Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 50 (2017)

Na diﬀusion in Nax FeO2J. Sugiyama,∗1 I. Umegaki,∗1 H. Nozaki,∗1 M. Månsson,∗2 O. K. Forslund,∗2 Y. Sassa,∗3 K. Ishida, ∗4
and I. Watanabe ∗4ν (μs-1)

32

NaFeO2
Na0.7CoO2

1(a)
00.5
0.4

Δ (μs-1)

In order to develop next-generation Na-ion batteries,1)1), we have attempted to measure Na diﬀusion iniesSR.2)2). The present
several candidate materials with μ+ SR
target compound, α-NaFeO2 , has a layered rocksalt
structure with the trigonal space group R3̄m (Fig. 1).
The isostructural compound, NaCoO2 , works as a positive electrode material owing to reversible Na+ deinterlattice.3)3). FurFurcalation from and intercalation into the lattice+thermore, μ SR clearly detected Na diﬀusive behavior
2), which is consistent with elecin Nax CoO2 at high TT,2)
results.4)4). Howtron diﬀraction and neutron scattering results
ever, α-NaFeO2 does not exhibit reversible Na deintercalation and intercalation behavior
behavior.5)5).
The following two explanations were proposed forsuch behavior in α-NaFeO2 , 1) Na ions do not diﬀuse,
or 2) Na ions diﬀuse, but electrons are not transferred
for charge compensation. Since α-NaFeO2 is an anti6,7), the second explanation is
ferromagnetic insulator,
insulator6,7)
more reasonable.
Figure 2 shows the T dependences of the ﬁeld ﬂuctuation rate (ν) and ﬁeld distribution width (Δ) for
α-NaFeO2 and Na0.7 CoO2 extracted from the μ+ SR
measured on
on ARGUS.
ARGUS. Here,
spectra measured
Here, νν corresponds
corresponds
to the spin-lattice relaxation rate (T1−1 ) of NMR, andΔ corresponds to the spin-spin relaxation rate (T2−1 ).
For Na0.7 CoO2 , ν increases with T particularly above
325 K, while Δ decreases with T . This is a typicalmotional narrowing behavior due to Na diﬀusion.
For α-NaFeO2 , both ν and Δ are scattered in thewhole T range measured. In particular, there is no ac-0.3
0.2
0.100(b)100 200 300 400 500
TEMPERATURE (K)Fig. 2. T dependences of the ﬁeld ﬂuctuation rate (ν)
and ﬁeld distribution width (Δ) for α-NaFeO2 andNa0.7 CoO2 .

ceptable reason for the increase in Δ above 300 K. Notethat even for stoichiometric NaFeO2 , there are interstitial sites between two adjacent Na+ ions, to which
LiNiO228).8). Back
Back
Na+ ions jump, as in the case for LiNiO
to the data analysis, the ZF- and LF-spectra wereﬁtted by a combination of an exponentially relaxing
dynamic Kubo-Toyabe signal and a time-independent
background signal from the muons stopped in the sample holder. This indicates the presence of two muonsites in the lattice, although the muon sites are unknown at present. However, owing to the large exponential relaxation of the two terms caused by theﬂuctuation of the Fe3+ moments, it is very diﬃcult toextract ν and Δ from the present μ+ SR spectrum withthe statistics of 18M events. Thus, we need additional
data with higher statistics.
References

Fig. 1. Crystal structure of α-NaFeO2 consisting of a twodimensional triangular lattice of FeO2 layers formed byedge sharing FeO6 octahedra separated by the Na layer.
∗1
∗2
∗3
∗4

Toyota Central Research and Development Labs., Inc.,
Department of Materials and Nanophysics, KTH Royal Institute of Technology,
Department of Physics and Astronomy, Uppsala University,
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