CHINESE JOURNAL OF PHYSICS VOL. 30, NO. 6 DECEMBER 1992
Orientation-dependent X-ray Absorption Fine Structure (XAFS) of the Bi-St=Ca-Cu-0 System
C. H. Chou, W. F. Pang’, J. B. Shi”, H. C. Ku3, and I. N. Lin4 Depurtment of Physics, Nutionul Tuiwun University, Tuipei, Tuiwun 107, R.O.C.
‘Department of Physics, Tumkung University, Tumati, Tuiwun 251, R.O.C. ‘Institute of Electronics, Nutionul Chiuo Tung University, Hsinchrc, Tuiwun 300, R.O.C.
3Depurtment of Physics, Nutionul Tsing Huu University, Hsinchu, Tuiwun 300, R.O.C. 4hfuteriuts Science Center, Nutionul Tsing Huu University, Hsinchu, Tuiwun 300, R.O.C.
(Received June 1,19!Z; revised manuscript received June 27,1992)
Using the transmission and fluorescence modes at the National Synchrotron Light Source (NSLS) X-23A2 beam line, we have measured polarized XAFS spectra of the Bi, Pb L3-edges and the Cu K-edge for the (Bit.8sPbo.ls)Ca2.2Srl.gCu3010+a sample and unpolarized XAFS spectra of the Bi, Pb L3-edges and the Cu K-edge for Bi2Cal,2Srl.@&.Os+~, Pbo.2Bil.gSrl.gCulO6t~ and (PbxBi2.x)Sr2Ca2Cu30Y (x = 0, 0.2, 0.4) samples. All XAFS spectra were measured at room temperature. The incommensurate modulations in the Bi-0 layers are discussed in terms of the XAFS technique. The XAFS spectra of Bi L3-edge do not vary substantially for different sample orientation with respect to the X-ray polarization vector E. There is evidence for the existence within the Bi-Sr-Ca-Cu-0 system of Bi atoms having the oxidation number exceeding 3. Pb ions predominantly substitute for Bi ions in the Bi-Sr-Ca-Cu-0 system because there is no significant difference behveen the XANES spectra of the Pb and Bi L3-edges, respectively. The oxidation numbers of substituted Pb in Bi-Sr-Ca-Cu-0 system differ much from those of Pb & PbO2 in which the oxidation number of all Pb atoms is 4. In agreement with other investigators, our results on the polarized XANES spectra of the Cu K-edge indicate that a satellite about 7 eV above the main K-XANES peak in both polarizations is a multielectron excitation to the final-state con-figuration 153d’~p’, and that a shoulder on the rising absorption edge, present for many Cu compounds and generally assigned to a shake-clown multielectron excitation, is a transition to a final state of the Cu compounds. We have also deduced that the structure of the Cu-0 planes in the Bi-Sr-Ca-Cu-0 samples is ordered.
I. INTRODUCTION
Following the discoveries of the oxide superconductors La-Sr-Ca-O1
and Ba-Y-Cu-0,” further superconductors Bi-Sr-Ca-Cu-0 (BSCCO) containing no rare-earth elements and with
Tc - 110 K and - SO K were reported by Maeda et a1.;3 the latter phases were rapidly identified
$61 01392 TIIE PlIYSICAL SOCIETY OF TI IE REPUBLIC OF CIIINA
862 OFUEh-I-ATION-DEPENDENTXX-RAYABSORiTONFINEFIM_JCTURE... VOL.30
as Bi&Ca$Zu3010+?j with T, (max) = 110 K and Bi2Sr2CaCu208+d with T, (max) = 95 K. Including Bi#r2Cu06+d with T, (max) = 32 K, previously discovered by Michel et al! and Akimitsu et al.,’ a series of compounds was formulated and expressed as BizSr2Ca,_
1Ch,O2,.,+4+~ with n = 1,2,3 (n is 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. Great ef-fort has been expended to prepare the 2223 phase in pure form. There are several procedures6 that have been found to increase the volume fraction of the 2223 phase. The key of these pro-cedures is the addition of Pb to the composition of Bi-Sr-Ca-Cu-0.7 The influence of Pb sub-stitution on the Bi-Sr-Ca-Cu-0 system is related closely to the location and oxidation number of the substituted Pb. Lead ions substitute mainly for bismuth ions in the Bi-Sr-Ca-Cu-0 system; this ‘deduction, which is still debated,819 originates from the result of local chemical electron microanalysis,l’ but relies on few experimental facts.‘1*‘2 In contrast with this evidence, from a neutron-diffraction study on powders the location of most Pb ions was assigned to calcium sites.13 The conclusions about the oxidation numbers of the substituted Pb are controversial.14 The extra oxygen is commonly observed with the appearance of incommensurate modulation
(vi& irzfra) in th e Bi-0 layers and creates holes that are considered possibly responsible for su-perconductivity. The electronic properties, such as metallic nature, of the Bi-0 layers seemingly plays an essential role in superconductivity.” However, because the structures of the Bi-0 layers are extremely complicated, the related properties of the Bi-0 layers have not yet been definitively elucidated. It is still desirable and proves challenging to lift the uncertainty about the issues mentioned above.
The X-ray absorption fine structure (XAFS)16 is conventionally distinguished into two regions: X-ray absorption near-edge spectrum (XANES) and extended X-ray absorption fine structure (EXAFS). XANES spectroscopy gives unique information on the local electronic structure at various atomic sites in these systems formed by diverse atoms with a complex struc-ture. Polarized XANES spectroscopy provides information about the orbital angular momen-tum of the unoccupied states according to the selection rules. Therefore the application of (polarized) XANES spectroscopy to high-T, superconductors is of interest. The photoelectron in the final state for E_LC polarization probes mainly the atomic distribution in the Cu-0 planes, whereas for E 1 1 C polarization it probes the atomic distribution on adjacent SrO and Ca planes. (E is the X-ray polarization.) Hence, polarized spectra are of key importance for a correct in-terpretation of XANES spectra of anisotropic crystals. In the EXAFS region, the strong anisotropy of the unbound final state of photoelectron for the polarized absorption spectrum of the K-edge allows us to employ spectra to probe orientation-dcpendcnlly the local structure with
intrinsic anisotropy around the absorbing atom.
