224 High-temperature superconductors
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© Woodhead Publishing Limited, 2011
may be explained partly from the stronger oxygen affinity of larger RE
3+
. RE
3+
is
coordinated (surrounded) by eight oxygen atoms, four of which are in the CuO
2
layer. Hence larger RE
3+
takes a firmer grip on weakly bound oxygen atoms in the
CuO
2
layer.
Next we see how the phase stability changes with Ce doping. Figure 6.9(b)
compares the decomposition lines of T
'
-RE
2
CuO
4
and T
'
-RE
1.85
Ce
0.15
CuO
4
for
RE = Nd, Sm, and Eu. In any RE, the phase stability field of T
'
cuprates is enlarged
by Ce substitution of x = 0.15. The enhanced phase stability by Ce substitution
can be explained similarly to the above RE dependence of the phase stability. Ce
is nearly tetravalent in T
'
cuprates, and takes a firmer grip on surrounding O
2–
ions
by stronger electrostatic force than trivalent RE.
6.3.2 Oxygen nonstoichiometry
Oxygen nonstoichiometry is a phenomenon common to cuprates, which is due to
the weak nature of a Cu-O bond and to the multivalent character of Cu (Cu
0
, Cu
1+
,
Cu
2+
, Cu
3+
, etc.). Oxygen loss in the CuO
2
layer starts to occur in low p
O2
prior to
decomposition, and this loss, in general, degrades and eventually kills high-T
c
superconductivity. In high p
O2
, excess oxygen atoms occupy the interstitial site in
the RE
2
O
2
layer in 214 cuprates: the tetrahedral site in T (the position close to O2
in T
'
) and the apical site (O
ap
) in T
'
. It is well known that interstitial excess oxygen
atoms in T-La
2
CuO
4
provide the CuO
2
layer with holes, achieving high-T
c
superconductivity without Sr substitution. In contrast, interstitial oxygen atoms in
T
'
cuprates are strong pair breakers, and harmful to high-T
c
superconductivity.
Therefore accurate data on oxygen nonstoichiometry in T
'
cuprates are required,
but not available. Although there are dozens of articles, the results do not concur
(Takayam-Muromachi et al., 1989; Kawashima et al., 1994; Moran et al., 1989;
Wang et al., 1990; Idemoto et al., 1990, 1991; Suzuki et al., 1990; Yamaguchi
et al., 1991; Kim and Gaskell, 1993; Klamut, 1993; Zhu et al., 1994a, 1994b,
1995; Radaelli et al., 1994; Schultz et al., 1996; Prado et al., 1995, 1999; Petrov
et al., 1999; Kang et al., 2007; Tanaka et al., 2008).
Even the presence of excess oxygen atoms is not supported by some articles.
Early iodometry titration experiments, employed for evaluation of the absolute
value (y) of oxygen in T
'
-(RE,Ce)
2
CuOy, often concluded y < 4.0. The behavior of
conductivity of T
'
cuprates, increasing monotonically with lowering p
O2
, is also
typical of n-type oxides with oxygen deficiencies. However, the presence of excess
oxygen atoms at the apical site (O
ap
) is in no doubt, as systematic photoemission
spectroscopy from Yamamoto et al. (1997) shows. The contradiction between
the conclusions by this study and others can be traced to the unique feature
of interstitial oxygen in T
'
cuprates. In fact, there is no space sufficient for a
large free O
2–
to reside at O
ap
, which may indicate the formation of a peroxide ion
(O
2
2–
) with neighboring regular oxygen atoms. In this case, interstitial excess
oxygen atoms will neither provide holes to the CuO
2
layer nor change the