Local electronic structure of interstitial hydrogen in MgH$_2$ inferred from muon study

TL;DR

This study uses muon spectroscopy to experimentally investigate the local electronic structure of interstitial hydrogen in MgH₂, providing insights that support theoretical models and implications for hydrogen storage kinetics.

Contribution

First experimental assessment of interstitial hydrogen electronic states in MgH₂ using muon spectroscopy, validating DFT-based models and revealing stabilization mechanisms affecting hydrogen kinetics.

Findings

Multiple muon states observed similar to wide-gap oxides

Electronic states linked to donor/acceptor levels in the ambipolarity model

Dehydrogenation stabilizes interstitial H⁻ states

Abstract

Magnesium hydride has great potential as a solid hydrogen (H) storage material because of its high H storage capacity of 7.6 wt%. However, its slow hydrogenation and dehydrogenation kinetics and the high temperature of 300 $^\circ$C required for decomposition are major obstacles to small-scale applications such as automobiles. The local electronic structure of interstitial H in MgH$_2$ is an important fundamental knowledge in solving this problem, which has been studied mainly based on density functional theory (DFT). However, few experimental studies have been performed to assess the results of DFT calculations. We have therefore introduced muon (Mu) as pseudo-H into MgH$_2$ and investigated the corresponding interstitial H states by analyzing their electronic and dynamical properties in detail. As a result, we observed multiple Mu states similar to those observed in wide-gap oxides, and found that their electronic states can be attributed to relaxed-excited states associated with donor/acceptor levels predicted by the recently proposed "ambipolarity model". This provides an indirect support for the DFT calculations on which the model is based via the donor/acceptor levels. An important implication of the muon results for improved hydrogen kinetics is that dehydrogenation, serving as a $reduction$ for hydrides, stabilizes the interstitial H$^-$ state.