VOL. 30 C. H. CHOU, W. F. PONG, J. B. SHI, H. C. KU, I. N. LIN 863
Great efforts have been expended to separate and to distinguish the features of XANES of the Cu K-edge of superconductors. After reliably interpreting the features of XANES of the Cu K-edge, which relates to peculiar aspects of the local electronic structure around the Cu atom within Cu-0 planes, one can utilize the versatile XAFS techniques to obtain desired experimen-tal data which are of benefit to probe the electronic properties of the Cu-0 planes of the prepared sample under consideration. Because various theories to explain the pairing mechanism depend heavily on the electronic properties, the understanding of the electronic properties of high-T, superconductors is of value to decide which theories are more likely to be correct, and would stimulate speculation about the possible mechanism of superconductivity.
By employing the XAFS techniques and by considering the results of auxiliary experiments on the samples, we have succeeded in clarifying some ambiguous statements concerning the properties of the Bi-0 layers and the influence of Pb substitution. Taking into account the theoretical and experimental data reported from recent studies, we have more conclusively in-terpreted the features of XANJZS spectra of the Cu K-edge of the Bi-Sr-Ca-Cu-0 system and provided XAFS evidence which is desirable to confirm the current conclusions about features of the Cu K-edge. In addition, some properties of the Cu-0 plane are also considered.
I I . E X P E R I M E N T A L P R O C E D U R E
II-l. Sample preparation and characterization
We synthesized two sets of superconducting samples using a solid-state reaction method. High-purity powders of Biz03, Pb304, CaC03, SrC03, La203 and CuO were used with the ratios (Bi+Pb):Sr:Cu = (1.9-t-0.2):1.9:1 for the (2201) sample, Bi:(Ca+Sr):Cu = 2:(1.2+ 1.8):2 for the (2212) sample and (Bi+ Pb):(Ca + Sr):Cu = (1.85 +0.15):(2.2 + 1.8):3 for the (2223) sample. Well-mixed powders were calcined at 800 “C in air for 1 day with several intermediate regrinding. These powders were then pressed into pellets and sintered at 850 “C in air up to 1 day and then either liquid nitrogen quenched or quenched in air (both were performed in this study) for the (2201) samples; at 850 “C up to 15 hours and then liquid nitrogen quenched for the (2212) sample; and at 859 “C up to 3 days and then furnace cooled for the (2223) sample. Sintering conditions were determined from differential thermal analysis (DTA) data by means of an ULVAC model 7000 symmetrical thermomicrobalance. Powder X-ray diffraction (XRD) data for random oriented powders and epoxy-embedded aligned powders were obtained using a Riguku D/MAX B diffractometer to identify the phases of the samples and to analyze the degree of c-axis alignment. Superconducting data of these samples were obtained by using a Quantum Design MPMS SQUID magnetometer with 2 K to 300 K and applied magnetic field up to _+ 5.5 T. For zero-field-cooled measurements, the “magnetic reset” option was used to quench the su-perconducting magnet and reduce the residual or remnant field to less than 1 G. For the double Cu-0 layers (n = 2) system with the BizSr~CalCu208+6_typc 2212 structure, the
86J ORIENTATION-DEPENDEN X-RAY ABSORFTION FINE !tTRUCTURE5... VOL. 30
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Bi#&a&u208+~ compound was chosen for the location of the composition inside the single-phase line Bi&_xCa~+xCu208+~ (0 I x I 0.75).17 For the three Cu-0 layers (n = 3) system with the Bi$Sr#@t3010 +&type 2223 structure, the (Bil.85Pbo.ls)Srl,8Ca2.2Cu30~0 +d compound was chosen to ensure the single-phase property of the 2223 structure. I8 For the single C u - 0 l a y e r s (n= 1) s y s t e m w i t h t h e Bi$+Cul06++type 2 2 0 1 s t r u c t u r e s , t h e (Bil.gPbo.2)Srl.gCulO6+~ compounds quenched in air and in liquid N2 were chosen to ensure the single-phase property of the 2201 structure. The measured data for the 2223 and 2212 samples were discussed in Ref. 19 in detail. The temperature dependence of electrical resis-tivities, measured by means of the four-probe method, shown in Fig. 1, reveals that T, = 13 K for the 2201 samples quenched in liquid N2 and that the sample quenched in air presents non-superconductivity. Fig. 1 also shows the magnetic susceptibility of the 2201 sample (T, = 13 K). All lines of the XRD pattern for the random oriented powders of the 2201 sample with T, = 13 K, shown in Fig. 2, can be indexed with 2201 structure. This indicates that excellent single-phase samples with single Cu-0 layer structure were prepared. The single-phase s a m p l e s e x t r a c t e d f r o m (Bil.8sPbo.ls)Srl.8Ca7.~Cu30*0+8, Bi$Sr&a&u208+~ a n d (Bil.gPbo,2)Srl,gCu106+~ compounds are denoted the 2223 sample, the 2212 sample and two 2201 samples (one with T, = 13 K, the other nonsuperconductive) respectively in what follows.
FIG. 1. Temperature dependence of electrical resistivities for the 2201 sample quenched in air and for the 2201 sample quenched in liquid Nl. The inset shows temperature dependence of magnetization M(T) for the 2201 sample quenched in liquid NL, field coolcd (FC) and zero-field cooled (ZFC) with applied field II = 20 G.
