Elucidation of superionic conduction in K2NiF4-type Ba–Li oxyhydrides from a first-principles molecular dynamics study

Abstract

Hydride-ion (H⁻) conductors have attracted considerable attention as charge carriers for hydrogen transport applications. To understand the microscopic H⁻ dynamics and elucidate the superionic conduction characteristics, we investigated K₂NiF₄-type Ba–Li oxyhydrides, Ba_1·75LiH_2·7O_0.9 (BLHO). Three microscopic structural models were constructed, Ba₅₆Li₃₂H₈₈O₂₈, Ba₅₇Li₃₂H₉₀O₂₈, and Ba₅₈Li₃₂H₉₀O₂₉, in which the H and O atoms at the apical sites were ordered in two ways (β- and δ-phase-like). By using first-principles electronic structure calculations, the constructed models showed band gaps of 2.2–2.7 eV, occupied H-1s orbitals, and Bader charges of Ba^1.37+, Li^0.85+, H^0.73−, and O^1.45−, which are consistent with those of pure H⁻ conductors and electrochemical H⁻ carriers observed in previous studies. Using first-principles molecular dynamics (FPMD) simulations, the H⁻ diffusion coefficients of the β- and δ-like models were in good agreement with those reported in previous quasi-elastic neutron scattering analyses. The activation energies of the ab-plane (0.52 and 0.55 eV in the β and δ models, respectively) and c-axis components (0.78 eV in the δ model) from FPMD simulations were found to be related to migration barriers of distinct H⁻ transfer pathways. Furthermore, the Lindemann index of H exceeded the threshold for a solid–liquid transition at temperatures above 700 K, suggesting sublattice melting of the H⁻ equatorial sites. Finally, above the sublattice melting temperature (T ≥ 700 K), the van Hove correlation function showed that the peak growth at the correlation hole position increased for a few picoseconds, and rearrangement indicator analysis showed that H⁻ hopping events tend to form a large cluster, indicating cooperativity in the H⁻ diffusion.