A combined deep inelastic neutron scattering and ab initio lattice dynamics study of the hydride anion dynamics and bonding in La2LiHO3 oxyhydride

Øystein S Fjellvåg,Ponniah Vajeeston,Jeff Armstrong,Matthew Krzystyniak,Anja O Sjåstad

doi:10.1088/2399-6528/ab4be6

Abstract

Kobayashi et al (6279) (Science 2016, 351) reported recently the existence of pure H- conductivity in the oxyhydride La2−x−ySrx+yLiH1−x+yO3−y, while demonstrating its functionality through a prototype solid-state Ti/La2LiHO3/TiH2 battery. In this study, we probe the atomistic motion of La2LiHO3 obtained by the promising halide salt flux method, via a combination of deep inelastic neutron scattering (DINS) and ab initio lattice dynamics (LD) calculations verified by vibrational inelastic neutron spectroscopy (INS). We successfully describe the measured momentum distributions from DINS via our LD calculations, without observing any diffusion activation over the temperature range reported by Kobayashi et al. This observation is corroborated by model predictions from our LD study, which reveals that the hydride anions remain bound within a 3D-harmonic potential. We conclude that with the current synthesis parameters, the method produces a vacancy free lattice, and that a necessary ingredient for diffusive motion of H- is the presence of a large population of vacancies. Based on the harmonic prediction for the hydrogen kinetic energy, we derive a picture of the evolution of the effective bonding potential for the hydride anions, and link this to the dynamics associated with decomposition of the oxyhydride.

Highlights

The oxyhydride La2LiHO3 has attracted significant attention since Kobayashi et al utilized the compound and the Sr-substituted analogues La2−x−ySrx+yLiH1−x+yO3−y as H−-conducting electrolytes in prototype solid-state Ti/La2LiHO3/TiH2 batteries operated at 573 K [1]
The fact that the measured standard deviations (SD) of the hydride anion momentum distribution strictly follow the lattice dynamics (LD) prediction within the harmonic approximation across the whole temperature range is a clear signature that the only potential energy surface shape, underlying the nuclear dynamics that can be plausibly fitted to the data, given their quality, is a symmetrical harmonic potential
A deviation from the LD prediction would have been an indirect consequence of the failure of a single-component model to fit a two-component hydride anion momentum distribution due to a build-up of a second much narrower Gaussian peak in the data

Summary

Introduction

The oxyhydride La2LiHO3 has attracted significant attention since Kobayashi et al utilized the compound and the Sr-substituted analogues La2−x−ySrx+yLiH1−x+yO3−y as H−-conducting electrolytes in prototype solid-state Ti/La2LiHO3/TiH2 batteries operated at 573 K [1]. Several methods are in use to prepare La2LiHO3. Both solid-state techniques (high-pressure methods or under a H2-pressure) and various halide salt flux approaches that give phase pure compounds, including our recent discovery to utilize the flux below its melting temperature [1,2,3,4,5,6]. A tetragonal analogue containing anion vacancies is reported by the high-pressure method [1]

Methods

Results

Conclusion