Characteristic mechanism for fast H(- )conduction in LaH2.5O0.25

A Iskandarov and T Tada and S Iimura and H Hosono, ACTA MATERIALIA, 230, 117825 (2022).

DOI: 10.1016/j.actamat.2022.117825

Solid hydrides are a new class of ionic conductors promising for future applications for hydrogen-carrier, catalysis, and energy conversion. The record-high H- conductivity reported for LaH2. 5O0. 25 is caused by extraordinarily large pre-exponential factor, which cannot be justified within any simple diffusion picture. To reveal H- dynamics in LaH2. 5O0. 25 on atomistic level, we developed DFT-based neural-network potential for large-scale molecular dynamics (MD) simulation and discovered the H- diffusion mechanism: the stoichiometric vacancies are ordered and almost immobile, but they can be rarely kicked-out and become fast mobile vacancies that migrate in a coupled manner with 3H- ring-like rotations. The MD and DFT phonon analysis reveal that the mobile vacancy formation gives rise to an extremely large entropy leading to the anomalously large pre-exponential factor, and that the anharmonicity of H- vibrations causes the large pre-exponential factor. A design concept for faster H- conductor is proposed based on the present finding. (C) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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