VOL. 30 C. H. CHOU, W. F. PONG, J. B. SHI, H. C. KU, I. N. LIN 865 a = 5.331 A b = 5.391 A c =24.64 i 0 1 0 20 30 4 0 50 60 2 8 (degree)
FIG. 2. X-ray-diffraction pattern for the powder sample Bil.9Pbo,2Srl.9CuO6+a with Tc = 13K.
We prepared superconducting compounds in another series having nominal composition (Pb,Bi~_,)Sr2Ca$&Oy with x = 0, 0.2, 0.4 (hereafter described as Pb-BSCCO), using PbO, Bi203, SrC03, CaC03 and CuO powders of at least 99.9 % purity. The mixed powders of ap-propriate proportions were calcined in air at 800 “C for 4 hours, ground, calcined again in air at 820 “C for further 4 hours, and then reground. The reground samples were pressed into pel-lets and sintered in air at 860 “C for 192 hours. Another sample with x = 0 were 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.) We determined the superconducting transition temperature (T,) of each sintered sample from the temperature dependence of the ac suscep-tibility (xac), using a Sumitomo Model SCR 204 T measuring system at 1 KHZ. The crystal struc-ture and phase constituents of the corresponding samples were determined by means of the XRD technique (Rigaku, Cu Ka). The experimental data reveal that almost only the 2212 phase is observed for the Pb-BSCCO samples (x = 0.0, 0.2) and, in contrast, the Pb-BSCCO sample (x = 0.4) are predominantly the 2223 phase, with a small proportion of 2212 phase coexisting. Detailed experimental procedures and characterization of the Pb-BSCCO (x = 0.0, 0.2, 0.4) samples were given in Ref. 20.
11-2. X-ray absorption measurements
X-ray absorption measurements were performed on beam line X-23A2 of the National Synchrotron light Source (NSLS) with a Si(220) double-crystal monochromator and ion-cham-bcr detectors. Mirror was used to remove the higher order harmonics. During this run, the NSLS WEIS operslcd at an energy 2.52s GeV and an electron current 234-195 mA. The photon cncrgy for each mcasurcment (Cu K-edge, Pb Lj-cdgc, and Bi LJ-cdgc) was simultnncously
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calibrated by means of Cu foil, PbO and Biz03, which were placed after the corresponding samples. The data were measured in the fluorescence or transmission mode. The energy resolu-tion AE/E was approximately equal to lOa 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.
To perform anisotropic X-ray-absorption measurements, we used the magnetic orientation technique of Farrel et al.*l A sintered single-phase superconducting pellet (the 2223 sample) was ground to powder with average microcrystalline grain size l-10 /Lrn, and mixed with SPAR 5-minute epoxy hardener in a quartz holder of diameter 8 mm in proportions appropriate for XAFS measurements (powder: epoxy ratio = 2:l). The mixture was spread onto an adhesive cellulose tape that was aligned in a 9.4-tesla superconducting magnet (Bruker, Germany) at room temperature; the alignment was checked by X-ray diffraction measurements. The (001) peaks of the XRD data are predominant in the aligned sample in which the degree of alignment is better than 90 %, and confirm the anticipated c-axis orientation along the applied field at room temperature.
III. DATA ANALYSIS AND DISCUSSION
III-l. Bi Lpedge
Because there are at least four times as many Bi atom as Pb atoms in the Pb-modified su-perconducting samples in our work, there is no significant contribution from XAFS of the Pb L3-edge which extends past the Bi L3-edge although the two edges are separated by only AE = 380 eV Neglecting the fine structure of the absorption spectra of the Pb L3-edge in the range above the Bi L3-edge and treating those as the background, we reliably obtain XAFS spectra of the Bi Lx-edge simply by removing the background.
We define the EXAFS function X(k) as”
X(k) =
P(E) - PO(E)
PO(E)
in which /L is the absorption coefficient, /CO is the structureless absorption coefficient of the iso-lated atom in question, and k is the wave vector of the ejected photoelectron. The X-ray cncrgy E is converted into the wave vector k by means of the equation
b = [2m,(E - Eo)/h”]:. (2)
Here Eu is the so-called threshold energy, m, is the mass of clcctron, and cncrgy and wave vector are conventionally cxpresscd in the units cV and A-’ respectively. As we mentioned prc-viously,“’ the existence of the modulation structures is monitored by means of the EXAFS
tech-VOL. 30 C. H. CHOU, W. F. PONG, J. B. SHI, H. C. KU, I. N. LIN 867
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nique. Here, we did similar measurements of unpolarized EXAFS spectra for Pb-BSCCO samples (x = 0, 96 h and x = 0, 192 h) and for the 2223, 2212 and 2201 (T, = 13 K, non-su-perconductive) samples. The magnitudes of the Fourier transforms of the k%(k) data sets of Bi203, the Pb-BSCCO samples (x = 0, 96 h and x = 0, 192 h) and of the 2223,2212 and 2201 (T, = 13 K, non-superconductive) samples, are collectively depicted in Fig. 3(a) and (b). The magnitude of each Fourier transform of k%(k) reveals peaks corresponding to the shells of neighbours around the absorbing Bi atoms. Curve (al) in Fig. 3(a) refers to the magnitude of the Fourier transform of k%(k) of B&03. Because there are oxygen atoms in two nonequivalent types of coordination23 around the bismuth atoms within BiI03 and because the Bi-0 coordina-tion in the structure is classified in two distinct ranges 2.08-2.29 A and 2.48-2.80 A, the first peak corresponding to the nearest oxygen atoms is broad; the peak located near 3.5 %, indicates the other Bi atoms around the Bi absorbing atoms. Curves (bl), (cl) in Fig. 3(a), and curves (dl), (el), (fi) and (gl) in Fig. 3(b) show the magnitudes of the Fourier transforms of the k%(k) data of the BSCCO sample (x = 0,96 h), the BSCCO sample (x = 0,192 h), the 2201 samples without superconductivity and with T, = 13 K, and the 2212 and 2223 samples, respectively. The first peak in each plot corresponds obviously to the nearest oxygen atoms in the Bi-0 layers. No ap-preciable overlap of different atomic species occurs before the second peak which contains
Bi-FIG. 3(a). 0.3 5 0.30 5 0.25 . -5 0.20 fl I I 5 - 0.10 g 0.05 (a,) t. 0.00 z (b,) Z ~~ 0.00 / Cc,)
o.ooo~
I 2 3 4 5 6 7 8 I I I IMagnitutics of I:ouricr transforms of the k%(k) data (I,~-edge absorption of Hi) of (at) Hi203, (bl) the RSCCO snmplc (x = 0.0,96 hrs), and (cl) the HSCCO sample (x = 0.0, 192 hrs), obtaincci by transforming over the k-ranges 2.13-11.23, 2.23-11.73, and 2.2Sll.SS A-’ rcspcctivcly, and by choosing I_0 at the half-height point of the absorption cocfficicnt/r.
