CHINESE JOURNAL OF PHYSICS
VOL. 29. NO. 3
JUNE 1991
Xray Absorption Fine Structure (XAFS) Study
of the High-Tc Superconductor Pb-Bi-Sr-Ca-Cu-0 System $
C. H. Chou ( JZj#EE >, W. F. Pong ( $jZ$E$$ )t
I. N. Lin ( $$$k% )tt and S. F. Tsai ( ?$@jsjx )
Department ofPhysics, National Taiwan tiniversity, Taipei 107, Taiwan, R.0.C
fDepartment ofPhysics, Tamhzng University
Tamsui 251, Taiwan, R.O.C.
f fMaterials Science Center, National Tsing Hua University
Hsinchu 300, Taiwan, R.O.C.
(Received Mar. 11, 1991)
Samples of (Pb,Bi,_x)Sr2Ca2Cu30, were prepared with appropriate proportions of PbO,
Bi,O,, SrCO,, CaCO, and CuO powders. For the high-Tc superconductive samples in which x =
0, 0.2, 0.4, 0.6, we have measured the XAFS spectra of the Cu K-edge, the Pb L,-edge and the
Bi L-,-edge at room temperature in the transmission mode at NSLS using the X-23 beamline.
The results of our study indicate that: (1) Pb atoms within PbO partially substitute for Bi
atoms in Bi-Sr-Ca-Cu-0 superconductors; (2) there is evidence for the existence of unreacted
PbO, particularly in the sample in which x = 0.6; (3) the oxidation number of most Pb ions in
the Bi site within the Bi-Sr-Ca-Cu-0 system remains 2; (4) a sufficient proportion of PbO is
essential for the formation of the high-Tc phase, (5) and the addition of PbO is responsible only
for the phase transformation of the low-Tc phase into the high-Tc phase. Moreover, the
electronic structure of the Cu-0 layers is essentially in,dependent of the Pb substitution. The
oxidation number of Cu ions in the high-Tc phase is slightly larger than that of Cu ions in the
low-T, phase, and the average oxidation number of Bi remains nearly constant independent of
either the phase transformation or Pb substitution. We have also deduced that the Cu-0 layers
have an ordered structure, and comment qualitatively about the incommensurate modulations
in the Bi-0 layers.
1. INTRODUCTION
Following the discoveries of the oxide superconductors La-Sr-Cu-0’ and Ba-Y-Cu-0,2
further superconductors Bi-Sr-Ca-Cu-0 (BSCCO) containing no rare-earth elements and with
T, - 110 K and - 80 K were reported by Maeda ef ~1.;~ the latter phases were rapidly
identified as Bi, Sr,
_ Ca,
_ Cu, Oy (high-Tc phase) and Bi, Sr, CaCu, Oy (low-Tc phase). Includ263
264
X R A Y A B S O R P T I O N FINk STRUCTURt (XAi-S) STUDY Ok: THE HIGH-T,.
ing a Bi-Sr-Cu-0 (BSCO) superconductor Bi2Sr2CuOY with Tc - 20 K. previously discovered by Michel et ~1.~ and Akimitsu et al.,” a series of compounds was formulated and
expressed as Bi2 Sr2Ca,_t Cu” OV with n = 1, 2. 3 (n being the number of consecutive Cu-0
layers). The members of this series are commonly designated 2201, 2212 and 2223
respectively according to the ratio of metallic cations in the chemical compositions. Under
normal conditions of preparation, the 2212 type appears to be the most stable and
commonly predominates even when the initial composition is intended to favor 2223. To
prepare the latter phase in pure form has therefore proved challenging, and great effort has
been expended for this purpose.
The principal procedures which have been found to increase the volume fraction of the
high-Tc phase are the following: (1) initial compositions with a greater proportion of Ca
and Cu than the stoichiometric ratio of 2223 compounds,6,7 (2) substitution of Pb for Bi
or the addition of Pb to the composition of BSCCO, *,’ (3) heat treatments involving either
prolonged sintering” or sintering under a small partial pressure of oxygen.11,12 In our
work we have applied and illuminated the second method. Although many investigations
of the lead-oxide-modified BSCCO system have been undertaken, neither the mechanism
of the promotion and stabilization in the method of Pb addition to prepare the pure highT, phase nor its influence on the BSCCO system has been definitively elucidated.
Based on the fact that in the mechanism of the transitions the predominant factors are
distinct, the xray absorption fine structure (XAFS) is conventionally distinguished into two
regions: xray absorption near-edge spectroscopy (XANES)‘3-‘g and extended xray absorption fine structure (EXAFS). 2o XANES can provide electronic and structural information
about an xray absorbing atom; EXAFS has proved a useful tool to determine the local
structure surrounding an xray absorbing atom. By employing these XAFS techniques and
by considering the results of auxiliary experiments on the samples, we have succeeded in
clarifying some ambiguous statements concerning the Pb-modified BSCCO system which
were proposed by other investigators based on different measurements. We also present the
XAFS evidence to reconfirm the established conclusions concerning the BSCCO system.
II. EXPERIMENTAL PROCEDURE
II-I. Sample preparation and characterization
We prepared a series of superconducting compounds having nominal composition
(PbxBi2_x)Sr2Ca2Cu30Y with x = 0, 0.2, 0.4, 0.6 (abbreviated hereafter “the Pb-BSCCO
samples”) , using PbO, Bi, 0, , SrCO,, CaCO, and CuO powders of at least 99.9% purity.
