The valence of Cu and Au in YBa2Au0.3Cu2.7O7-delta was investigated using x-ray absorption near edge structure (XANES). X-ray and neutron diffraction studies indicate that Au goes on the Cu(1) site and Cu K-edge XANES shows that this has little effect on the oxidation state of the remaining copper. The Au L3 edge develops a white line feature whose position lies between that of trivalent gold oxide and monovalent potassium gold cyanide, and whose height relative to the edge step is smaller than in the two reference compounds. The appearance of the Au L3 edge suggests that fewer Au 3d states are involved in forming the Au-O bond in YBa2Au0.3Cu2.7O7-delta than in trivalent gold oxide

Hepp, Aloysius F.

Ruckman, Mark W.

English

NASA Technical Reports Server

NASA Technical Memorandum 105220
....... Near-Edge Study Of Gold-Substituted
YBa2Cu307_ 8
Mark W. Ruckman
Brookhaven National Laboratory
Upton, New York
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Aloysius E Hepp
_w_'s Research Center
Cleveland, Ohio
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NEAR-EDGE STUDY OF GOLD-SUBSTITUTED YBa2Cu307. _
Mark W. Ruckman
Brookhaven National Laboratory
Physics Department
Upton, New York 11973-5000
and
Aloysius F. Hepp
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
Abstract
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The valence of Cu and Au in YBa2Au0.3Cu2.707_ _ was investigated
using x-ray absorption near-edge structure (XANES). X-ray and neutron
diffraction studies indicate that Au goes on the Cu(1) site and Cu K-edge
XANES shows that this has little effect on the oxidation state of the
remaining copper. The Au L 3 edge develops a white line feature whose
position lies between that of trivalent gold oxide (Au203) and monovalent
potassium gold cyanide (KAu(CN) 2) and whose height relative to the edge
step is smaller than in the two reference compounds. The appearance of
the Au L3-edge suggests that fewer Au 3d states are involved in forming
the Au-O bond in YBa2Auo.3Cu2.707__ than in trivalent gold oxide.
PACS Nos: 74.70.Vy, 78.70.Dm
Substitution of many metals in YBa2Cu307_ s (abbreviated as 123 in
the following) 1-4 reduces the superconducting transition temperature Tc.
Metals like Fe, Co, or AI replace linear chain site copper, the Cu(1) site,
and depress Tc more slowly than Zn or Ni which replace copper located on
the CuO 2- planes, the Cu(2) site. X-ray absorption near-edge structure
(XANES) studies5, 6 indicate that transition metal substitutions
sometimes change the oxidation state of Cu or oxygen. An apparent
exception to the above generalization is the behavior of Ag and Au.
Considerable amounts of Au or Ag can be put into 123 before Tc begins to
decrease. 7 Streitz et al.8 examined the microstructure of Au/123
composites and found that separate Au and Au-containing 123-like phases
existed after heat treatment in oxygen. Hepp et al.9 and Cieplak et al.l 0
investigated YBa2(AuxCUl_x)3OT_ 6 using x-ray diffractiong,lo and neutron
scatteringlO and found that Au went into the Cu(1) site.
In this communication, we report the results of an examination of
the Cu K and Au L3 x-ray absorption edges for YBa2(AuxCUl_x)307_8
designed to determine the valences of Cu and Au. We find that an Au
substitution of 8 mole percent has no measurable effect on the oxidation
state of Cu in 123 which is divalent. The appearance of the Au L3 edge
suggests that the valence of gold is less in the superconductor than in
trivalent Au20 3.
Samples used in this study were synthesized and characterized at
NASA Lewis Research Center; synthetic details are discussed in
reference 9. For YBa2(Auo.ICUo.9)307_8, x-ray diffraction (XRD) patterns
and x-ray photoemission (XPS) suggest that "trivalent" Au goes into the
Cu(1) site. When this occurs the a and b axes remain unchanged but the c
axis expands from 11.69 to 11.75 A. This is in accord with the well known
2
structural chemistry of Au (see reference 9 for relevant discussion). Hepp
et al.9 found no evidence for the presence of secondary phases in their x-
ray data for Au substitutions less than 10 mole percent. The formation of
a second phase was readily detected in the x-ray diffraction data when
more than 10 mole percent was put in 123. Tc was observed to be 89 K for
the 8 mole percent gold containing material and 91 K for the parent 123
material made without Au20 3. It should be emphasized at this point that
the x-ray-based results of Hepp et al. have been confirmed by detailed
neutron diffraction studies, lo
The x-ray absorption measurements were made in the transmission