-868 ORIENTATION-DEPENDENT X-RAY ABSORMION FINE STRUCTURE... VOL. 30 0.40, I I I I I I I 0.35 0.30 0.2 5 -0.20 0.1 5 0.10 (d,) 0.05 0.00 5-:-_ 0.00 0 1 2 3 4 5 6 7 6
FIG. 3(b). Magnitudes of Fourier transforms of the k%(k) data (L3-edge absorption of Bi) of the 2201 samples (dt) non-superconductive and (el) with Tc = 13 K, (fl) the 2212 sample, and (gl) the 2223 sample, obtained by transforming over the k ranges 11.13, 10.98, 11.03 and 2.23-11.03 A-’ respectively, and by choosing Eo at the half-height point of the absorption coefficient/c.
Sr(Ca) and Bi-Bi components and would be expected to be located near 3.5 A in each plot. Of particular interest is that the second peak shows an exceptionally small amplitude. The disap-pearance of the second peak in the Fourier transform of the k%(k) data of each sample implies that there occurs an anomalous static disorder of the distances between the absorbing Bi atoms and the nearest neighboring Bi, Ca, Sr atoms. The incommensurate modulations within the Bi-0 layers in the Bi-Sr-Ca-Cu-0 family have been investigated by means of X-ray single-crystal dif-fraction, X-ray and neutron diffraction of powders and transmission electron microscopy (TEM).24a2s Each Bi-0 layer contains occupational and positional 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. Schncck et al.“’ and Wen et a1.27 reported that the basic structure of the incommcnsuratc modulation within the lead-free compounds is prcservcd in the Pb-modified Bi-Sr-Ca-Ca-0 system. The reduction of the second peak in the Fourier transform of thcX(k) data of each sample is caused by periodic displacements of the corresponding metallic atoms from their avcragc position in the modulated structure and is the signature of the occurrence of the modulation structures in all samples. For
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the Bi Ls-edge of the BSCCO sample (x = 0, 96 h) the relative height of the second peak of the magnitude of the Fourier transform of the X(k) data measured in our previous work2’ is greater than that of the second peak of the Fourier transform of theX(k) data measured recentiy for the Bi Lsedge of the same sample. This result indicates the progressive appearance of the modulation structure in the BSCCO sample (x = 0,96 h) and reflects that the modulation struc-tures seem more stable. Fig. 4. illustrates the total phase of the Fourier-filtered X(k) data from the first peak located in the range of R 0.8-2.01%, in each plot, depicted in Fig. 3(a), (b). Ac-cording to the beat-node method, the minimum point (beat node) of the amplitude of the Fourier-filtered X(k) data, and the associated kink in the total phase, reveal the difference AR of the bond distances of a coordinate shell from the equation kb = (n/2AR). Because of the variation of the bond distances of the nearest oxygen atoms around the Bi atom in Bi203, the beat nodes, indicated by arrow mark, appear in the total phase. Because no salient beat node appears in the total phases of the Fourier-filtered X(k) data from the first peak corresponding to the nearest oxygen atoms around the Bi atom in each BS(C)CO sample within the k range with k,,, = 10 A-‘, we suggest that most nearest neighbor oxygen atoms around the Bi atoms within Bi-0 layers are located at almost the same distance (AR I n/2k,,, = 0.16 A) from the Bi atom.
The XANES spectrum of the Cu L3-edge for the Bi-Sr-Ca-G-0 system has enabled the successful determination of the occupancy of the d,2_y2 and d3z2_r2 orbital? of Cu. The
orien-- 2223 -‘-‘- 22t2 -..-..- BSCCO (x= h)0,96 --- BSCCO (x =O ,192h) - - - - 2201 (T,=13K) . 2 2 0 , I I I I 2 4 6 k (A-Y 10 12
FIG. 4. Total phasesofthe inverse transforms (the Fourier-filteredX(k) spectra) from the first peaks, located in the K-range 0.8-2.01 A, of the Fourier transforms of the XAFS spectra depicted in Fig. 3 (a) (b)..
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tation dependence of XANES spectra of the Cu L3-edge depends on the other 3d orbitals being occupied. In the EXAFS region in which the final states of photoelectron ejected by a monochromatic X-ray are unbound and empty completely, or in the case of XANES of the material in which the orbitals of d symmetry are totally unoccupied or totally occupied, it be-comes more difficult and complicated to employ XAFS data of the L3 edge to investigate the geometry of the local structure and electronic states around the absorbing atoms. The general EXAFS expression for L2 and L3 edges is2’
xw =
CCAT(“){;(
1 + 3 COS2 Ckij)M,2, sin[2rER+ + $$]i j
+( 1 - 3 COS’ cYij)Mo1M21 Sin[2kRi + $b2] (3)
.
