Excited configurations of hydrogen in the BaTiO$_{3-x}$H$_x$ perovskite lattice associated with hydrogen exchange and transport

TL;DR

This study investigates excited hydrogen configurations in BaTiO$_{3-x}$H$_x$ using muon spin relaxation, revealing two metastable states linked to hydrogen transport and exchange, including a mobile protonic state and a complex with H$^-$ ions.

Contribution

The paper provides experimental evidence of hydrogen configurations in oxyhydrides, identifying a mobile protonic state and a complex with H$^-$ ions, advancing understanding of hydrogen transport mechanisms.

Findings

Identification of a highly mobile interstitial protonic state.

Observation of a complex involving H$^-$ ions at oxygen vacancies.

Detection of Mu$^+$ diffusion and retrapping above 100 K.

Abstract

Excited configurations of hydrogen in the oxyhydride BaTiO$_{3-x}$H$_x$ ($x=0.1-0.5$), which are considered to be involved in its hydrogen transport and exchange processes, were investigated by positive muon spin relaxation ($\mu^+$SR) spectroscopy using muonium (Mu) as a pseudoisotope of hydrogen. Muons implanted into the BaTiO$_{3-x}$H$_x$ perovskite lattice were mainly found in two qualitatively different metastable states. One was assigned to a highly mobile interstitial protonic state, which is commonly observed in perovskite oxides. The other was found to form an entangled two spin-$\frac{1}{2}$ system with the nuclear spin of an H$^-$ ion at the anion site. The structure of the (H,Mu) complex agrees well with that of a neutralized center containing two H$^-$ ions at a doubly charged oxygen vacancy, which was predicted to form in the SrTiO$_{3-\delta}$ perovskite lattice by a computational study [Y. Iwazaki $et$ $al$., APL Materials 2, 012103 (2014)]. Above 100 K, interstitial Mu$^+$ diffusion and retrapping to a deep defect were observed, which could be a rate-limiting step of macroscopic Mu/H transport in the BaTiO$_{3-x}$H$_x$ lattice.