The mixed powders of appropriate proportions were calcined in air at 800 “C for 4 hours,
ground. calcined again in air at 820 “C for a further 4 hours, and then reground. The
reground samples were pressed into pellets and sintered in air at 860 “C for 192 hours.
Another sample with x = 0 was prepared by the same process, except that the duration of
sintering was 96 hours. (We refer below to this sample by specifying its sintering duration.)
-..
c. H. CHOU, W. F. PONG, I. N. LIN AND S. 1.. TSAI
265
An xray powder diffraction experiment showed that the PbO powders contain two phases
with the red lead II oxide being the major component, whereas the B&O3 powders exist as
the a-phase.
We determined the superconducting transition temperature (Tc) of each sintered
sample from the temperature dependence of the ac susceptibility (x,), using a Sumitomo
Model SCR 204T measuring system operated at 1 kHz. The crystal structure and phase constituents of the corresponding samples were determined by means of the xray-diffraction
(XRD) technique (Rigaku, Cu K,). The XRD patterns, shown in Fig. 1 (a) and (b), reveal
that only the 2212 phase is observed for the Pb-BSCCO samples (x = 0,0.2). The propor-
o 2212 phase
BSCCO (x=0)
0
0
0
0
em-
o 2212 phase
28 (degree)
Cc)
28 (degree)
Pb-BSCCO (x=0 2)
2
2 600 -
Pb-ESCCO
(x.0.6)
Ii
0
z
;;
0
2 LOO:
z
200 -
Cd;
(b)
28
16
2‘
28 (deg reel 34
44
(degree)
FIG. 1. Xray diffraction patterns in the range of 28 = 4’ - 54’ of the Pb-BSCCO samples in which (a) x
= 0. (b) x = 0.2, (c) x = 0.4, (d) x = 0.6.
tion of secondary phases was too small to be detected. In contrast, the Pb-BSCCO samples
(x = 0.4, 0.6) are predominantly the 2223 phase. with a small proportion of 2212 phase
coexisting (Figs. l(c) and 1 (d)). The beneficial effect of PbO addition to enhance the
formation of the 2223 phase is thus clearly demonstrated. This result agrees with previous
studies.**’ T h e x, measurement (Fig. 2) shows that there are 3 transition temperatures.
51
-.-. ___
266
XRAY ABSORPTION I-'INE STRUCTURE IXAFS)STUDY OFTHE HIGH-Tc...
superconducting transition of the Pb-BSCCO sample (x = 0) occurring at 70 K indicates
that only one superconducting phase exists in this sample. whereas the transitions of the PbBSCCO sample (X = 0.2) occurring at 106 K and 79 K, respectively, indicate that there are
The
- -0.006
GY
5 -0.010
V
g -0.014
*
3 -0.018
E
-0.022
-0.026
-0.030
10
30
50
70
90
110
TEMPERATURE (K)
FIG. 2. ac susceptibility data for the Pb-BSCCO samples (x = 0,0.2,0.4,0.6).
two superconducting phases coexisting in this sample. One of them, the same phase as in
the Pb-BSCCO sample (x = 0), is identified as the 2212 phase, indicated by the XRD
patterns shown in Fig. 1 (a), (b). The other is identified as the 2223 phase, because the
superconducting transition occurs at the same temperature as for the Pb-BSCCO sample (x =
0.4). The Tc onsets are 106 K and 92 K for the Pb-BSCCO samples (x = 0.4, 0.6) and
correspond to the 2223 and 2212 phases, respectively. The mechanism that explains why
the superconducting transition of the 2212 phase in the Pb-BSCCO samples (x = 0, 0.2)
occurs at lower temperature than that of the 2212 phase in the Pb-BSCCO samples (x = 0.4,
0.6) is not well understood. The discrepancy in the composition between the starting
mixture Bi:Sr:Ca:Cu = 2:2:2:3 and the resulting phase Bi:Sr:Ca:Cu = 2:2:1:2 seems to be
the main source. This effect might decrease the superconducting transition temperature via
the formation of non-superconducting secondary phases enveloping the superconducting
grains.
n-2. Xray absorption measurements
Xray absorption measurements were performed on beam line X-23 of the National
Synchrotron Light Source (NSLS) with a Si (220) double-crystal monochromator and ionchamber detectors. During this run, the NSLS was operated at an energy 2.5 GeV and an
electron current 225 - 1 17 mA. The photon energy for each measurement (CU K+dge, pb
L,-edge, and Bi L,-edge) was simultaneously calibrated by means of CuO, PbO and Bi, 0, ,
-
I,.
,~
-_
C. H. CHOU, W. F. PONG, I. N. LIN AND S. F. TSAI
261
which were placed after the corresponding samples. The data were measured in the transmission mode. The energy resolution AE/E was approximately equal to 10 -4 for each
measurement. The XAFS samples were ground into fine powders (-400 mesh), and then
spread uniformly onto adhesive cellulose tape which was then folded in order to provide the
desired thickness. All spectra were measured at room temperature.