The summation extends over all j atoms in the i’th neighbor shell at a distance Ri and with an effective scattering amplitude Ai*. aij is the angle between the electric field vector E of the X-ray and the internuclear axis from the absorbing atom to the backscattering atom j in the i’th shell. Mel and M2l are radial dipole matrix elements for transitions from I = 1 initial to I = 0 and I = 2 final states, respectively. The scattering phase shifts q(k) are those experienced by the photoelectron from the absorbing (6) and backscattering (t) atoms:
ft(l;) =
2Sl(rl-) +
T;(k), (4)&(il-) = 6&q + 62@) + r;(k). (5)
The influence of atoms beyond the first oxygen shell on the polarized XAFS spectrum of the Bi L3-edge is reduced due to the peculiar structure of the incommensurate modulations in the Bi-0 layer; the local coordination geometry of the nearest oxygen atoms around the Bi atom has isotropic symmetry despite the strong two-dimensional character of the structure of the Bi-Sr-Ca-Ca-0 system. For these two reasons, neither polarized XANES spectra of the Bi L3-edge, which is caused by the transition to the completely empty states with d symmetry, nor the polarized EXAFS spectra of the Bi L3-edge, which would be expected to reflect no orientation dependence according to the formula derived by Stohr et al.,“’ exhibit the orientation depend-ence. We have measured the El ) C, EJ_C polarized and unpolarized XAFS spectra of the Bi L3-edge of the 2223 sample. The experimental XANES spectra reveal no prominent difference from each other. The magnitudes of the Fourier transforms of the polarized (E 1 1 C, EIC) and unpolarized EXAFS spectra of the Bi L3-edge for the 2223 sample are also similar. Similarly, in the case of the Pb L3-edge, the orientation dependence of the XAFS spectra of the polarized 2223 sample is also not detectable. Employing polarized XAFS spectra of the Bi L3-edge (or
VOL. 30 C. H. CHOU, W. F. PONG, J. B. SHI, H. C. KU, 1. N. LIN 871
of the Pb L3-edge) to probe the orientation dependence of the electronic and structural features around the Bi atom (or the Pb atom) within the Bi-Sr-Ca-Ca-0 system is futile.
Because the XANES spectrum of the Bi L3-edge for each of the 2223, 2212,220l (T, = 13 K and non-superconductive) samples, and the Pb-BSCCO samples with x = 0.2,O.O strongly resemble the XANES spectra of the others, it is difficult to monitor the possible minute varia-tions of the average oxidation number of Bi between these samples based on XANES data. It is beneficial to examine the derivative of the XANES spectra of the samples shown in Fig. 5. Based on band-structure calculations, we expect the low-lying states to consist of Bi(6s) and O(2p) states in the samples. For the Bi L3-edge the dipole selection rules allow direct transitions
0.00
0.0:
0.0:
0.0;
0.0;
O.O( O.O( 0.01 -ENERGY (eV>FIG. 5. Derivatives of the normalized X-ray-absorption near-edge structures measured at the Bi L3-edge of (a) Biz03, (b) the 2223 sample, (c) the 2212 sample, the 2201 samples with (d) T, = 13 K and (e) non-superconductive, and the Pb-BSCCO samples with (f) x = 0.2 and (8) x = 0.0. The zero of the energy scale is at the vicinity of the raising edge of each spectrum of the Bi L3-edge. For comparison, the derivative of the normalized X-ray-absorption near-edge structure measured at the Pb Lj-edge for (h) PbxOJ is illustrated simultaneously. The energy zero in the case of the Pb LJ-edge refers to 13035 eV.
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to empty Bi(6s) states; the occurrence of a low-lying bump corresponding to the transition to the Bi(6s) orbitals thus indicates the presence of empty Bi(6s) states and that the oxidation num-ber of Bi in the compound exceeds 3. Reports by Salem-Sugui et al. and Heald et al.% support this interpretation. Because for Bi3+, like the Bi cation within Biz03, these states are complete-ly filled, the low-complete-lying bump corresponding to the transition to Bi(6s) states is suppressed (curve (a)). The consistent existence of the bump located at about -6 eV, easily observed in each deriva-tive of XANES spectra of the samples shown in Fig. 5 (peaks located at about -7 eV in curves (b)-(g)), reveals that within the Bi-Sr-(Ca)-Cu-0 system there are some Bi cations with oxidation number exceeding 3. Similarly, because in Pb304 there are Pb2+ and Pb4+ (Pb2+ cations in the majority),31 the low-lying bump relating to the transition to empty Pb(6s) state is observed. The similarity of the derivative of the lowienergy region of XANES spectrum of the Pb L3-edge of Pb304 and the derivatives of the low-energy region of the XANES spectra of the Bi La-edge of the samples (cf. curve (h) and curves (b)-(g)) confirms that there is a proportion of Bi with oxidation number exceeding 3.
111-2. Pb Ls-edge
Examining the entire absorption spectra within the XANES range of the Pb-modified samples and of the lead-oxide compounds, we are able to find that the XANES spectra of the Pb Ls-edge of PbO and Pb02 differ from those of the Pb L3-edge of the samples; for this reason, the proportions of PbO and Pb02 in each sample must be extremely small.
Figure 6(a) presents the XANES spectra of the Bi Lx-edge of the 2223, 2201 (T, = 13 K, non-superconductive) and the Pb-BSCCO (x = 0.2, 0.4) samples and those of the Pb Ls-edge of the same Pb-modified samples. Because there is no substantial change of XANES between the Pb and Bi’La-edges for each sample, following the interpretation due to Boyce et a1.32 of superconductive BaBia.zSPb1.750y, we suggest that most lead atoms in the additional lead-oxide compounds are situated in Bi-0 layers substituting for a portion of the Bi atoms. The conclusion is confirmed by the reproducible similarity of the first derivative of the high-energy region of the XANES spectra, which are sensitive to the local structure, of the Bi and Pb Ls-edges of each of the 2223, 2201 (T, = 13 K, non-superconductive) samples and the Pb-BSCCO (x = 0.2,0.4) samples (shown in Fig. 6(b)). Despite the similarity of the experimental spectra of the Pb Ls-edge of Pb304 and of the Pb-modified samples, we eliminated the co-existence of the Pb304 phase in the samples by means of the XRD data of them. Therefore, the XAFS data without supplementary data from auxiliary measurements provide just the necessary evidence, but not sufficient evidence, of Pb substitution for Bi in the Bi-Sr-(Ca)-Cu-0 samples.
