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Oxidation State of the Unusual Rare Earth (R= Ce, Pr and Tb) in Double-Layer High-Tc Superconductors

01 Apr 1997-Journal De Physique Iv (EDP Sciences)-Vol. 7, Iss: 2

Abstract: X-ray absorption and magnetization experiments are used to describe the oxidation states of the rare earth ions R=Ce, Pr and Tb in the double layer high-T c superconducting series RBa 2 Cu 3 O 7 and Pb 2 Sr 2 R 1-x Ca x Cu 3 O 8 . We obtained the same oxidation states for the rare earths in these two classes of compounds, namely, R=Ce tetravalent and R=Pr and Tb trivalent.

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Oxidation State of the Unusual Rare Earth (R= Ce, Pr
and Tb) in Double-Layer High-Tc Superconductors
U. Staub, L. Soderholm, S. Skanthakumar, M. Antonio
To cite this version:
U. Staub, L. Soderholm, S. Skanthakumar, M. Antonio. Oxidation State of the Unusual Rare Earth
(R= Ce, Pr and Tb) in Double-Layer High-Tc Superconductors. Journal de Physique IV Proceedings,
EDP Sciences, 1997, 7 (C2), pp.C2-1077-C2-1079. �10.1051/jp4:19972141�. �jpa-00255203�

J
PHYS
IV
FRANCE
7
(1
997)
Colloque
C2,
Suppl6ment au Journal de Physique
I11
d'avril 1997
Oxidation State of the Unusual Rare Earth
(R
=
Ce,
Pr and Tb) in Double-
Layer High-Tc Superconductors
U.
Staub,
L.
Soderholm*, S. Skanthakumar* and
M.R.
Antonio*
Swiss Light Source Project, P. Scherrer Institute,
5232
Villigen PSI, Switzerland
*
Chemistry Division, Argonne National Laboratory ZL
60439
Argonne, U.S.A.
Abstract.
X-ray absorption and magnetization experiments are used to describe the oxidation states of the
rare
earth
ions R=Ce,
PI
and
Tb
in
the double layer
high-Tc superconducting series RBa2Cu307 and Pb2Sr2R1-,CaXCu3O8. We obtained the same oxidation states for the rare
earths in these two classes of compounds, namely,
R=Ce tetravalent and R=Pr and
Tb
trivalent.
1.
INTRODUCTION
It is well known that the compounds
RBa2Cu307 (R=Y or rare earth ion)
are
superconducting below 92 K, except for the rare
earth
R=Ce,
Pr
or Tb. The reason for each exception is different. PrBa2Cu307 forms as a "single phase" material but does not
superconduct
[I]. Neither Ce nor Tb form this phase. For small doping concentrations Ce suppresses superconductivity in a
manner similar to
Pr
whereas
Tb
does not influence T, at all [2]. It is interesting to note that all these three rare earths are
redox active, and can be found in their trivalent and tetravalent oxidation states in nature. Ce has the lowest redox potential and
therefore is expected to have the most stable tetravalent oxidation state.
Pr
and
Tb
have similar redox potentials and therefore
may be expected to have the same oxidation state within a given structural series. However, the
Pr
4f wave functions are far
more extended than those of Tb and therefore both hybridization with higher shells and
bonding effects are more likely for the
Pr
ions.
The double layer
high-Tc superconductors PqLSrzRl.xCaxCu,08 form single phase compounds for all the rare earths,
including Ce and Tb. The
Pr
and Tb analogs are both superconducting, whereas the Ce analog is not [3,4]. In addition to its
superconductivity, the
R=Tb analog also exhibits unusual magnetic properties 151.
Here we present X-ray absorption and susceptibility results comparing the oxidation states' of Ce, Pr and Tb in the
RBa2Cu307 and Pb2Sr2R1.xCaxC~308 series.
2.
EXPERIMENTS
Details about the sample preparation is published elsewhere [4,6]. The single phase character of the samples was checked by
X-ray powder diffraction
andlor by neutron powder diffraction. X-ray absorption near edge structures (XANES) for the R L3-
edge were collected at ambient temperature. Beam line X-23A2 at the National Synchrotron Light Source (NSLS) and the
wiggler beam line 4-1 at the Stanford Synchrotron Radiation Laboratory (SSRL) were used.
X-23A2 at the NSLS is equipped
with a
Si<311> double-crystal monochromator (AE/E=2.9~10-~) and the beam line 4-1 at SSRL is equipped with a Si <Ill>
doublecrystal monochromator that gives an energy resolution of 14.1~10-~. The magnetic susceptibilities were obtained from
a superconducting quantum interference device (SQUID) magnetometer over the temperature range 10 to 300K using an
applied field of 500 Oe.
3.
RESULTS
AND
DISCUSSION
3.1
Ce
doped
compounds
Figs. la and b show the measured L3 X-ray absorption edges obtained from Ce in Ceo.2Yo,8Ba2Cu307 (a) and
Pb2Sr2Ceo,sCao.sCu308
(b)
compared with trivalent {Ce3+Ti03 (a); Ce3+F3
(b)]
and a tetravalent (Ce4+02) standards. The L3-
edge for the tetravalent oxidyzed Ce is composed of at least three transitions, as can
be
seen from the tetravalent standard [7].
The fingerprint of a trivalent oxidation state of Ce would
be
a
single line at lower energies. Hence,
in
contrast to the other rare
earth incorporated in these double layer perovskites, Ce is in a tetravalent oxidation state in both
RBa2Cu307 and
Pb2Sr2Rl-,CaXCu3O8 series This is also confirmed by susceptibility results, where no effective magnetic moment is observed
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19972141