III. DATA ANALYSIS AND DISCUSSION
III-1 . Cu K-edge
The near-edge structures measured in the vicinity of the Cu K-edge for the Pb-BSCCO
samples (x = 0, 0.2, 0.4, 0.6) and for CuO are illustrated in Fig. 3. Although the features of
the XANES spectra are not readily separated and identified without both a detailed know-
Pb- BSCCO
samples
ENERGY (eV)
FIG. 3. Xray-absorption near-edge spectra, measured at the Cu K-edge and normalized to the edge step, for
7 0.4,0.6. The energy zero corresponds to 8980
CuO and Pb-BSCCO samples in which x = 0. O.i,
eV.
le?lge of the materials and sophisticated calculations, there have been published several
valuable studies21-26 on the interpretation of the features of the Cu K-edge XANES spectra.
The edge features, in particular of the peaks B and D, are assigned as follows. (1) The samll
pre-edge feature A indicates the Is-to-3d quadrupole transition. (2) The set of features
falling between the labels B and C is dominated by an overlap of the dipole-allowed direct
transitions to Cu 4p-like final states which are split into out-of-plane (4~~) and in-plane (4p, j
states by crystal-field effects. The shoulder at 4p, *. clearly observed in the plot of XANES
268
.
XRAY ABSORPTION FINESTRUCTURt(XAFS) STUDY OFTHEHICH-Tc..
for CuO. is attributed to transitions to the 4pn * state. The asterisk indicates that the lsto-4pn transition is accompanied by a shake-down transition associated with charge transfer
from the ligand to the metal; this charge transfer enhances the screening of the core hole
and decreases the transition energy from that associated with the p states. The main peak C
at the top of each edge is attributed to the transition to 4p0 states. (3) The edge feature D,
which is the signature of EXAFS mutiple scattering (MS) within the Cu-0 plane,
is associatI
ed with EXAFS contributions from high-shell neighbours. It is salient that there is no
significant difference among the XANES spectra of the Pb-BSCCO samples (x = 0, 0.2, 0.4,
0.6). Hence the electronic and local structures around the Cu atom in the Pb-BSCCO
samples (x = 0, 0.2, 0.4, 0.6) are almost equivalent to each other regardless of the phase
composition or the addition of PbO, despite the different valence states which are indicative
of the edge shift. The strength of the feature 4pn * in the case of the Pb-BSCCO samples (x
= 0, 0.2, 0.4, 0.6) is significantly less than that found in CuO. In previous work, there was
the hypothesis that there is a lack of delocalized Cu p states close to the Fermi leve1.27 A
more likely interpretation is that holes exist in the 0 2p band; such holes would be expected
to reduce the probability of charge transfer from the 0 2p band, as such transfer would
further deplete the band.22
Despite the vagueness of identification of the specific features of the absorption edge,
some reliable conclusions about the valence of Cu may be drawn from the position shift of
the absorption edge. In the xray-absorption measurement, the position of an edge is equal
to the difference of the energies of the initial and final states involved in the transition. If a
valence electron is removed from the absorbing atom, the effective potential seen by the
initial and final states changes; the change in the transition energy, which is displayed as the
shift of the absorption edge due to the removal of an electron, is approximately equal to the
difference in the energy variation of the two states, on account of the screening change.
The method of measuring the shift of the absorption edge caused by the variation of the
electronic configuration of the absorbing atom, known as the chemical shift, has long been
used to establish the oxidation number in unknown materials.28
For the Cu K-edge, the position of the absorption edge shifts to greater energies as the<
number of valence electrons removed increases with a resulting increase of the oxidation
number of the Cu atom. Based on the calculation provided by Davenport et al,,29 it is
evident that the shift in energy of the edge position is not linearly proportional to the
number of electrons removed. The addition of a 3d electron to, or removal from, a copper
atom causes a shift approximately 10 eV in the transition from 1s to 4p; changing the
occupation of a 4s or 4p state by unity results in a shift of only - 2 eV. Non-integral 3d
occupancy, as expected for itinerant electrons in a solid, can result in a continuous variation
of edge position.
Figure 4 presents an expanded view of the vicinity of the positions of the Cu K-edge
for the Pb-BSCCO samples (x = 0,0.2, 0.4, 0.6) and for CuO. This figure shows that the Cu
K-edge positions for the Pb-BSCCO samples (x = 0, 0.2, 0.4, 0.6) shift to energies greater
than that of CuO; the Cu K-edge positions for the Pb-BSCCO samples (x = 0.4, 0.6) shift
C.H.CHOU,W.F.PONG,I.N.LIN
---
AND
S.F.TSAI
269
BSCCO
ENERGY (eV)
FIG. 4. Xray-absorption near-edge structure in the vicinity of the Cu K-edge for CuO and for Pb-BSCCO
.
samples in which x = 0,0.2,0.4,0.6. The curves have been shifted vertically for clarity, and the
energy zero refers to the K-absorption edge (8980 eV) of pure Cu.