Figure 7 shows the low-energy region of the XANES-spectra, which is particularly sensitive to the electronic structure, of the Pb Lx-edge of the 2223, 2201 (T, = 13 K, non-superconduc-tive) and Pb-BSCCO (x = 0.2, 0.4) samples; for each sample, the XANES spectra of PbO,
VOL. 30 L. H. LHUU, W. t’. I-UNCI, J. H. SHI, tl. L. KU, 1. IV. LIN 875
o.oot
Y :’,y
&y/lr
i! .’ F/ 0.00 /’/ (,!’ .I’ /’ ‘-/.’d
0.00 /./‘/ /’ / 0.00 -20 -10 0 - - - PbO ... Pb304 -.-.- PbOz 10 ENERGY (eV)FIG. 7. X-ray-absorption near-edge structures in the vicinity of the Pb Lx-edge for the 2223 sample (a), the 2201 samples with T, = 13 K (b) and non-superconductive (c), and the Pb-BSCCO samples in which x = 0.2 (d) and O.-l (e). The curves superimposed over each curve of the data of these samples present the X-ray absorption near-edge structures of the Pb L@ge of PbO, PbOz, Pb-,OJ for comparison. The curves have been shifted vertically for clarity and the the energy zero refers to 13035 eV.
spectra.) Besides, for the Pb LJ-edge of the Pb-modified samples there are no significant shifts toward higher energy compared with those of PbO and PbJOd, whereas the aspects of the XANES spectra of the Pb Lx-edge of the samples are similar to those of the spectrum of PbjO+ Hence, the oxidation number of the total substituted Pb in the Pb-modified samples cannot bc 4 but the existence of some Pb with oxidation number exceeding 2 cannot bc precluded.
1113. Cu K-edge
The orientation-dcpcndcnt near-edge structures mcasurcd in the vicinity of the Cu K-cdgc for the aligned 2223 sample and for an unpolarized CuO sample and their dcrivativcs arc il-lustrated in Fig. 8 and 9 respcctivcly. Although thcrc have been publishccl scvcrsl
intcrprcta-876
FIG. 8.
ORIENTATION-DEPENDEN X-RAY ABSORPTION FINE STRUCTURJZ... VOL. 30
-20.00 0.00
iii-ENERGY (eV)
Unpolarized XANES spectra of the Cu K-edge for CuO (a) and for the 2223 sample (b) alongwith polarized EIC (c), L EC < 45’ (cl) and El 1 C (e) XANES spectra of the Cu K-edge for the 2’223 sample. The curves have been shifted vertically for clarity and the energy zero refers to the
K-ab-sorption edge (89SO eV) of pure Cu.
tions33-35 of the features of Cu K-edge XANES spectra, it was premature to separate readily and to identify definitely the features of the XANES of the Cu K-edge without both sophisticated calculations and improved experimental data. In preliminary works the commonly accepted in-terpretations of the features,36120 in particular of the peaks A and C in curve (b) in Fig. 8, are assigned as follows. (1) The small pre-edge feature Q indicates the ls-to-3d quadrupole tran-sition. (2) The set of features falling between the labels A and B 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 (4~0) states by crystal-field effects. The shoulder at 4pn*, clearly observed in the curve of XANES for CuO and corresponding to A in the curve of XANES of the 2223 sample, is attributed to transitions to the 4pn* state. The asterisk indicates that the Is-to-4p.z transition 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 p states. The main peak B at the top of the edge
VOL. 30 C. H. CHOU, W. F. PONG,J. B. SHI, H. C. KU, 1. N. LIN
b)
II I
0.00 20.00 40.00 6 ENERGY (eV) 877 IOFIG. 9. Derivative of the spectra in Fig. S for CuO (a) and for the 2223 samples of which the c axis is random (b), parallel to E (e) and vertical with respect to E (c).
is attributed to the transition to 4pa states. (3) The edge feature C, which is the signature of EXAFS multiple scattering (MS) within the Cu-0 plane, is associated with the signature of EXAFS contributions from high-shell neighbors.
Recent work has included the application of the multiple-scattering approach in real space for XANES spectra of the Cu K-edge,3728 simple Hartree-Fock cluster calculations,39 and ex-perimental studies of related supercnducting compounds-particularly the orientation-depend-ent measuremorientation-depend-ents!’ By these means we understand the nature of absorption processes41V37 more clearly than before and WC are thus able to reinterpret the features of XANES spectra of
the Cu K-edge.
In the process of X-ray absorption, a photoelectron moves outward and a core hole is created suddenly by the incident X-ray. The system may undergo a complicated relaxation processes, which are the so-called many-body effects induced by the excited photoelectron-hole dipole and which arc certainly important and cvcn dominant in the threshold region in XAFS,
878 ORIENTATION-DEPENDENT X-RAY ABSORPTION FINE .SI-RUCITJRE... VOL. 30
including shake-up or shake-down. In the high T, copper oxide superconductor, both 0 2p holes and Cu’3d holes are present. These holes, which are initially shared between the Cu and 0 atoms, interact strongly with the core-hole or the slow photoelectron present in the final state, either moving almost onto the oxygens to form a well-screened Cu 3d”L configuration (for which L indicates a hole on the ligand oxygens) or shifting almost completely onto Cu to yield a poorly screened Cu 3d9 configuration. Therefore, the many-body relaxation, subject to con-flicting interpretation, results in a change of the admixture of the electron configurations in the final states. The admixture of the valence configurations of Cu, determined by the polarized XANES, has been influenced by many-body effects and cannot reveal directly the inherent electron configuration of Cu before the interaction with the incident X-ray.
