Physica B 163 (1090) 340-342 North-Holland

RESISTIVITY ANOMALIES IN ELI-BASED VALENCE FLUCTUATING SYSTEMS

Sujata PATIL”. R. NAGARAJAN, L.C. GUPTA, B.D. PADALIA’ and R. VIJAYARAGHAVAN Tuta Insiitute of Fundamental Research, Romhay, India

“Indian Institute of Technology. Romhay. Indiu

The resistivity measurements of three Eu-based valence fluctuating systems, EuIr,Si,, EuzNi,Sii and EuNiSi,. are

presented. It is shown that the anomalous resistivity behaviour of valence fluctuating systems can bc understood in terms of

dynamic alloy mode;.

1. Introduction

Resistivity anomalies [I, 21 are known to exist in valence fluctuating (VF) systems containing Ce, Eu and Yb ions. The anomalies, as has been discussed and analysed by Wohlleben and Wittershagen 131, in these systems are intimately coupled with the tempera- ture dependent fractional valence. V(T) and the val- ence fluctuation temperature. T,,, or the spin fluctua- tion temperature T,, of the unstable rare earth ion in question. Both of these quantities are strongly tem- perature dependent in general and, through their tem- perature dependence. give rise to various anomalies observed in these materials.

In order to understand these anomalies, Wohlleben and Wittershagen introduced [3] what they called the dynamic alloy model. According to this model. these anomalies arise because the conduction electrons “see” the unstable rare earth ion in two valence states, with different outer charge clouds and undergo nearly elastic Coulomb scattering. At high temperatures. the rare earth lattice appears to be a collection of rare earth ions “frozen” in two valence states. At low temperatures, the “extra” resistivity arising due to this effect vanishes. This is because the lifetime of a Bloch state is much longer than the residence time of the rare earth ion in any of the two valence states and thus the conduction electrons “see” an average uniform background. At a certain intermediate temperature. corresponding to valence fluctuation temperature T,, , there is a crossover from the low temperature be- haviour to the high temperature behaviour.

The two main ingredients of this model, as men- tioned above. arc the average valence and the valence fluctuation temperature. Both of these quantities are easily determined in the case of Eu-based intermediate valence compounds. We have been working on Eu- based VF systems for the past few years and have

OY21-4S26/YO/$O3.S0 0 El. wvier Science Publishers B.V. (North-Holland)

identified a number of interesting systems such as EuNiSi,, EuIrlSi, and Eu,Ni,Si,. In this paper. we report the results of our resistivity measurements in these materials and discuss them in the light of the dynamic alloy model.

2. Experimental

The samples were prepared by melting stoichiomet- ric amounts of the constituent elements of high purity in an arc and/or induction furnace under an inert atmosphere of argon. The loss in weight during melt- ing was made up by adding extra Eu and remelting the sample. All the samples were annealed at around 1000 K in vacuum sealed quartz tubes. Formation of single phase samples was checked using a Sicmens X-ray diffractometer. Eu intermetallics investigated here are highly brittle and it was difficult to make regular shaped samples for resistivity studies. There- fore. only the relative resistivity (say. with respect to

300 K) of the material could be measured. DC electri- cal resistivity measurements were made by using a standard four probe technique. Highly conducting sil- vcr epoxy was used to attach copper leads to the samples for current and voltage contacts. A Keithley nanovoltmeter and a high stability Lakeshore constant current source were used in these measurements. In order to avoid possible complications arising from thermal emfs, each reading was repeated with the current in the forward and the reverse directions and the average of the two values was taken. The mcasure- ments were performed over the temperature range 4.2~300 K using a home made liquid helium cryostat. Carbon glass resistance (4.2-M K) and platinum re- sistance (30-300 K) thermometers were used as tem- perature sensors.

S. Patil et al. I Resistivity of Eu-based valence fluctuating systems 341

3. Results and discussion

The results of the resistance meaurements (4.2 K < T s 300 K) of EuIr,,/Si,, Eu,Ni,Si, and EuNiSi, are shown in figs. 1-3, respectively. In these figures the average valence of Eu is also shown as determined by isomer shift (IS) measurements [4-61. It is clear that the temperature dependence of the resistance of these materials is anomalous. The most striking is the be- haviour of EuIr,Si, in which dR/dT is negative in the temperature range 150 K s T s 300 K. Even though

-I .

0 35 70 105 140 175 210 245 280 315

TEMPERATURE (K)

2.20

2.30

2.40 :

2.50 t

;: 260 ’

Z-70

2.80

z.90

Fig. 1. R(T)/R(300 I<) and average valence of Eu in EuIrlSiz at various temperatures. The value of average val-

ence and spin fluctuation temperature T,, are taken from ref.