to -3 eV above those for the Pb-BSCCO samples (x = 0, 0.2). It is widely believed3’ that
the charge carriers in the copper-oxide high-temperature superconductors, including the
BSCCO ceramics, are holes located in the Cu-0 planes which move in a band formed predominantly from 0 2p states near the Fermi energy. Therefore, taking into account our
results of XRD and x, measurements and applying the interpretations of Transquada et
aL31 to the XANES spectra of La2_x(Ba,Sr)xCuO+Y, we draw the following two conclusions. First, the oxidation number of the Cu cation in CuO, of which the valence configuration is 3d,9 is 2. This value is slightly smaller than the oxidation number of the Cu
cation within Pb-BSCCO samples (x = 0, 0.2, 0.4,0.6), of which the valence configuration32
is 3d91, (I, indicates the hole in the ligand) with a resulting occupation of the 3d states of
Cu midway between 8 and 9, because the 3d electrons are itinerant in the solid. Second, the
oxidation number of Cu in the Pb-BSCCO samples (x = 0, 0.2), of which 2212 is the major
phase, is slightly smaller than the oxidation number in the Pb-BSCCO samples (x = 0.4,0.6),
of which 2223 is the major phase. The reason is that there is slightly greater probability of
itinerancy in the ligand of 3d electrons with an accompanying greater concentration of
mobile holes in the 0 sites of the Cu-0 planes within the Pb-BSCCO samples (x = 0.4, 0.6).
We extracted the EXAFS function x(k) of the Pb-BSCCO samples (x = 0,0.2, 0.4,0.6)
from the absorption spectra of the Cu K-edge by choosing E, at the half-height point of the
absorption edge, and by performing background subtraction and normalization. We define
the EXAFS function x(k) as”
x(k) =
ME) - p,,(E)
F,(E)
(1)
---i
XRAY ABSORF’TION FINE STRUCTURE (XAFS) STUDY OF THE HIGH-T, .
270
in
which /.I is the absorption coefficient, /.L, is the structureless absorption coefficient of the
isolated atom in question, and k is the wave vector of the ejected photoelectron. The xray
energy E is converted into the wave vector k by means of the equation
k = [2m,(E - E,)/h’ ] 1’2
(2)
Here E, is the so-called threshold energy, m, is the mass of electron, and energy and wave
vector are conventionally expressed in the units eV and A-’ respectively. The magnitude of
the Fourier transform of k3x(k), obtained by transforming over the k-range 2.63 - 11 .18
A-l , shows peaks corresponding to shells of neighbours around the Cu absorbing atom, as
shown in Fig. 5. The peak positions are shifted a few tenths of an angstrom from the
actual interactomic distance due to the EXAFS phase shift. Because of the complexity of
7
26
- x=0.4
5 5
d
$4
V
0
0
1
2
3
4
R L$
FIG. 5
Magnitudes of the Fourier transforms of the EXAFS interference functions k3 X(k) over the krange 2.63 - 11 .I8 a-l converted by choosing Ee at the half-height point of the absorption edge
for the Pb-BSCCO samples (x = 0,0.2,0.4,0.6) and the Pb-BSCCO sample sintered for 96 hrs.
The peaks appearing near 0.9 a in the plots for the Pb-BSCCO samples (x = 0.2,0.4) are caused
by the residual background in the spectra and lack physical meaning.
the known structure of the Bi-Sr-Ca-Cu-0 system at room temperature, the transforms
cannot resolve the contributions of all the different atoms into well separated peaks of only
one atomic species. The first peak located at - 1.5 A, appearing in each plot of the magnitudes of the Fourier transforms, is predominantly due to the four 0 nearest neighbours
within the Cu-0 plane. The peak located at - 2.7 A corresponds to the Ca and Sr atoms
around the Cu absorbing atom. It is notable that the third peak (Cu-Cu peak)33 located at
- 3.7 A, appearing in each plot of the magnitudes of the Fourier transforms of k3 x(k) of
w-
C. H. CHOU, W. F. PONG, I. N. LIN AND S. F. TSAI
271
the Pb-BSCCO samples (x = 0, 0.2, 0.4, 0.6) and of the BSCCO sample (x = 0; 96 hrs), has
a great amplitude because of focused multiple scattering via the intervening oxygen atoms.
The appearance of the focusing effect indicates that the Cu-0 planes within each Pb-BSCCO
sample (x = 0, 0.2, 0.4, 0.6) and the BSCCO sample (x = 0,96 hrs) have an ordered planar
structure, viz., not only is the distance between each Cu atom and the nearest Cu atoms
equivalent, but also the positions of the Cu atoms and the intervening oxygen are colinear.
111-2. Pb L, -edge
The XANES spectra of the Pb L,-edge of the Pb-BSCCO samples (in which x = 0.2, 0.4
and 0.6) and of the PbO and Pb foil are illustrated in Fig. 6; the energy zero in this plot
refers to 13035 eV. Because the XANES spectrum of the Pb L,-edge of the Pb foil is
I
I
I
I
(a) -
ENERGY (eV)
FIG. 6. Xray-absorption near-edge spectra, measured at the Pb L,-edge and normalized to the edge step,
for (a) Pb metal, (b) PbO and for the Pb-BSCCO samples in which (c) x = 0.2, (d) x = 0.4, (e) x =
0.6. The energy zero corresponds to 13035 eV.
clearly different from the spectra of the Pb L,-edge of the Pb-BSCCO samples (x = 0.2,0.4
and 0.6), we confirm that the proportion of metallic lead in each Pb-BSCCO sample is extremely small. Fig. 6 also shows that the XANES spectrum of the Pb L,-edge of PbO is
different from the spectra of the Pb L,-edges of the Pb-BSCCO samples (x = 0.2 and 0.4)
and is slightly different from that of the Pb L,-edge of the Pb-BSCCO sample (x = 0.6).