C2-1078
JOURNAL
DE
PHYSIQUE
IV
except the contributions from the Cu spins indicating no unpaired 4f electrons in both compounds.
A
tetravalent oxidation state
were also found
in
Nd~-~Ce,Cu04
[8].
5715 5725 5735 5745
energy [eV]
571 5 5725 5735 5745
energy [eV]
Figs.
la
and
b:
Ce L3-edge XANES of Ceo.2Yo.8Ba2Cu307 (Ce123), CeTi03 and Ce02 (left) and PbzSrzCel.,C~Cu30s
(x=O
and
0.5),
CeF3 and Ce02 (right) obtained through electron-yield (a) and fluorescence
@)
detection
at
ambient temperature.
3.2
Tb
doped compounds
Figs. 2a and
b
show the measured L3 x-ray absorption edges obtained from Tb
in
Tb0,1Y09Ba2C~307 (a) and Pb2SrzTbCu308
(b)
compared with trivalent Tb
in
(Tb3+Cb), a mixed valent standard
(
Tby0,) and a tetravalent standard (SrTbQc03). The L3-
edge in the tetravalent oxidyzed
Tb
is composed again of at least three transitions, two strong ones in the mixed-valent
compound as can
be
seen from the standards. The fingerprint of trivalent Tb is a single line at lower energies. Hence, Tb is
in
a
trivalent oxidation state in both the
RBa2Cu307 and Pb2Sr2RI.,C&Cu3O8 compounds. This is also confirmed by the
susceptibility results, where an effective magnetic moment of
9.7
pB
is observed, which is very close to the value expected for
trivalent Tb.
energy [ev] energy [eVl
Figs.
2a
and
b:
Tb L3-edge XANES of Tb0.1Y0.9Ba2C~307. TbC13, Tb4O7 and SrTb03 (left), Pt~Sr~TbCu~0~. TbC13, Tb4O7
and
SrTb03
(right) obtained through electron-yield detection at ambient temperature.
3.3
Pr
doped compounds
Fig.
3
shows the measured
Lg
X-ray absorption edges obtained from Pr in Pb2SrzPr1.,C&Cu3O8 compared with a trivalent
Pr
in
(Pr3+F3) and a tetravalent Pr standard WO2). The
Lg
edge in the tetravalent oxidyzed Pr is composed of at least two
transitions as can be seen from Fig.
3.
The fingerprint of a trivalent oxidation state of
Pr
is a single line at lower energies.
Hence,
Pr
is in a trivalent oxidation state in the Pb2Sr2R1-xC&C~308 structure. Here, we did not show any results on

