Resistivity and LIIIedge absorption studies in valence fluctuation system
Ce2Ni3Si5
Chandan Mazumdar, R. Nagarajan, C. Godart, L. C. Gupta, B. D. Padalia et al.
Citation: J. Appl. Phys. 79, 6347 (1996); doi: 10.1063/1.362691
View online: http://dx.doi.org/10.1063/1.362691
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Published by the American Institute of Physics.
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Resistivity and L III-edge absorption studies in valence fluctuation
system Ce2Ni3Si5
Chandan Mazumdara)
Department of Physics, Indian Institute of Technology, Bombay 400 076, India
R. Nagarajan
Tata Institute of Fundamental Research, Bombay 400 005, India
C. Godart
L.C.M.S.T.R., U.P.R. 209-C.N.R.S., 92195 Meudon, Cedex, France
L. C. Gupta
Tata Institute of Fundamental Research, Bombay 400 005, India
B. D. Padalia
Department of Physics, Indian Institute of Technology, Bombay 400 076, India
R. Vijayaraghavan
Tata Institute of Fundamental Research, Bombay 400 005, India
From our x-ray ~L III-edge! absorption ~XAS! investigations of Ce2Ni3Si5 , we show that Ce-valence
is temperature dependent; it is 3.07 and 3.11 at 280 and 8 K, respectively. We also report on our
resistivity measurements of two related materials Ce22x Rx Ni3Si5 ~R5Y, Gd and x50.1!. Absence
of any qualitative difference in the resistivities of these two samples suggests that the enhancement
of resistivity at low temperature on introduction of impurity atoms is due to Kondo hole scattering
implying that Ce2Ni3Si5 is a concentrated Kondo system. © 1996 American Institute of Physics.
@S0021-8979~96!25208-3#
Cerium based compounds often exhibit anomalous
physical properties, such as Kondo behavior, valence fluctuation ~VF!, heavy fermion, heavy fermion superconductivity,
etc. The close proximity of the cerium 4 f -level w.r.t. the
Fermi level is responsible for the origin of such behaviors.
We had shown earlier that Ce in Ce2Ni3Si5 is in the VF state1
by resistivity, magnetic susceptibility and specific heat measurements. As the resistivity shows a Kondo type behavior at
high temperatures and decreases because of onset of coherence at low temperature, it is of interest to study the effect of
magnetic and nonmagnetic impurity on the coherence effect.
Here we present the results of our investigations on dilute
substitution of Ce by Y ~nonmagnetic! and Gd ~magnetic! in
Ce2Ni3Si5 . We also present the results of x-ray absorption
~L III-edge! spectroscopic measurements which confirm the
VF behavior of Ce ions in this compound.
Ce22x Rx Ni3Si5 ~R5Y, Gd; x50, 0.1! were prepared by
melting high purity ~.99.9%! constituent elements by standard arc melting procedure.1 Room temperature powder
x-ray diffraction pattern was obtained using Cu K a radiation
on an x-ray diffractometer ~JEOL, Japan!. X-ray absorption
studies at the L III-edge, for investigation of valence state of
rare earth ions, were carried out at the French synchrotron
facility, Laboratoire pour l’Utilisation du Rayonnement Electromagnetique ~LURE! at Orsay, France. The details of the
experimental setup has been given elsewhere.2
The L III-edge spectra of the material taken at 280 K and
8 K ~Fig. 1! show a double edge structure as expected for a
VF system. The two edges correspond to Ce31 and Ce41
states. In order to determine the relative population of the
two valence states, the observed spectra were deconvoluted
using the procedure given in Ref. 3. The average valency of
Ce in this compound thus deduced is 3.07 and 3.11 at 280 K
and 8 K, respectively. The temperature dependence of the
intensity of the two edges confirms that the observed double
edge is due to VF phenomenon ~and not due to XANES or
impurity phases! as, in the case of VF, a change of relative
population of the two valence states with temperature is expected. We should point out that x-ray absorption spectroscopy measurement underestimates Ce valence. This is due to
a!
FIG. 1. X-ray absorption spectroscopy ~L III-edge! measurement of
Ce2Ni3Si5 at 280 and 8 K.
Present address: Solid State Physics Group, Tata Institute of Fundamental
Research, Bombay 400 005, India.
J. Appl. Phys. 79 (8), 15 April 1996
0021-8979/96/79(8)/6347/2/$10.00
© 1996 American Institute of Physics
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6347
FIG. 2. Resistivity of Ce2Ni3Si5 ~solid line! as a function of temperature.