These results indicate that, in the Pb-BSCCO samples (x = 0.2, 0.4 and 0.6), a substantial
proportion of the Pb initially contained in the added PbO is no longer present in the PbO
phase. Unfortunately the signal-to-noise ratio of the XAFS spectra of the Pb-BSCCO
samples (x = 0.2. 0.4 and 0.6), containing a small concentration of Pb. is too small to allow
--..-- .
-
.
212
us
.
.
XRAY ABSORPTION FINESTRUCTURE CXAFS)STUDY OFTHEHIGH-Tc...
to extract information about the local structure around Pb by analyzing the EXAFS
spectra of the Pb La -edge.
By comparing the XANES spectrum of the Pb L,-edge of PbOz reported by Boyce et
al,34 with the XANES spectra of the Pb La-edges of the Pb-BSCCO samples (x = 0.2, 0.4
and 0.6) (shown in Fig. 6), we find that each XANES spectrum of the Pb La-edge of the PbBSCCO samples does not exhibit the absorption peak corresponding to the transition from
2p to 6s. This peak can be observed in the XAFS measurement of samples containing Pb
cation with oxidation number exceeding 2, such as PbO;! . The peak would be expected to
be located near -10 eV in Fig. 6. (The bump near -10 eV appearing in the Pb La-edge of
the sample in which x = 0.2 is an instrumental artefact.) Furthermore there is no evidence
that the energy of the average edge position of the Pb La-edge of the Pb-BSCCO samples (x
= 0.2, 0.4 and 0.6) exceeds the energy of the average edge position of the Pb L,-edge of
PbO. (As Pb atoms in the metal and in the oxides are not in an equivalent environment, it is
futile to compare the energies of the absorption edges of lead oxide and of Pb foil.) Based
on the fact that these results are consistent, we therefore assert that the oxidation number
of a large proportion of the Pb cation subtituting for the Bi sites remains 2. Thus we disagree with Dou et ~1.~~ who believe that the Pb appears to be tetravalent. Because the
XANES spectrum of the Pb La-edge of the Pb-BSCCO sample (x = 0.6) is similar to that of
the Pb L,-edge of PbO (compare curves (b) and (e) in Fig. 6), we propose that the PbBSCCO sample (x = 0.6) contains some proportion of PbO. The first derivative of the
XANES spectrum of the Pb La-edge of the Pb-BSCCO sample in which x = 0.6 resembles
the first derivative of the XANES spectrum of the Pb La-edge of the PbO. This fact
supports our proposal that some PbO is present in the Pb-BSCCO sample (x = 0.6).
Figure 7 (a) presents the XANES spectra of the Bi La-edge of the Pb-BSCCO samples
5i?
= 0.12
I
1
5
/h___'(a)
_J__
ENERGY (eV)
FIG. 7(a) The Bi Ls-edge absorption near-edge spectra for the Pb-BSCCO samples in which (a) x = 0 and
(b) x = 0.2; the energy zero in the two spectra refers to 13415 eV. For comparison, the Pb L,edge absorption near-edge spectrum of the Pb-BSCCO sample (x = 0.2) is shown in (c); here the
energy zero refers to 13033 eV. The vertical axis for each curve is normalized to give the same
step height in arbitrary units.
C.H.CHOU,W.F.PONG.I.N.LIN
AND S.F.TSAI
213
(x = 0 and 0.2) and the XANES spectrum of the Pb La-edge of the Pb-BSCCO samples (x =
0.2). The XANES spectra of the Bi La-edges of the Pb-BSCCO sampls (x = 0 and 0.2) are
similar; they are discussed in detail in the following section. Because there is no substantial
change of XANES between the Pb and Bi La-edges, we, following the interpretation which
Boyce et al.34 gave to their XAFS studies of the superconducting BaBio,25 Pb0,75 OY, claim
that almost all lead atoms in the additional PbO are situated in the Bi-0 layers substituting
for a portion of the Bi atoms. This conclusion is confirmed by the similarity of the first
derivatives of the high-energy region of the XANES spectra, which are sensitive to the local
structure, of the Bi and Pb L,-edges of the Pb-BSCCO sample(x = 0.2) (shown in Fig. 7(b)).
5 0.0
i=
a
8
m
0
2
20
60
80
ENERGY (eV)
FIG. 7(b) Derivatives of the spectra of the Bi L3-edge and of the Pb L,-edge for the Pb-BSCCO sample
(x = 0.2) in Fig. 7 (a).
Although the presence of the PbO phase becomes more apparent as the concentration of the
additional PbO increases (because the XANES spectra of the Pb-BSCCO samples (x = 0.2,
0.4, 0.6) increasingly exhibit the features of the XANES spectrum of PbO), we similarly
believe that lead atoms also substitute for some Bi atoms in the Bi-0 layers of the PbBSCCO samples (x = 0.4 and 0.6).
Recent studies indicate that the role of lead oxide in the formation of high-Tc superconducting single-phase Bi-Sr-Ca-Cu-0 ceramics includes several effects which are mainly the
results of the substitution of Pb for Bi and of the catalytic influence of the PbO added to
the initial mixtures.36 Among the effects of substitution of Pb for Bi which have been proposed are the following: 1) modifying the incommensurate modulations (vi& infra) in the
Bi-0 layers,37938 2) promoting and stabilizing the poorly crystallized high-Tc phase,39p35 and
3) transferring charge from Cu-0 planes and therefore influencing the concentration of holes
in the Cu-0 planes and the superconducting properties of the Bi-Sr-Ca-Cu-0 systems.“’
Kijima et al.41 reported that large proportions of the high-Tc phase are produced by repeating the cycle of the disproportionation reaction of the low-Tc phase which occurs in the
PbO flux. Konstantinov et al.42 suggested that it is the catalysis of PbO - rather than the
274
.