PrBazCu307, because these XANES results are published in the literature [9,10]. The susceptibility results, are consistent with
an observed effective magnetic moment of
2.7
pB,
intermediate between the free-ion expectation values for trivalent an
tetravalent
PI. However, this intermediate moment is well understood in terms of the crystalline electric field interaction,
which split the
3H4
ground-state of the trivalent
Pr
ions [ll].
5950 5960 5970 5980 5990
Energy
(eV)
Fig.
3:
Pr
L3-edge XANES
of
Pb2Sr2~1~x~axC~308,
(x=O, 0.5)
PrF3 and ProP obtained through fluorescence detection at ambient
temperature.
4.
CONCLUSIONS
We present XANES and susceptibility results on the "exceptions" Pr, Ce and
Tb
in
the two superconducting series RBa2Cu307
and PbzSr2Pr,.xCaxCu30,. These results strongly indicate that Ce is tetravalent in both series whereas Pr and Tb are both
trivalent. The tetravalent oxidation state of Ce explains why superconductivity is suppressed upon Ce doping (reduction of the
canier concentration) and also explains why CeBa2Cu307 does not form. The trivalent oxidation state of
Tb
is in accordance
with the fact that
T, is not suppressed upon
Tb
doping. The reason why TbBa2Cu307 does not form lies in the chemical
stability of the
TbBaO,, which is formed during the normal solid state reaction
[6].
For PI, which is also in the trivalent
oxidation state, the much more extended 4f wave functions (compared to
Tb)
allow hybridization with the Cu02 bands, which
effectively suppresses superconductivity.
ACKNOWLEDGMENT
The authors wish
to
thank M. Guillaume for loan of the
"R123"
samples. This research is supported by
the
U.S.
Department of
Energy, Basic Energy Sciences, Chemical Sciences, under contract W-31-109-ENG-38. The
SSRL is operated by the
Department of Energy, office of BES and the NSLS is supported by the U.S. Department of Energy, Divisions of Materials
Sciences and Divisions of Chemical Sciences.
1
L. Soderholm, K. Zhang,
D.
G. Hinks, M.
A.
Beno, J.
D.
Jorgenson, C.
U.
Segre, and
I.
K.
Schuller, Nature
328,604 (1987).
2
C.
R. Fincher, Jr., and G. B. Blanchet, Phys. Rev. Lett.
67,2902 (1991).
3
U.
Staub,
L.
Soderholm, S. Skanthakumar, and M. R. Antonio, Phys. Rev.
B
52,9736 (1995).
4
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B
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5
U.
Staub,
L.
Soderholm, S. Skanthakumar, S. Rosenkranz, C. Ritter, and
W.
Kagunya, Europhys. Lett.
34,447 (1996).
6
U.
Staub, M. R. Antonio,
L.
Soderholm, M. Guillaume,
W.
Henggeler, and
A.
Furrer, Phys. Rev.
B
50,7085 (1994).
7
M.
Gasgnier, G. Schiffmacher, L. Albert, P.
E.
Caro,
H.
Dexpert,
1.
M. Esteva,
C.
Blancard, and
R.
C.
Kamatak, J. Less-Common Met.
156,59 (1989).
8
M.
Nagoshi,
Y.
Fukuda, and
T.
Suzuki, J. Elcctron Spec.
66
257 (1994)
9
F.
W.
Lytle,
G.
v. d.
Laan,
and R. B. Greegor, Phys. Rev. B
41,8955 (1990).
10
E. E. Alp, L. Soderholm, G.
K.
Shenoy,
D.
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11
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Staub, S. Skanthakumar, L. Soderholm, and
R.
Osbom, J. Alloys Cmpds. (to be published).
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