Defect Chemistry of High Tc Superconductors
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DEFECT CHEMISTRY OF HIGH Tc SUPERCONDUCTORS KAZUO FUEKI AND YASUSHI IDEMOTO Science
University of Tokyo,
2641 Yamazaki,
Noda,
Chiba
278,
Japan
ABSTRACT The oxygen deficiency 6
in
YBa
2
Cu 3 O7 _, was determined as
a
function of oxygen partial pressure and temperature. It was found that all the 6-log Po 2 data points fall on a single smooth curve (master curve) when the points for each temperature were shifted in
parallel with each other in
by a certain distance.
A theoretical
the direction of abscissa
equation describing the 6 -
log P0 2 relation was derived on the basis of mass
action
law and site conservation.
electroneutrality,
The
theoretical agreed well with the experimental hL.Lster curve.
curve
1.Introduction The oxygen nonstoichiometry produces electronic defects as well as ionic ones in oxides and makes them conductive, in some cases superconductive. nonstoichiometry, so nonstoichiometry in Generally,
High Tc superconductors have large oxygen it is important to study the oxygen
high Tc superconductors.
the oxygen nonstoichiometry of oxides depends on
the oxygen partial pressure and temperature.
Most of oxides have
small
concentrations
oxygen
electronic
nonstoichiometry,
and
approximation
is
defects.
However,
oxides is
large.
deficiency
6
ionic defects
and
the
are low.
So the
dilute
of
solution
well applied to interpret the properties due to the oxygen nonstoichiometry of superconducting In
the case of YBa 2 Cu
amounts
3
O7 _6,
the maximum
to 11% of total oxygen content,
oxygen and
the
linear relationship between log &(or loga ) and log Po 2 no longer holds in the whole P0 2 range. High Tc superconductors are complex oxides containing copper, they decompose into other kinds
of
pressures.
oxides and oxygen at relatively high oxygen partial For example, Ndl. 8 5 Ce 0 . 1 5 Cu04_ 6 decomposes below Mat. Res. Soc. Symp. Proc. Vol. 209. @1991 Materials Research Society
784
log Po 2 (atm.)=-4.5 at 1000"C[lI]. Namely, the chemical stability region is narrow with regard to oxygen partial pressures. The purpose
of this study is
to propose a method for obtaining
&-log P0 2 relation for LnBa 2 Cu a
O7 _6 (Ln:
3
the
rare earth elements)
wide oxygen pressure range and interpreting
it
from the
in
view
point of defect thermodynamics. 2.
The 6-logPo 2 master curve. In
this experiment,
oxalate route YBa content log
was
6 was measured
O7 _6 samples.
3
determined
curves.
P0 2
previous
2 Cu
by iodemetry.
Fair agreement
gravimetrically Figure 1
was found
shows
with
the & -
the
data
by
investigators[2]-[4].
The point of log P0 2 against l/T at constant parallel
straight
Smyth[51.
It
dissolution straight
is
lines
as
already
AB
This means
pointed
6 values by
in it
independent of 6.
Fig.2 is shifted from
should coincide with definite
that the6 -log Po
2
curve of T1
in
of
So if
T1
gave
Melta
concluded that the enthalpy change
into the oxide is
line
constant 6 lines, CD.
using
The reference point of
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