XRAYABSORFTION FINESTRUCTURE(XAFS)STUDYOFTHEHIGH-Tc
introduction of Pb in the Bi-Sr-Ca-Cu-0 lattice - which facilitates the high-Tc phase.
There have been few studies that address the electronic structure of these Pbsubstituted materials; hence it is not definitely known whether the substitution of Pb for Bi
perturbs the electronic states important for superconductivity in this system. Furthermore,
there has been no conclusive explanation of the exact mechanism of the substitution of Pb
for Bi, which enhanced the development of the high-Tc phase, nor has there been much
literature available concerning the influence of PbO added to the sample.
Considering the results obtained from the XAFS spectra of the Cu K-edge and Pb L3edge in tandem with those from the x, and XRD measurements, we formed two conclusions. First, the phase composition (the low-Tc phase is in the majority) of the PbBSCCO sample (x = 0.2) is still similar to that of the lead-free BSCCO sample. Second, the
electronic structure of the Cu-0 planes of the lead-free BSCCO sample and the electronic
structure of the Cu-0 planes of the lead-substituted BSCCO sample (x = 0.2) are almost
equivalent although Pb cation has substituted for Bi cation in the Bi-0 layrs within the PbBSCCO sample (x = 0.2). Thus we have proved that the incorporation of Pb to replace Bi
does not perturb the electronic structure near the Fermi energy of the Cu-0 planes, i.e. Pb
cation substituting for Bi cation does not act as donor or acceptor of holes for the Cu-0
planes. This result is consistent with the results reported by Zhang et al.43 but differs from
those reported by Wells et al4 (It proves worthwhile to compare the two.) Obviously, the
strong correspondence between the phase composition and the superconductivity of the
BSCCO ceramics implies that the Ca cation exerts a strong influence on the electronic
structure of the Cu-0 planes within these superconducting materials as reported by Kanai
et aZ.45 Furthermore, the results of the x ac and XRD measurements and the results of the
analysis of XANES spectra of the Pb L,-edge also indicate that the variation of the phase
composition is not proportionally related to the substitution of Pb for Bi; these results also
indicate that the high-T, phase, as the content of the additional PbO reaches a certain proportion in the starting mixtures, profoundly increases so as to become the majority phase.
In conclusion, the influence of the substitution of Pb for Bi contributes only to structural
changes, such as the modification of the incommensurate modulations in the Bi-0 layes and
to the stabilization of the lattice structure of the high-Tc phase, as reported by Wen et aZ.46
The addition of PbO, in conjunction with an appropriate fabrication process, plays a crucial
role in creating a reaction path favorable to the growth of the high-Tc phase; if the substitution of Pb for Bi is however to play a critical role, the proportion of the substituted Pb must
exceed 10 per cent.
111-3. Bi L, -edge
Because there are at lest 2.33 times as many Bi atoms as Pb atoms in the Pb-BSCCO
samples, there is no observable contribution from XAFS of the Pb L,-edge which extends
past the Bi L,-edge although the two edges are separated by only AEz380 eV. Therefore,
neglecting the contributions of the XAFS spectra of the Pb L,-edge and treating those as
the background, we reliably obtain the XAFS spectra of the Bi L,-edge simply by removing
C. H. CHOU, W. F. PONG, I. N. LIN AND S. F. TSAI
215
the background.
The edge spectra of the Pb-BSCCO samples (x = 0,0.2,0.4,0.6) and of BiZ O3 are compared in Fig. 8. There are no significant shifts to higher energy, but there are observable
-20.00
0.00
20.00
40.00
6(
.oo
ENERGY (eV)
FIG. 8. Normalized xray-absorption near-edge structures, measured at the Bi La-edge of (a) Biz03 and the
Pb-BSCCO samples in which (b ) x = 0, (c) x = 0.2, (d) x = 0.4, (e) x = 0.6. Plot (a 1) in the case
of the Pb-BSCCO sample (x = 0) presents the edge of Bi, 03. The solid line (b 1) in each case of
the Pb-BSCCO sample (x = 0.2,0.4,0.6) presents the edge of the Pb-BSCCO sample (x = 0) for
comparison. The energy zero refers to 1341.5 eV. The curves have been shifted vertically for
clarity _
changes in the fine structure of the edge by comparison with the BiZ03 standard. Of
particular interest are the small bumps near -8 eV appearing in the XANES spectra of the
Bi L,-edge of the Pb-BSCCO samples (x = 0,0.2, 0.4, 0.6). These bumps are observed more
easily in Fig. 9 which shows derivative spectra of the low-energy region; these bumps are
largely absent from the XANES spectrum of the Bi L,-edge of BiZOs. Based on bandstructure calculations,47 we expect the low-lying states to consist of Bi(6s) and O(2p) states.
For the L,-edge, dipole selection rules allow direct transitions to unfilled Bi s and d states.
