Excited configurations of hydrogen in the oxyhydride ${\mathrm{BaTiO}}_{3\ensuremath{-}x}{\mathrm{H}}_{x}$ ($x=0.1\char21{}0.5$), which are considered to be involved in its hydrogen transport and exchange processes, were investigated by positive muon spin relaxation spectroscopy… Click to show full abstract
Excited configurations of hydrogen in the oxyhydride ${\mathrm{BaTiO}}_{3\ensuremath{-}x}{\mathrm{H}}_{x}$ ($x=0.1\char21{}0.5$), 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 ${\mathrm{BaTiO}}_{3\ensuremath{-}x}{\mathrm{H}}_{x}$ 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 ${\mathrm{H}}^{\ensuremath{-}}$ ion at the anion site. The structure of the (H,Mu) complex agrees well with that of a neutralized center containing two ${\mathrm{H}}^{\ensuremath{-}}$ ions at a doubly charged oxygen vacancy, which was predicted to form in the ${\mathrm{SrTiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ perovskite lattice by a computational study [Y. Iwazaki et al., APL Mater. 2, 012103 (2014)]. Above 100 K, interstitial ${\mathrm{Mu}}^{+}$ diffusion and retrapping to a deep defect were observed, which could be a rate-limiting step of macroscopic Mu/H transport in the ${\mathrm{BaTiO}}_{3\ensuremath{-}x}{\mathrm{H}}_{x}$ lattice.
               
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