Chapter 18
Effects of Oxygen Stoichiometry on Structure and Properties in B a Y C u O 2
3
x
D. W. Murphy, S. A. Sunshine, P. K. Gallagher, H. M. O'Bryan, R. J. Cava, B. Batlogg, R. B. van Dover, L. F. Schneemeyer, and S. M. Zahurak
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AT&T Bell Laboratories, Murray Hill, NJ 07974
Results of a series of studies on the oxygen stoichiometry in Ba YCu O , 6.0 ≤ x ≤ 7.0, are reported. The structure at x=7 contains layers of corner-shared, square pyramidal copper and chains of square planar copper. Oxygen is removed from the chains leaving two-fold coordinate copper at x=6. Superconductivity is optimal for x=7 and for x=6 the material is a semiconductor. A valence description ascribes Cu to the Cu in the layers for all x with the Cu in the chains going from Cu at x=7 to Cu at x=6. A model for twinning based on oxygen defects similar to those in Ba YCu O is discussed. 2
3
x
+2
+3
+1
2
3
6
T h e recent discovery of s u p e r c o n d u c t i v i t y above 9 0 K i n a number of cuprate perovskites (1-6) has made these materials the focus of intense scientific effort. T h e general class of compounds exhibiting these high T ' s is B a M C u O ( M = Y , L a , N d , S m , E u G d , D y , H o , E r , T m , Y b ; x ^ 7) (710). Soon after their discovery it became evident t h a t oxygen s t o i c h i o m e t r y plays a v i t a l role i n the structure a n d properties of these materials (11-15). T w o of these studies o n the prototype phase B a Y C u O . have revealed a s t a b i l i t y range of 6.0 < χ < 7.0 (11,15). Reversible changes i n crystallographic s y m m e t r y a n d physical properties have been found to occur over this range of composition. D e t a i l e d studies of the structures, oxygen m o b i l i t y , a n d properties of B a Y C u 0 for 6 . 0 < χ < 7.0 have provided insight i n t o the chemical nature of these materials a n d i n t o possible c
2
3
x
2
2
3
3
a
J P
0097-6156/87/0351-0181$06.00/0 © 1987 American Chemical Society
Nelson et al.; Chemistry of High-Temperature Superconductors ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS
182
mechanisms for s u p e r c o n d u c t i v i t y a n d for crystal t w i n n i n g . these studies are s u m m a r i z e d below. STRUCTURES OF Ba YCu O 2
The
crystallographic
Ba YCu 0 2
3
3
unit
PHASES
z
cells
of
the
are presented i n F i g u r e
6
end
members
analysis indicates t h a t the unit cell of B a Y C u 0 2
Ba YCu 0 2
3
6
is
tetragonal
c=11.8194(3)A (20,21). oxygen
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3
3
is o r t h o r h o m b i c ( P m m m )
7
(P4/mmm) structures
are
with
(17-19) while t h a t
a=3.8570(l)A
conveniently
c o o r d i n a t i o n numbers
of
Cu
from
o c t a h e d r a l c o o r d i n a t i o n of a stoichiometric perovskite. oxygen
and
7
and
described
as
ordering
T h e ordered oxygen vacancies i n these structures result i n
r e d u c t i o n i n the
two
2
deficient perovskites w i t h t r i p l e d unit cells due to B a - Y
along the c axis. a
These
Ba YCu 0
1. N e u t r o n powder diffraction profile
w i t h a = 3 . 8 1 9 8 ( l ) A , b = 3 . 8 8 4 9 ( l ) A , a n d c=11.6762(3)A of
T h e results of
vacancies
result i n four-fold
coordinate (Cu(2)) C u atoms. m a d e of layers a n d chains.
coordinate
the
ideal
six-fold
F o r B a Y C u 0 , the 2
3
(Cu(l)) and
7
five-fold
T h i s c o m p o u n d c a n be thought of as being
The Cu(l)
atoms form linear chains of corner
s h a r e d square planes oriented along the b axis a n d the Cu(2)
atoms
form
t w o - d i m e n s i o n a l layers of corner s h a r e d square p y r a m i d s as shown i n F i g u r e 2.
T h e 0(1)
a t o m from the c h a i n also serves as the a p i c a l oxygen a t o m for
the square p y r a m i d a l Cu(2). of
the
layers
and
T h e relative i m p o r t a n c e to s u p e r c o n d u c t i v i t y
chains is one
of the
c e n t r a l questions
that
must
be
answered to u n d e r s t a n d s u p e r c o n d u c t i v i t y i n these materials a n d to a i d i n the design of new phases.
W e discuss later the dependence of T
c
o n oxygen
s t o i c h i o m e t r y w h i c h m a y provide some insight into this question. T h e s t r u c t u r e of B a Y C u 0 2
the r e m o v a l of 0(4) coordination
about
coordination.
The
3
differs from t h a t of B a Y C u 0
6
2
from along the b axis. Cu(l) five-fold
from
3
by
7
T h i s results i n a change i n the
square
planar
to
c o o r d i n a t i o n about Cu(2)
a
linear
two-fold
is m a i n t a i n e d .