The occurrence of a low-lying bump thus indicates the presence of empty Bi(6s) states. For
Bi3+, like the Bi cation within Biz 03, these states are completely filled; hence the existence
of these bumps reveals that the oxidation number of some Bi cation within the Pb-BSCCO
samples (x = 0, 0.2, 0.4, 0.6) exceeds 3. Support for this interpretation is found in the
report by Heald et ~1.~~ who present systematic XANES spectra of standard materials in
which the oxidation number of the Bi cation varies from 3 to greater than 3. However,
there are no significant shifts of the Bi L,-edge of the Pb-BSCCO samples by comparison
with the Bi I_,-edge of Biz03. Thus, we claim that the oxidation number of a significant
proportion of the Bi cation within the Pb-BSCCO samples is still 3. The main part of rhe Bi
216
XRAY ABSORPTION FINE STRUCTURE (XAFS) STUDY Ok THE HIGH-Tc
-20
-10
0
10
ENERGY (eV)
FIG. 9. Derivatives of the spectra in Fig. 8 for Bi,03 (a) and the Pb-BSCCO samples in which x = 0 (b), x
= 0.2 (c), x = 0.4 (d), x = 0.6 (e).
L,-edge, located at about 20 eV in Fig. 8, should be dominated by transitions to d states
and also be sensitive to structural changes. Some differences concerning the main part of
the edge, between the XANES spectra of the Pb-BSCCO sample and the XANES spectrum
of the L,-edge of Biz 03, are observed, as structural differences occur between Bi, O34g and
Pb-BSCCO (x = 0, 0.2, 0.4, 0.6). A detailed interpretation of these features without consideration of multiple-scattering processes is, in the final analysis, lacking in certitude.
Figure 8 and Fig. 9 also reveal that the positions, structures and the first derivatives of
the XANES spectra of the Bi La-edge of Pb-BSCCO (x = 0, 0.2, 0.4, 0.6) are all similar.
These results, considered in combination with the results of the XRD and x, measurements,
reveal that for the Pb-BSCCO samples (x = 0, 0.2,0.4,0.6) the average oxidation number of
the Bi cation remains nearly constant (but we have insufficient evidence to probe reliably
the possible minute variations of the average oxidation number of Bi between samples of PbBSCCO (x = 0, 0.2, 0.4, 0.6)); the local structures of the Bi cation are similar, regardless of
either the composition of the low- and high-Tc phases or the substitution of Pb.
We extracted the EXAFS interference functions x(k) of the Bi, O,, BSCCO and the PbBSCCO samples (x = 0, 0.2, 0.4, 0.6) by choosing E, at the half-hieht point of the Bi i,edge absorption coefficient p(E), and by performing background subtraction and normalization. We obtained the magnitudes of the Fourier transforms of the x(k) data sets of Bi, 0, ,
BSCCO (x = 0, 96 hrs), and the Pb-BSCCO samples (x = 0,0.2, 0.4, 0.6), shown in Fig. 10
(a) , (b), by multiplying the x(k) data by kZ and by transforming over the k-ranges 2.23 11.23 A-‘, 2.23 - 1 1.78 A-‘, and 2.23 - 11.08 A-‘, respectively. The magnitude of each
_-
‘,’
._,_.
.)
C. H CHOU, W. 1:. PONG, I. N. LIN AND S. I’. TSAI
R
211
(6)
FIG. 10(a) Magnitudes of Fourier transforms of the k2X(k) data (La-edge absorption of Bi) of (a) Bi20,,
(b) the Pb-BSCCO sample (x = 0,96 hrs), (c) the Pb-BSCCO sample (x = 0, 192 hrs), obtained
by transforming over the k-ranges 2.23 - 11.23,2.23 - 11.78 and 2.23 - 11.08 8-l respectively, and by choosing E, at the half-height point of the absorption coefficient 1-1.
Fourier transform of k’x(k) shows peaks corresponding to shells of neighbours around the
Bi absorbing atoms. Plot (a) in Fig. 10 (a) refers to the magnitude of the Fourier transform
of k2 x(k) of Bi, 03. Because within Bi2 0, there are two nonequivalent sites of Bi and
because the Bi-0 distances fall in the range 2.08 - 2.80 &49 the first peak corresponding to
the nearest oxygen atoms in Bi,O, is broad; the peak located near 3.5 Bi indicates the other
d
L
0.15
2 0.10
$
lkll
: 0.05
z 0.00
(e)
h+
5.I
0 . 0 074
6
I 0
FIG 10(b) Magnitudes of Fourier transforms of the k2 x(k) data (L s-edge absorption of Bi) over the
range 2.23 - 11.08 a-’ converted by choosing E, at the half-height point of the absorption
edges for the Pb-BSCCO samples in which (d) x = 0.6. (e) x = 0.4, if) Y = 0.2.
-_.
__L
278
XRAY ABSORPTION FINESTRUCTURE(XAI:S)STUDY
OF- THEHIGH-Tc..
Bi. atoms around the Bi absorbing atoms. Plot (b) shows the magnitude of the Fourier
transform of k2x(k) of BSCCO (x = 0. 96 hrs) and is in agreement with the plot made by
Maeda” et al. who analysed the EXAFS data of Bi-Sr-Ca-Cu-0. No appreciable overlap of
different atomic species occurs before the peak located near 3.5 A in plot (b) in Fig. 10 (a).
