Abstract
We report measurements of the thermopower and Co
K
-edge x-ray absorption spectroscopy on bilayer and monolayer
(
Na
,
K
)
x
(
H
2
O
)
y
Co
O
2
−
δ
. According to the Co
K
-edge x-ray absorption spectra, the Co oxidation number for bilayer hydrate
Na
0.33
K
0.02
(
H
2
O
)
1.33
Co
O
2
−
δ
is higher than that of monolayer hydrate
Na
0.07
K
0.21
(
H
2
O
)
0.63
Co
O
2
−
δ
with a chemical shift of
3.5
eV
, which is in sharp contrast with Co
K
-edge x-ray absorption spectroscopy and cerimetric titration results for
Na
x
(
H
2
O
)
y
Co
O
2
−
δ
, where there is essentially no difference of the oxidation number of Co between bilayer and monolayer sodium cobalt oxyhydrates. According to thermopower analysis, band narrowing is observed when
γ
-
Na
0.7
Co
O
2
is transformed to potassium sodium cobalt oxyhydrates.