First published URL: https://doi.org/10.1080/27660400.2026.2676562

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Updated at: 2026-10-01 16:17:50 +0900

Published on MDR: 2026-10-01 18:35:21 +0900