mode using powdered samples dispersed on adhesive tape. Au or Cu foil
absorbers were placed after the samples to run in conjunction with the
samples to maintain calibrated energy scale. The work was done at the X-
11A beamline at the Brookhaven National Synchrotron Light Source (NSLS).
The technical details of this beamline are published in reference 11. The
resolution of the monochromator is estimated to be --1.0 eV at the Cu k-
edge and ~1.2 eV at the Au L3 edge. The samples were crushed into powder
and screened through 400-mesh before dispersal onto adhesive tape. The
near-edge data for the Cu K and Au L3 edges was obtained and analyzed
using standard procedures. A linear background was removed from the
edge before normalization. Energy calibration of the edge was ensured by
measuring the L 3 edge from a gold foil and the k-edge from a Cu foil
simultaneously with the superconducting samples.
The Cu k-edges for the 123 material (solid line) and the 8 mole
percent Au sample (dashed line) are shown in figure 1. The shape of the Cu
k-edge is complex and several interpretations of it exist. 12-16 XANES
results are now available for highly oriented powders or single crystal
3
materials using polarized x-rays and provide the most reliable data for
the Cu k edge. 17-20 The Cu k near edge structure arises from dipole
transitions from the Cu Is core level to the low-lying copper valence or
conduction band states with p or = symmetry and to transitions from the
Cu Is to continuum final states that are modified by multiple scattering
(shape resonances). The transitions to bound final states are related to
the electron density of states and are sensitive to changes in the chemical
state of the Cu while the shape resonances are sensitive to structural
modification. Heald et al. 17 examined the Cu k-edge from 123 powders
oriented such that the x-ray polarization vector _ was either parallel or
perpendicular to the c axis. The position of a weak pre-edge feature
marked "A" due to Is to 3d quadrupole transitions is marked on figure 1
and it is directly related to the valence of the Cu. In oxygen deficient 123
material, this peak grows in proportion to the removal of holes from the
oxygen site and the formation of Cu1+. 21,22 Peak "B" is due to transitions
from the Is to 4p= band accompanied by shake down transitions (4p=*) and
peak "C" contains contributions from the Is to 4p= and Is to 4p(_
transitions from the Cu(2) and the Cu(1) sites, The feature marked "D" is
identified as a shape resonance. Figure 1 shows that the Cu-k edges for
YBa2Cu307_ 8 (solid line) and YBa2(Auo.ICUo.9)307_ 8 (dashed line) are
virtually identical which indicates that Au substitution has little or no
effect on the valence of copper.
The Au L3 edges for YBa2(Auo.ICUo.9)307__, Au foil, monovalent
KAu(CN)2 and trivalent Au20 3 are shown in figure 2. The spectra for the
reference compounds and Au substituted 123 have been normalized to the
edge step of the Au absorption edge. The near-edge structure of the L2 and
L3 edges in 5d transition meia_compounds is _ominated by 2pl/2 to 5d3/2
4
and 2P3/2 to 5dl/2 transitions (white line feature). 23 The L3 edge also
has a contribution from the 2P3/2 to 5d3/2 level but it is much weaker
than the 2Pl/2 to 5d3/2 and 2P3/2 to 5dl/2 transitions. The intensity of
the white line feature is thought to provide a good indication of the 5d
occupation. 24 For Au (small dashed curve), the 5d band is filled and no
white line feature is observed. The oxidation of Au to the mono or
trivalent state creates the white line feature. We find that the white line
area is larger for Au20 3 (dot-dashed line) than KAu(CN)2 (dashed line) and
also find that the white line feature shifts to lower photon energy when
the oxidation number increases from 1+ to 3+. It should be noted that the
ratio of the white line areas for Au20 3 and KAu(CN)2 is less than the ratio
of d-electron removal suggested by the valence. The white line area for
Au in 123 (solid line) is considerably smaller than either of the reference
compounds and lies at lower photon energy than KAu(CN)2 but at a higher
photon energy than Au203.
The Au L3 near edge data for YBa2(Auo. 10u0.9)307_ 5 suggests that the
valence of Au in the 123 material differs from that of Au in trivalent
Au20 3. Iron also replaces Cu on the chain site and is trivalent. However,
the substitution of 8 mole percent Fe depresses Tc by 55 K25 rather than
the 2 K found for the equivalent gold substitution. Yang et al. 5 found that
Fe substitution modifies the O k-edge and reduces the number of 2p holes
on the oxygen. We suggest, based on the reduction in the apparent number