Dashed line represents the 4 f contribution of Ce to the resistivity obtained
by subtracting the resistivity data of Y2Ni3Si5 from the resistivity data of
Ce2Ni3Si5 . Inset: Normalized resistivity of Ce22x Rx Ni3Si5 ~R5Y, Gd; x50,
0.1!. The agreement of the resistivity of the three samples in the high temperature region and the resistivity of the substituted samples below ;100 K
is to be noted.
the existence of a shake-down satellite located at the same
energy as the main peak of Ce31.4
Resistivity, r (T), of Ce2Ni3Si5 ~Fig. 2! exhibits anomalous behavior:
~i! Magnitude of r (T) in Ce2Ni3Si5 is rather high ~'300
mV cm at 300 K! than that observed in Y2Ni3Si5 and
Gd2Ni3Si5 ~'40 mV cm at 300 K!.
~ii! r (T) in Ce2Ni3Si5 is nearly temperature independent
in the interval 100 K <T<300 K.
This is to be compared with the temperature dependence
of r (T) of nonmagnetic Y2Ni3Si5 ~Ref. 1! and magnetic
Gd2Ni3Si5 ~Ref. 5! which exhibit normal metallic behavior.
These considerations suggest that the Kondo scattering
dominates the resistivity of Ce2Ni3Si5 . A similar overall resistivity behavior has been earlier observed in other known
VF systems, such as, CeRhIn,6 CeIr2Si2 ,7 CePd3 .8 r 4 f (T),
the 4 f -contribution of Ce to the resistivity, obtained by subtracting the phonon contribution ~using the resistivity data of
Y2Ni3Si5 , considering both the materials to have similar phonon contributions! has a negative slope with a broad maximum centered around 150 K and falls relatively sharply
below 100 K.1 Such a behavior is typical of a concentrated
Kondo system. The resistivity drop observed at low temperatures, seen in other concentrated Kondo systems also, has
been ascribed to the onset of coherence.1,9 In order to obtain
further insight into the behavior, we investigated the effect of
impurities on the resistivity behavior. For this purpose, we
substituted 5% of Ce by Gd ~magnetic! and Y ~nonmagnetic!
ions in Ce2Ni3Si5 .
For comparison, considering that effects of dilute substitution may not affect resistivity significantly, the resistivity
data of the substituted samples are plotted in inset of Fig. 2,
6348
normalized to room temperature value. From the figure we
see that the normalized resistivity for all the three sample,
viz., Ce2Ni3Si5 , Ce1.9Y0.1Ni3Si5 , and Ce1.9Gd0.1Ni3Si5 , are
similar in the temperature range 70–300 K. This is to be
expected because, at high temperatures, there is no coherence
and the scattering due to impurity will be negligible compared to scattering by Ce ions. At low temperature end, both
the Y and Gd doped samples show a similar increment in the
resistivity with respect to undoped material which is remarkable considering that Gd ions are magnetic. That implies that
the increase in resistivity at low temperature is due to a common effect in Ce22x Yx Ni3Si5 and Ce22x Gdx Ni3Si5 . A similar
behavior has also been seen in the system Ce12x Rx Pd3
~R5Y, Gd!.8
The absence of any qualitative difference in the resistivities of the samples doped with small concentration of a
magnetic ion ~Gd! and nonmagnetic ion ~Y!, shows that the
impurity-spin scattering does not have much effect on resistivity. We understand this in terms of Kondo hole as explained in the case of CePd3 ~Ref. 10! where it was shown
that coherence is destroyed by the creation of holes in an
otherwise regular Ce lattice by removing cerium atoms and
substituting other atoms in their place. Such removed Ce
ions are called Kondo holes. The scattering potential due to
the impurity is small in comparison to that of the ‘‘Kondo
hole’’ term and, as a result, the total resistivity at low temperatures is insensitive to the nature of solute. The total resistivity is primarily controlled by the integrity of the cerium
lattice.
In conclusion, we have confirmed here the VF nature of
Ce in Ce2Ni3Si5 from the temperature dependence of x-ray
L III edge results. The alloying of Ce2Ni3Si5 , by magnetic Gd
or nonmagnetic Y, exhibits a Kondo hole pattern in resistivity and strengthens our earlier interpretation of Kondo coherence in resistivity at low temperature in this material.
1
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1713 ~1981!.
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C. Mazumdar, R. Nagarajan, L. C. Gupta, R. Vijayaraghavan, C. Godart,
and B. D. Padalia, J. Appl. Phys. 75, 7155 ~1994!.
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D. T. Adroja, S. K. Malik, B. D. Padalia, and R. Vijayaraghavan, Phys.
Rev. B 39, 4831 ~1989!.
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B. Buffat, B. Chevalier, M. H. Tuilier, B. Lloret, and J. Etourneau, Solid
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therein.
J. Appl. Phys., Vol. 79, No. 8, 15 April 1996
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Mazumdar et al.