After a careful analysis in terms of one-electron transitions, which are calculated on the basis of the various electronic configurations of the absorbing Cu atom resulting from many-body effects, one can identify the presence of specific many-electron features in polarized XANES spectra. As illustrated in curves (c) and (e) in Fig. 8, for X-ray radiation polarized along the crystal c axis and in the ab plane, the satellites A3 and Bz are assigned to the poorly
. screened core hole with 3d9 electronic configuration of the central Cu ion, i.e., to & 3d9, q1
final state, whereas the main peaks A2 and B1 are assigned to the fully relaxed and well screened core hole with Cu 3d”L electronic configuration, i.e., to the final state configuration b3d1’,p1. Here, EP means the p-like (I = 1) states projected on the Cu site with orbital angular momentum ml = 0 in the E 1 1 C spectra, and the ml = 1 for the EIC spectra and the underline denotes a hole in corresponding state. The peaks Al and A? are assigned as the transitions, determined by resonance of the mutiple scattering pathways involving mainly Ca, Sr, O(apex) together within Ca and SrO layers in real space, to states delocalized over a large cluster of atoms. The peaks Al and A2 appearing in the E 1 1 C polarized spectrum are proportional to the partial density of unoccupied states, whereas the variation of the relative heights of the peaks Al and A2 does ltot reveal the influence of many-body effects. The peaks B1 and B3 of E_LC polarized XANES for the 2223 sample are due to the transitions, given by resonance of the multiple scattering of the photoelectron by O(plane) and Cu ions in the four neighboring Cu sites in the Cu-0 plane, to the final states delocalized over a large cluster of atoms.
The peak Bz in rhe EIC polarized spectra of the Cu K-edge, which was previously assigned only to EXAFS multiple scattering within the Cu-0 plane in copper-oxide superconductor, is assigned to a many-body peak. The peak A3 in the E 1 / C polarized spectra is also attributed to a many-body peak. Several conflicting hypotheses concerning the peaks A3 and Bz deserve at-tention. Li et a1.40 proposed that the intense multielectron-excitation satellites about 7 eV from the main peak in XANES of the Cu K-edge seem characteristic of a largely non-isotropic Cu site. Guo et a1.4’ suggested that these multielectron excitation features involve the dipole-al-lowed relaxation channels for the Cu 3d + 4p transition in the sample with certain symmetry of the copper sites for X-ray radiation polarized along the crystal c axis and in the ab plant.
Tolen-I
VOL.30 C. H. CHOU, W. F. PONG, J. B. SHI, H. C. KU, I. N. LIN a79
tino et a1.41 advocated the need to take into account the role of the slow photoelectron in the many-body relaxation which leads to electron transfer from 0 2p to fill a hole in Cu 3d in cuprates.
Bianconi et al. found no obvious A3 peak in the polarized XANES spectrum of the Cu K-edge measured for Bi2CaSrZCuz0s.37 In contrast, we observe clearly the peak A3 in the E 1 1 C
spectra, according to the polarized XANES curve in Fig. 8 measured for the 2223 sample. The peak A3 is also observed from the derivative of the E 1 1 C polarized XANES spectrum shown in Fig. 9. If the feature Al appearing in the E 1 1 C spectrum is indeed due to the shake-down effect, then in the El-C spectrum a similar shake-down peak on the rising edge may also be present. In the EIC polarized XANES spectra of the Cu K-edge of the 2223 sample no identification of such a peak, which one expects to be located on the rising pre-edge according to this interpreta-tion, is observed. The derivatives of thepofatized XANES spectra of the Cu K-edge of the 2223 sample (cf. curves (c) and (e) in Fig. 9) clearly reveal that the bump, which corresponds to the 3d quadrupole transition located at about -6 eV, appears in the derivative of only the E_LC
polarized XANES spectra of the Cu K-edge for the 2223 samples, because only in the condition
E_LC the quadrupole transition to the empty 3d,z+ orbitals is allowed. This result is consistent with the well established conclusion, based on the XANES data of the Cu L3-edge,28 that most holes at the Cu site are located in 3dx2_,,2 orbitals oriented parallel to the Cu-0 plane.
In Fig. 10 the XANES spectra of the 2201, 2212 and 2223 samples are illustrated. The in-tensity of Al is enhanced going from the 2223 sample with two Ca layers, the 2212 sample with one Ca layer to the 2201 samples without Ca layer and indeed depends strongly on the Ca and SrO layers. The intensities of the peaks C of the XANES spectra of the Cu K-edge for the 2223 sample and for the 2201 sample without superconductivity are extremely different from each other. Fig. 11 shows the magnitudes of the Fourier transforms of k3X(k) of the two samples. 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
- 2.7 A corresponds to the Ca and Sr atoms around the Cu absorbing atom. The third peak &-Cu peak) located at - 3.6 A, appearing in each curve of the magnitudes of the Fourier-transforms, has a great amplitude because of focused multiple scattering via intervening oxygen atoms.43 The appearance of the focusing effect indicates that the Cu-0 planes within the samples have an ordered planar structure, viz., not only is the distance between each Cu atom and the nearest Cu atoms almost equivalent, but also the positions of the Cu atoms and the in-tervening oxygen are nearly collinear. Although the ordered structure of the Cu-0 planes has been confirm4 by the XAFS data, we find the Cu-Cu peak in the curve of the magnitude of k3%(k) for the 2223 sample is broader and lower relative to that in the curve of the magnitude
of k3;C(k) for the 2201 sample without superconductivity (Fig. 11). This discrepancy of the line shapes indicates that in the 2223 sample thcrc is more significant distortion of the internuclear Cu-Cu line via the intcrvcning oxygen atom or, more likely, a more significant displacements of
880 ORIENTATlON-DEPENDENXX-RAYABSORFJJONFWJESTRUCTURE... VOL.30 0.00 ( 0.00 ( 0.00 -20.00 0.00 20.00 40.00 60.00 ENERGY (eV)
FIG. 10. X-ray-absorption near-edge spectra, measured at the Cu K-edge and normalized to the edge step, for the non-superconductive 2201 sample (a), the 2201 sample with T, = 13 K (b), the 2212 sample (c) and the 2223 sample (cl). The energy zero corresponds to S9SO eV.