The
C u ( l ) - 0 ( 1 ) distance decreases a n d the C u ( 2 ) - 0 ( l ) distance increases m a k i n g Cu(2) more square p l a n a r as seen i n F i g u r e 3.
T h e structure of B a Y C u 0 2
c a n be described as being made of the same type of layers as i n the phase, but w i t h isolated, two-coordinate
Cu(l)
3
6
x=7
atoms replacing the chains
(Figure 4). A n e u t r o n structure s t u d y of B a Y C u 0 2
3
6 8
(20) showed t h a t the
a d d i t i o n a l vacancies were r a n d o m l y distributed on the 0(4) atoms along the b axis.
Another
study
on
a
composition w i t h
r a n d o m oxygen v a c a n c y on 0(4)
(22),
but
as we
x=6.5 showed
the
same
indicate later, this m a y
depend o n the details of sample p r e p a r a t i o n . T h e structures of the x=6
a n d x=7
valence description of the C u on the different
phases lend themselves to a crystallographic sites.
The
k e y to this description is t h a t the linear two-fold c o o r d i n a t i o n observed for Cu(l)
at x=6
is t y p i c a l of C u
+ 1
compounds (23,24), whereas C u
m a y be four, five, or six coordinate.
+ 2
and
Cu
+ 3
T h e average o x i d a t i o n state of C u in
Nelson et al.; Chemistry of High-Temperature Superconductors ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
18.
Effects of Oxygen Stoichiometry
MURPHY E T AL.
183
A — ~ 7 \
© Cu(2)
Cu(2) 0(2)
0(2)
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P(1) 0(4) Cu(1)
Cu(1) YCu 0 3
Figure
1.
Schematic
Ba YCu 0
i
7
drawings
3
2
of the unit
6
cells
of B a Y C u 0 2
3
7
and
Ba YCu 0 . 2
3
6
Ο BARIUM Ο YTTRIUM •
COPPER
-α
F i g u r e 2. A perspective view of B a Y C u 0 2
Cu(l)
3
emphasizing the chains of
7
a n d layers of Cu(2). 9/1
0/1.795
Ο Κ
. 2.295 (2.469)
1.930 (1.941)
Ô Cu1 Ba YCu 0 2
Figure Cu(l)
3.
3
Cu1 Ba YCu 0
6
2
T h e coordination
a n d Cu(2) in B a Y C u 0 2
3
3
Cu2 Ba YCu 0 (Ba YCu 0 )
7
environments 7
2
3
7
2
3
6
a n d bond
lengths
( A ) for
and B a Y C u 0 . 2
3
6
Nelson et al.; Chemistry of High-Temperature Superconductors ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
184
CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS
the end members is 2.33 for χ = 7 a n d 1.67 for x=6.
Since Cu(2) is relatively
u n c h a n g e d between end members a n d there are two Cu(2) per f o r m u l a unit we
assign it as C u
limiting
formulas
members
and
that
the
Cu
+ 3
2
2
+ 2
0
and
7
T h i s reasoning
Ba YCu 2
B a ^ C u ^ j C u ^ C u ^ O ^
stoichiometrics. along the
in b o t h end members.
+ 2
Ba YCu
Thus, C u
6 axis a n d C u
for
Cu
atoms w i t h the layers.
T h e change i n weight of a sample of B a Y C u 0
7
to C u
occurs
+ 1
i n going
from
0
to
7
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to
is shown
lose
transition
i n F i g u r e 5.
oxygen
occurs.
and
This
end
oxygen indicates
is p r i m a r i l y
a
at C u ( l ) . 2
pure oxygen
the
T h i s further 6
+ 3
leads to the for
6
intermediate
0
r e d u c t i o n of C u
begins
0
+ 2
2
atoms are associated w i t h the chains oriented
+ 3
+ 2
reduction that
+ 1
at
At
686 C
the
0
corresponds
3
when heated in
high temperatures
to
this
compound
to
tetragonal
orthorhombic
an
oxygen
stoichiometry
of
6.6
(11,14,16). A s noted i n F i g u r e 5, b o t h the oxygen loss a n d s y m m e t r y change are reversible even under r a p i d cooling ( l O O ° C / m i n ) . from
high
temperatures
may
preserve
oxygen
Q u e n c h i n g a sample
deficiency,
but
a
more
controlled w a y to assure a p a r t i c u l a r stoichiometry is to heat to the desired weight i n a n inert gas. T h e evolution of lattice parameters a n d cell volume in air as a f u n c t i o n of temperature is displayed in F i g u r e 6. that
the
orthorhombic
to
tetragonal
transition
It c a n be seen
occurs
at
a
lower
t e m p e r a t u r e (610 ° C ) i n air t h a n i n oxygen but the t r a n s i t i o n still occurs at the same oxygen stoichiometry.
T h e stoichiometry at w h i c h the t r a n s i t i o n
occurs does, however, depend on how the sample is p r e p a r e d . a sample
of B a Y C u 0
Ba YCu 0
6
2
3
2
3
prepared
6 5
and B a Y C u 0 2
3
7
combining equal
F o r example,
mole portions of
a n d heating i n a sealed tube at 450 ° C a n d a
series of samples w i t h 6.6 < gettering
by
χ
