Excited configurations of hydrogen in the BaTiO3-x Hx perovskite lattice associated with hydrogen exchange and transport

Excited configurations of hydrogen in the oxyhydride BaTiO3-xHx (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 spectroscopy using muonium (Mu) as a pseudoisotope of hydrogen. Muons implanted into the BaTiO3-xHx 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-12 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 SrTiO3-δ perovskite lattice by a computational study [Y. Iwazaki, APL Mater. 2, 012103 (2014)2166-532X10.1063/1.4854355]. 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 BaTiO3-xHx lattice.