of unoccupied Au 5d states for the Au-O bond in 123, when compared to
Au20 3 and the small Au-induced change in T c, that little or no change
i
occurs in the number of the oxygen 2p holes. This is supported by our data
for the Cu k-edge which shows no change in the Cu-O bonding like that
observed when the high T c material becomes oxygen deficient and holes
5
are removed from the oxygen site. 21
The near-edge data suggests that small amounts of Au incorporated
in 123 at the Cu(1) have less 5d involvement in the Au-O bond in the
superconductor than in Au20 3. We believe this is related to the doping of
the oxygen site with holes. Gold has less effect on the superconducting
properties than Fe presumably because Au does not localize the holes on
itself. There are no obvious changes in the Cu k-edge and we conclude that
Au substitution has little or no effect on the chemical state of Cu or
oxygen.
Acknowledgment
The authors thank Professor M. Croft of Rutgers University for help
in obtaining some of the EXAFS data and S. M. Heald, J. Jayanetti, and E.
Barrera for their help in performing and analyzing the XANES
measurements. M.W.R. acknowledges the support of the U. S. Department
of Energy, Division of Materials Sciences under Contract DE-AC02-
CH00016. A. F. H. acknowledges support from the Space Electronics
Division at NASA Lewis Research Center. The X-ll beamline and the
National Synchrotron Light Source are supported by the U. S. Department of
Energy under _ontracts DE-AC05-80-ER10742, DE-AC02-CH00016,
respectively.
6
References
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7
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38, 8893, (1988).
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Hirabashi, K. Murata, N. Terada, T. Yao, H. Iwasaki, and Y. Kimura. Jpn.
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23. B. Qi, I. Perez, P. H. Ansari, F. Lu, M. Croft, and D. Wieliczka. Phys. Rev.
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24. I. Perez. B. Qi, G. Liang, F. Lu, M. Croft, and D. Wieliczka. Phys. Rev. B
38, 12233 (1988).
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Zolliker. Phys. Rev. B 39, 6667 (1989).
8
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Figure 1.-Cu k-edge XANES for YBa2Cu307_ 6 (solid
curve) and Y_2Auo.3Au2.707_ 8 (dashed curve).
The energy reference was maintained using a Cu
foil.
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Figure 2.-Au L3 absorption edges for gold (dotted curve), a
monovalent gold compound - KAu(CN)2 (dashed curve),
trivalent gold oxide - Au203 (dot-dashed curve) and
YBa2Au0.3Au2.707. 6 (solid). A censlstant energy reference
was maintained by examining the L3 edge of a gold foil
simultaneously with these samples.
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September 1991 Technical Memorandum
4. TITLE AND SUBTITLE
Near-Edge Study of Gold-Substituted YBa2Cu307_ 8
6. AUTHOR(S)
Mark W. Ruckman and Aloysius F. Hepp
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135- 3191
9. SPONSORING/MONITORING AGENCY NAMES(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Washington, D.C. 20546- 0001
5. FUNDING NUMBERS
WU-506-41-11
II. PERFORMING ORGANIZATION
REPORT NUMBER
E-6509
!10. SPONSORING/MONITORING
AGENCY REPORT NUMBER
NASA TM- 105220
11. SUPPLEMENTARY NOTES
Mark W. Ruckman, Brookhaven National Laboratory, Physics Department, Upton, New York 11973-5000; Aloysius F.
Hepp, NASA Lewis Research Center. Responsible person, Aloysius F. Hepp, (216) 433-3835.
12a. DISTRIBUTION/AVAILABILITY STATEMENT
Unclassified - Unlimited
Subject Categories 25 and 76
12b. DISTRIBUTION CODE
13. ABSTRACT (Maxlmum 2OOwords)
The valence of Cu and Au in YBa2Au0.3Cu2.7OT_swas investigated using x-ray absorption near-edge structure (XANES).
X-ray and neutron diffraction studies indicate that Au goes on the Cu(1) site and Cu K-edge XANES shows that this has
little effect on the oxidation state of the remaining copper. The Au L 3 edge develops a white line feature whose position
lies between that of trivalent gold oxide (Au203) and monovalent potassium gold cyanide (KAu(CN)2) and whose height
relative to the edge step is smaller than in the two reference compounds. The appearance of the Au L3-edge suggests that
fewer Au 3d states are involved in forming the Au-O bond in YBa2Au0.3Cu2.707_ _ than in trivalent gold oxide.
14. SUBJECT TERMS
Gold-substituted YBa2Cu307_8; XANES; Valence of gold; Copper
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Near-edge study of gold-substituted YBa2Cu3O(7-delta)

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