[41

bO5- I I I I I I I -2.58 Eu,Ni, Si,

O-90 - - 2’6’ _ .**

- I

g 075

m

IK 0.60 t

.- C.

: :,” x

0 35 70 105 140 175 210 245 280 315

TEMPERATURE (K)

Fig. 2. R(T)IR(300 K) and average valence of Eu in

EuzNi,Si, at various temperatures. The values of average

valence and spin fluctuation temperature T,, are taken from

ref. [6].

EuNiSi,

090

6 0.75

$ ; 0.60

+..**= l .**

2.64 x

8 .*

.** 2.67

i”+ 1s~ - 200K 2,73 2.70 ti ~

\ F, 0.45 a

0.30

0.15 1:’ 1

-f .

/

x

X--r

J 2.76 9

2.79

2.82

2.85 xx

ooo / / 1 / I 1 2.88 0 35 70 105 140 175 210 245 280 315

TEMPERATURE (K)

Fig. 3. R(T)IR(300 K) and average valence of Eu in

EuNiSiz at various temperatures. The values of average

valence and spin fluctuation temperature T,, are taken from

ref. [5].

we are unable to make corrections due to the electron phonon scattering and extract the contribution due to Eu valence fluctuations (since we could not measure the absolute resistivity), the negative dR/dT is quite evident.

Likewise, the near saturation of resistivity in the case of Eu,Ni,Si, at high temperatures suggests that the Eu induced contribution to the resistivity should also behave the same way, i.e. dR/dT < 0. The resis- tivity of EuNiSi, also exhibits a tendency to saturate at high temperatures. We have obtained the values of t,r, the spin fluctution temperature, of all three materials from the analysis of their IS and magnetic suscep- tibility data. The values are: T,, = 148 K, 200 K and 235 K for EuIrZSi,, EuNiSi, and Eu,Ni,Si, respective- ly [4-61. These temperatures have been marked in the corresponding figures. This confirms the suggestion of Wohlleben and Wittershagen that the crossover tem- perature should be nearly the same as the valence or spin fluctuation temperature. In the case of EuIr,Si,, it is not difficult to obtain the temperature T, (the

temperature at which the resistive anomaly has fully developed). From the observed temperature depen- dence TC is estimated to be -140 K which is nearly the same as the T,, value of 148 K. In the case of ELI,- Ni,Si,, the resistance shows a tendency to saturate at temperatures close to -290 K. However, it is clear that if the phonon contribution could be subtracted the anomaly temperature would be lower than -290 K. Similar remarks may be made in the case of EuNiSi,.

342 S. Patil et al. i Resistivity of Eu-based valence fluctuating system

4. Conclusions References

We have carried out resistivity, R(T), measure- ments of three Eu-based VF systems, EuIrZSi,. Eu,Ni,Si, and EuNiSj?,, in the temperature range 4.2-300 K. The resistwlty of EuIr,Si, saturates at -150 K. In EuZNilSi5 it exhibits a tendency to satu- rate at -290 K. The resistivity of EuNiSi, also indi- cates a tendency to saturate at higher temperature.

These data can be understood in terms of the dynamic alloy model introduced by Wohlleben and Witters- hagen. The variety of the behaviour of R(T) in these three samples reflects the increasing value of the charge fluctuation temperature TC in the sequence: T(EuIr,Si,) < (T,(Eu,Ni,Si,) < T,(EuNiSi,).

Ill

PI

[31

[41

[jl

[61

Proc. Int. Conf. on Valence Fluctuations. B. Roden, E.

Muller-Hartman and D. Wohlleben, eds. (North-

Holland, Amsterdam. 1984).

Valence Instabilities, P. Wachter and H. Boppart. eds.

(North-Holland, Amsterdam, 1982).

D. Wohlleben and B. Wittershagen. Adv. Phys. 34

(1985) 403.

Sujata Patil, R. Nagarajan. L.C. Gupta, R. Vij-

ayaraghavan and B.D. Padalia, Solid State Commun. 63

(1987) 955.

R. Nagarajan, Sujata Patil, L.C. Gupta and R. Vii-

ayaraghavan, J. Magn. Magn. Mat. 54-57 (1986) 349.

Sujata Patil, R. Nagarajan, L.C. Gupta, C. Godart, R.

Vijayaraghavan and B.D. Padalia, Phys. Rev. B 37

(1988) 7708.