First-Principles Molecular Dynamics Analysis of Hydride-Ion Diffusion in Ba1.75LiH2.7O0.9
Authors: Haruyama Jun , Takeiri Fumitaka , Kobayashi Genki , Sugino Osamu 
Published: December 2024

Introduction

In Solid State Ionics, a negatively charged hydrogen, that is, hydride (H–), diffuses in solids and contributes to ionic conduction had been a long-standing problem because it is difficult to distinguish between H– and other ions (O2–, H+, etc.). In 2016, it is shown that the ion conductivity of La2LiHO3 can be attributed to pure H– conduction, not mixed with electron, by demonstrating the electrochemical reaction of H– charge transfer in an all-solid-state TiH2/La2LiHO3/Ti cell.1 Since the milestone study, the material investigation of hydride-ion conductors has been actively conducted because of their potential to enable high-performance electrochemical device based on hydrogen. Recently, we reported that the ionic conductivity of Ba1.75LiH2.7O0.9 shows 10–2 S·cm–1 at around 300 °C after a structural phase transition.2 Their results show conductivity jump at 300 ℃ and ultra-low activation energy above the transition temperature, but this behavior has not been understood correctly. In this study, we conduct first-principles molecular dynamics (FPMD) simulations to clarify the origin of high H– conductivity and the mechanism of H– conduction in Ba1.75LiH2.7O0.9. The results of H– diffusion coefficients are compared with quasi-elastic neutron scattering (QENS) analysis. We further discuss about sublattice melting and H– collective motion in Ba1.75LiH2.7O0.9.

Methods and Models

K2NiF4 type structures of Ba1.75LiH2.7O0.9 at low- (β) and high-temperature (δ) phases obtained by neutron diffraction (ND) data2 were adopted as crystal structure models used in FPMD simulations. As the certain anion (and cation) sites is partially occupied by H and a small amount of O (and Ba) according to Rietveld analysis, 2×4×1 and 4×4×1 supercells were employed to reproduce the occupancy ratios to some extent by arranging H, O, and Ba atoms and their vacancy defects. It is noted that the high ionic conductivity of Ba1.75LiH2.7O0.9 is observed at an intermediate (γ) phase, and the crystallographic difference between β and γ phases can be attributed to the partial occupancy ratios of equatorial anion sites. Therefore, we consider that the phase transition from β to γ could be included in FPMD simulations at high temperatures even if we consider only the structures of β and δ phases. FPMD simulations in thermal equilibrium conditions at the temperatures of 500, 700, 900, 1100 K were performed for several hundred ps. Then, the self-diffusion coefficients, Lindemann indices (the indicators of liquid-solid transition), and van Hove time correlation functions of H were calculated from the atom trajectories of FPMD simulations.

Results

We obtained the self-diffusion coefficient Dab
and Dc
, which decomposed in ab plane and c-axis direction, from the mean square displacements (MSD) of H atoms in the β and δ phases. The diffusion process in ab plane corresponds to the H– hopping among equatorial sites. The value of diffusion coefficients Dab
estimated by the extrapolation under high temperature conditions (700, 900, 1100 K, we excluded the simulation of 500 K because of the insufficient number of trajectory ensembles) are consistent with that obtained by the QENS analysis (3–4 × 10–7 cm2·s–1).2 However, the activation energy E
a estimated from calculated Dab
is 0.50–0.51 eV for both phases, which values are different from the estimated values from experimental ionic conductivity: E
a =1.0 eV (β phase) and 0.0 eV (γ phase). From the comparison, it is indicated that the rate-determining process of conductivity is not the ab-plane diffusion of H–, but rather to the c-axis diffusion, ion movement at crystallite interfaces, or collective motions that cannot be accounted for our short time scale simulations. Furthermore, the Lindemann indices were calculated for Ba, Li, O, and H atoms. We found that the Lindemann index of H exceeds the threshold for a solid-liquid transition at high temperatures above 700 K, which suggests the sublattice melting of anion equatorial sites and the superionic conduction of H– in Ba1.75LiH2.7O0.9. In the presentation, we will further discuss the collective motion and diffusion mechanism of H− from the time correlation function.

References

1. G. Kobayashi, Y. Hinuma, S. Matsuoka, A. Watanabe, M. Iqbal, M. Hirayama, M. Yonemura, T. Kamiyama, I. Tanaka, and R. Kanno, Science
351 1314, (2016).
2. F. Takeiri, A. Watanabe, K. Okamoto, D. Bresser, S. Lyonnard, B. Frick, A. Ali, Y. Imai, M. Nishikawa, M. Yonemura, T. Saito, K. Ikeda, T. Otomo, T. Kamiyama, R. Kanno, and G. Kobayashi, Nat. Mater.
21, 325 (2022).