Research Group for Nanoscale Structure and Function of Advanced Materials
Group Leader : Shin-ichi Shamoto
Members : Wataru Higemoto, Yuki Fukaya, Satoshi Yasuda, Takashi U. Ito, Yuki
Uozumi, Masahiro Yano, Eiko Torikai, Kohei Oshima, Koji Nakashima, Yosuke
Uchibori

The research objective of Nanoscale Structure and Function ofAdvanced Materials group is to study nanoscale structures offunctional devices and materials under external fields by using
high-intensity quantum beams of muons, positron, neutron, andsynchrotron light. As a highlighted result in our group, anatomic configuration of germanene, i.e., a single layer ofgermanium, is experimentally revealed to have an asymmetricsheet structure by Total-Reflection High-Energy Positron
Diffraction method [1]. The phase separation near a metalinsulator transition of Mott system has been revealed as aninternational collaboration of REIMEI research by using
positive muon spin rotation/relaxation spectroscopy (µ+SR) [2].
In this activity report, our recent progress in the research of amixed electron/hydrogen ion conductor with microscopic
insights into hydrogen transport is described from a uniqueapproach using µ+SR.
Excited configurations of hydrogen in the BaTiO3-xHx
perovskite lattice associated with hydrogen exchange andtransport [3]Oxyhydrides of perovskite titanates ATiO3−xHx (A=Ba, Sr, Ca)have attracted much attention because of their fascinating
properties, which are associated with the lability of H− ions [4].
These are obtained as O2−/H− solid solutions and can contain alarge amount of hydrogen up to x~0.6. Macroscopic gas analysis
revealed that the hydrogen is mobile and exchangeable in its
gaseous environment at temperatures above 400 °C. The H
substitution changes the parent band insulators into
paramagnetic metals, as evidenced by electrical resistivity andmagnetic susceptibility measurements. These transport andhydrogen exchange characters make these materials potentiallysuitable for application in mixed electron/hydrogen ion
conductors and hydrogen membranes. However, experimental
insights into the hydrogen dynamics in the ATiO3−xHx lattice arestill quite limited.
In this study, we investigated excited configurations ofhydrogen anions in BaTiO3−xHx (x=0.1–0.5) by µ+SR
spectroscopy, which have been predicted from a computational
study to be involved in its hydrogen transport and exchange
processes [5]. In standard µ+SR measurements, spin-polarized
positive muons with a kinetic energy of ~4 MeV are implanted
into a solid-state specimen and the energy is lost mainly byionization. They are then trapped at local potential minima, notnecessarily at the global minimum. Finally, the electronic
structure of muonium (a hydrogen-like µ+−e− bound state: Mu)or its ionized species (Mu+ or Mu−) is probed by the µ+SR
technique, which is supposed to be very similar to that ofhydrogen. In general, the as-implanted mixture of Mu states isfar from equilibrium and can involve metastable excited states[6]. Therefore, the µ+SR method is suitable to obtainmicroscopic insights into excited configurations of hydrogen in
the BaTiO3−xHx lattice.
Positive muons implanted into the powder samples ofBaTiO3−xHx were mainly found in two metastable states. Onewas assigned to a highly mobile interstitial protonic state asillustrated in Fig. 1(a), which is often observed in perovskite
oxides. The other was found to form an entangled two spin-1/2
system with the nuclear spin of H− ion in the host lattice asshown in Fig. 1(b). The structure of the (H, Mu) complex agrees
well with that of a neutralized center containing two H− ions at
an oxygen vacancy VO2+, which was predicted to form inoxygen-deficient SrTiO3−δ from another computational study [7].
These observations are mostly in line with a theoretical proposal
on hydrogen transport and exchange involving charge-statetransitions [5,7,8]. In addition, we observed interstitial Mu+
diffusion and re-trapping at a deep defect above 100 K as
evidenced from the temperature dependence of a muon spin
relaxation rate associated with the interstitial Mu+ component.
This could be a rate-limiting step of macroscopic Mu/H
transport in the BaTiO3−xHx lattice.

Fig. 1. (a) Atomic configuration for the highly mobile interstitial Mu+.
The first and second nearest neighbour anion sites (1 nn and 2nn) are fixed to be O2− and the third nearest neighbour (3 nn) andfurther anion sites are randomly replaced with H− with aprobability of x/3. (b) Atomic configuration for the (H−, Mu−)
complex at an oxygen vacancy, where hydrogen exchange canoccur.

References[1] Y. Fukaya et al., 2D Mater. 3, 035019 (2016).
[2] B. A. Frandsen et al., Nature Commun. 7, 12519 (2016).
[3] T. U. Ito et al., Phys. Rev. B 95, 020301(R) (2017).
[4] Y. Kobayashi et al., Nat. Mater. 11, 507 (2012).
[5] J. Zhang, G. Gou, and B. Pan, J. Phys. Chem. C 118, 17254(2014).
[6] R. L. Lichti, K. H. Chow, and S. F. J. Cox, Phys. Rev. Lett.
101, 136403 (2008).
[7] Y. Iwazaki, Y. Gohda, and S. Tsuneyuki, APL Mater. 2,012103 (2014).
[8] Y. Iwazaki, T. Suzuki, and S. Tsuneyuki, J. Appl. Phys. 108,083705 (2010).