the Cu atoms from their average position. Aparfiaf contribution of the feature Bz is reproduced in the mutiple-scattering calculation by taking into account larger cluster of at least eight shells, whereas the features B1 and B3 are reproduced in the mutiple-scattering calculation by consider-ing smaller cluster of lesser shells. For this reason the features B1 and B3 seem less sensitive to the faint variation of the Cu-0 planar structure; hence the position and relative height of the peaks Bt and B3 in the XANES spectra of the 2223 sample are roughly similar to those of the peaks B1 and B3 in the XANES spectra of the 2201 sample with non-superconductivity. In con-trast, the Csatelhte, which comisfs of mpeyosiriorl predornirtantly of B2 artdpartially ofA3, moves slightly toward lower energy and its intensity obviously increases going from the 2223 sample to the 2201 sample without superconductivity. Following Bianconi et a1.,37 we believe that the in-crease of the intensity of C in the unpolarized spectra reflects mainly the inin-crease of that ofpeaks
B2, due partially IO the many-body effects and partially to the resotlauce of the ~llltltiVle-scatten’tlg pathways‘mai,lly involvitlg the Cu and 0 atom itt the Cu-0 plane. This result is consistent with
the result reported by Bianconi et al.“‘who observed the intensity of the peak B, of the XANES spectra of the insulating Bi$rzYCu,Os+g phase to increase relative to the superconducting Bi$SrzCaCuz08 +J phase at room temperature. However, it is difficult to determine whether the predominant factor, which causes the variations of peaks C of the samples, is the many-body
VOL. 30 C. H. CHOU, W. F. PONG, J. B. SHI, H. C. KU, I. N. LIN 881
(~i~.85Pbo.15)~r~.8~2.2C~301D1c
-.-.- (Bi,.sPbo.2)Sr,.SCu106d quenched in air
R
<A>
. FIG. 1 1 . Magnitudes of the Fourier-transforms of k%(k) over the k-range 2.79-11.14 A-’ converted by
choosing I!$ at the half-height point of the Cu K-edge for the 2223 sample and the non-supercon-ductive 2201 sample.
effects possibly relating to the change of the isotropy of the structure around the Cu site or the multiple-scattering resonance being sensitive to the distortion of the Cu-0 plane. Nevertheless, we believe the observable variations-particularly the variation of the Cu-Cu peak-of the mag-nitude of the Fourier transform ofX(k) of the corresponding sample cannot be ignored in clarify-ing the origin of the variations of peaks C in the unpolarized XANES spectra (i.e. peaks B2 in the E_tC polarized XANES spectra) of the 2223,2212,2201 (T, = 13 K, non superconductive) samples. The detailed description of the many-body final states A3 and Bl is beyond the scope of our work and is not yet carried out although several conflicting hypotheses conducting the nature of the many-body effects have been proposed.
IV. CONCLUSIONS
Prudently using the XAFS technique and taking into account its restrictions, we have ex-tracted information about the properties of the Bi-0 layers and the substituted Pb atoms in the Bi-Sr-Ca-Cu-0 system. We have deduced the ordered structure of the Cu-0 planes which are responsible for the superconductivity of the copper-oxide high T, superconductors.
Specifically, in all families of the Bi-Sr-Ca-Cu-0 system the modulation structures in the Bi-0 layers are present. Because of the symmetry of the coordination geometry of the nearest oxygen atoms around the Bi cation, it seems less reliable to extract related structural and
882 ORIENTATION-DEPENDENT X-RAY ABSORPTION FINE FI-RUCTURE... VOL. 30
electronic information about the Bi cation by means of polarized XAFS data for these Bi-Sr-Ca-Cu-0 superconductors. Despite the oxidation number of some Bi cations being 3, that of the remaining Bi cations exceeds 3. The XAFS data provides the necessary evidence, which reveals that the lead atoms in the added lead oxide substitute for some Bi atoms in the Bi-0 layers of the Bi-Sr-Ca-Cu-0 system. The oxidation number of all substituted Pb cation in the Bi site cannot be 4, and the oxidation number of a fraction of the substituted Pb cations is 2.
We have found peak A3 in the polarized E 1 1 C spectrum of the Cu K-edge for the 2223 sample. We have noticed the variation of the features At and B2 (A$ going from the 2223 samples to 2201 samples. The main open problem concerns the detailed nature of the many-body effects involving manifold relaxation channels. Consequently, further experimental and theoretical work is needed to determine how the admixture of the inherent electronic configura-tions of Cu within the Bi-Sr-Ca-Cu-0 system combined with the many-body effects affect the change of the admixture of the electronic configuration of the absorbing atom after the excita-tion of incident X-ray. The change results in the variaexcita-tion of the relative posiexcita-tions and par-ticularly of the relative intensities of the main peaks and corresponding satellites in XANES of the Cu K-edge.
A C K N O W L E D G M E N T S
We are grateful to Dr. Chau-Ting Chang for his gracious help. We also express our ap-preciation to Prof. P. K. Tseng and Dr. B. A. Bunker for their help to obtain the beam time. This work was supported by the National Science Council of the Republic of China under grant No. NSC 81-0208-M-032-13 to W. F. Pong.
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