The first peak in the Fourier
This peak contains Bi-Sr(Ca) and Bi-Bi components.
transform of each k2x(k) of the Pb-BSCCO samples (x = 0, 0.2, 0.4,0.6), illustrated by the
plot (c) in Fig. 10 (a) and the plots (f), (e), (d) in Fig. 10 (b), respectively, corresponds
obviously to the nearest oxygen atoms in the Bi-0 layers. Of particular interest is that the
second peak, located at - 3.5 A in the Fourier transform of k’x(k) of each Pb-BSCCO
sample (x = 0, 0.2, 0.4, 0.6), shows an exceptionally large reduction of amplitude by comparison with the second peak of the Fourier transform of k’x(k) of the BSCCO sample (x =
0) sintered for 96 hrs. This effect reveals the special character of the Bi-0 layers.
Several investigations of the modulated structures of the Bi-0 layers within both the
low-T, and high-T, phases of the BSCCO system have been conducted recently by means
of xray single-crystal diffraction, xray and neutron powder diffraction and transmission
electron microscopy (TEM). 51-55 It has been ascertained that each Bi-0 layer consists not
of perfect two-dimensional sheets, but contains occupational and positiona fluctuation of
Bi sites which results in “Bi-concentrated bands” and “Bi-deficient bands” and an accompanying displacement of atoms in the Bi-0, Sr-0, Cu-0 and Ca layers from their average
position. A partial substitution of Sr or Cu for Bi, insertion of the extra oxygen in the Bideficient region, the modulation waves of occupation probabilities for the Bi and oxygen
atoms, and ordering of Sr vacancies were variously proposed as inferred origins of the
modulated structure 56-58 The Bi-0 layers within the Pb-modified BSCCO system have also
been considered.59,6b Schneck et al. 61 reported that the addition of lead affects the
modulated structure, whereas the basic structure of the lead-free compounds is preserved.
Wen et aL6’ reported that three modulated structures have been observed in the Pb-BSCCO
superconducting oxides.
In general, it is observed and well known that for a given material the peaks of the
Fourier transform of x(k) of the amorphous phase which refer to more distant shells, are
largely absent compared with the corresponding peaks in the Fourier transform of x(k) of
the crystalline phase. The reason is that in the amorphous phase the distance between the
absorbing atoms and the more distant atoms, with the exception of the nearest atoms, is
variable. Consequently the disappearance of the second peak in the Fourier transform of
k’x(k) of each Pb-BSCCO sample (x = 0, 0.2, 0.4, 0.6) sintered for 192 hrs implies that
there occurs an anomalous static disorder of the distances between the absorbing Bi atoms
and the nearest neighbor Bi, Ca, Sr atoms, This effect is caused by the periodic displacements of the corresponding metallic atoms from their average position in the modulated structure. In contrast, within each Pb-BSCCO sample (x = 0, 0.2, 0.4, 0.6) the average
distance between the Bi atom and the nearest neighbor 0 atoms in the modulated structures
remains roughly constant.
Our work iS apparently the first in which the XAFS technique has been used to detect
C. H. CHOU, W. t:. PONG, I. N. LIN AND S. I;. TSAI
279
the presence of modulated structure in the Bi-0 layers of the Pb-modified BSCCO system.
However, because the modulated structures in the Bi-0 layers are complex, quantiative probing of the modulated structure by means of the XAFS technique requires extreme
versatility.
IV. CONCLUSIONS
We have demonstrated the power of the XAFS technique to probe the electronic
structure near the Fermi energy of Cu-0 planes which is responsible for the superconductivity of the copper-oxide high-temperature superconductors. We have also proved how
the XAFS technique can detect the existence of modulated structures in the Bi-0 layers and
to resolve their properties.
Specifically, the oxidation number of the Cu cation in the samples (x = 0,0.2, 0.4, 0.6)
depends predominantly on the phase constituents but is unlikely to be related to the substitution of Pb for Bi. The oxidation number of the Cu cation within the samples in which
the 2223 phase is in majority is greater than that of the Cu cation within the samples in
which the 22 12 phase is in majority. In spite of the appearance of the modulated structures
in the Bi-0 layers, the change of phase composition and the substitution of Pb for Bi, the
Cu-0 planes preserve an ordered planar structure. Some Pb within the additional PbO
partially substitutes for Bi in the BSCCO superconductors whereas the oxidation number of
most substituting Pb atoms remains 2. The addition of PbO is responsible only for the
phase transformation favoring the 2223 phase. If, instead of the catalysis of the PbO flux,
the substitution of Pb for Bi is to play a critical role in the mechanism of the phase transformation the proportion of substituted Pb must exceed 10 per cent. The oxidation
number of some Bi cations within the Pb-BSCCO samples (x = 0, 0.2, 0.4, 0.6) exceeds 3.
In the Pb-BSCCO samples (x = 0, 0.2, 0.4, 0.6) sintered for 192 hrs there exist modulated
structures in the Bi-0 layers.
ACKNOWLEDGMENTS
We thank Dr. S. M. Peng and Dr. Y. Wang for their help and Dr. M. K. Wu for valuable
information about superconductors. This work was supported by the National Science
Council of the Republic of China under contract No. NSC 80-0208-M032-15Y to W. F.
Pong.
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XRAY ABSORPTION FINE STRUCTURL (XAFS) STUDY 01: THL
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