Resistivity and thermopower studies on La3X (XAl, Sn, In, Ru, Ir, Co, Ni, Ge, Ga) systems

Journal of Alloys and Compounds, 198 (1993) 165-172 JALCOM 680

165

Resistivity and thermopower studies on La3X (X-= A1, Sn, In, Ru, Ir, Co, Ni, Ge, Ga) systems C. S. G a r d e ,

J. R a y a n d G . C h a n d r a

Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400 005 (India)

(Received October 2, 1992; in final form February 5, 1993)

Abstract

We report resistivity (p) and thermopower (S) measurements on the La3X (X=-AI, Sn, Ru, Ir, Co, Ni, Ge, Ga) and La3_xGdxIn (x=0, 0.02, 0.06, 0.14, 0.20) systems between 1.7 and 300 K. All the systems except La3Ir and La3_xGdxIn (x=0.14, 0.20) become superconducting with Tc below 10 K. The x=0.14 and 0.20 systems order magnetically at 4.7 and 7.1 K respectively. The p curve of all the compounds shows a strong negative curvature at intermediate temperatures followed by its saturation at higher temperatures. Further, some of the compounds exhibit a T z dependence at low temperatures. The low temperature T 2 dependence and the high temperature saturation effect in the p behaviour of these compounds have been suggested to arise from the scattering of conduction electrons from the spin fluctuations in the d-band. The low temperature extremum features in the S data could also arise from spin fluctuation effects. A sudden change in the slope of the p curve for some alloys, e.g. La3A1 and La3In, at Ts=27 K has been observed, which may be related to lattice transformation effects.

1. Introduction

Lanthanum and its compounds are known to exhibit complex transport properties. Both the f.c.c, and d.h.c.p. phases of La as well as compounds of the type LaX3 (X = In, Sn, Ga) and LaA12 show anomalous resistivity (p) behaviour [1, 2]. One of the characteristic features in the p curves of these systems is the occurrence of a strong negative curvature (at intermediate temperatures) and a tendency towards saturation at higher temperatures. This behaviour cannot be explained on the basis of the traditional theories [3, 4], which predict a linear temperature dependence for p at high temperatures. Such a p anomaly observed for LaAlz has been attributed [2, 5] to the mixed valent (MV) nature of La in this compound. The valency of La in LaA12 has been deduced [6] to be around 2.9 (from L~II edge spectral), indicating a partial occupation of the 4f shell of the La ions. Although the La atom has a completely empty 4f shell, a partial occupancy of this shell could take place when La is embedded in a metallic environment. The fractional valency of La thus points to its possible MV behaviour. This viewpoint is also supported [2] by a resistivity study in which rare earth (R) impurities (e.g. R - P r , Nd and Gd exhibiting a fixed valency) have been substituted at the La site in LaA12 and the p(T) curves of La~_xRxAl= compared with that of pure LaAl2.

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Further, La metal under pressure exhibits [7] an anomalous p behaviour at the structural transformation temperature Ts. This structural transformation is believed to arise from the presence of the soft phonon modes in pure La. In addition, transport studies [8-12] on LaAg and LaIn~Agl -x not only exhibit at Ts a marked variation in Op/OT in the p curve but also thermal hysteresis effects. In view of the wide variety of behaviour exhibited by these La-based compounds, it is felt that a further study of other La compounds would be useful. While some analyses of p data are presented in the literature, there is hardly any satisfactory explanation of the thermopower (S) behaviour in these classes of compounds. T h e r e f o r e the p and S behaviour of compounds of the type La3X ( X - A 1 , Sn, In, Ga, Ge, Co, Ni, Ru, Ir) have been examined in this study. The selection of the element X includes four characteristic types: (1) X - A 1 , Sn, In, Ga and Ge which belong to the sp class of elements, (2) X - C o and Ni of the 3d type, (3) X = R u of the 4d type and (4) X = I r of the 5d type. The valence electrons of the sp elements will form broad (4-5 eV) conduction bands, whereas the d-elements will give rise to narrow (1-2 eV) d-bands in their respective La3X systems. The aim of the present study is thus to investigate a wide class of La3X compounds with different band structures and carry out a comparative study of these intermetallic compounds. Prior to this study there has been a brief report [13]

© 1993- Elsevier Sequoia. All rights reserved

166

c. S. Garde et al. / Resistivity and thermopower of La3X

on the p behaviour of La3In and La3Co only. As far as we know, no S(T) study has been reported so far on any of the compounds investigated by us. In addition, magnetic impurities such as Gd have been substituted at the La site in pure La3In with a view to (1) understanding the influence on the p behaviour of increasing the Gd content in the La3_~Gd~In series and (2) investigating the dependence of Ts with increasing x.

The r.m.s, deviation X is defined as N

X [j~lXj21/2

=l--;-)

Here N is the number of data points in the region of interest and

pj( T)fit - Pj( T) =

2. Experimental details All the intermetallic La compounds have been prepared in an arc furnace under flowing argon and repeatedly melted to ensure sample homogeneity. The samples are drawn into cylindrical rods with an approximate size of 3 cmx0.2 cm. The La3X ( X = R u , Ir, Ge, Co, Ni, Ga) samples are annealed at 500 °C for 1 week, whereas the d.h.c.p. La sample is annealed at 200 °C for 1 week. All the other samples are heat treated at 200 °C to relieve internal strains in the solid. In particular, two heat treatments have been applied to the La3In sample. The compound has been annealed at (1) 200 °C for 1 week and (2) 500 °C for 1 week. In both the annealed and unannealed cases the low as well as the high temperature features are found to be well reproduced. The absolute value of p (for La3In) is found to vary at most by 10% on application of the above heat treatments. The La3Al, La3Sn and La3_xGdxln samples [14] are found to be highly reactive to air. For these cases the powdering of the samples was performed in a glove-box filled with argon gas and the X-ray analysis was carried out after coating the powder with a fine layer of grease. The rest of the samples were found to be relatively stable in air. The La3Sn, La3Ga and La3_xGd~In systems are found to occur in the cubic Cu3Au phase, whereas La3A1 crystallizes [14] in the hexagonal MgaCd type of structure. Further, all the compounds with X = 3 d , 4d and 5d elements are found [15] to crystallize in the orthorhombic Fe3C type of structure. La3Ge is the only compound with X =sp element which stabilized with the above crystal structure. All the compounds are found to be single phase (within the 5% resolution) using a Siemens X-ray diffractometer. The values (Table 1) of the lattice parameters for all the compounds are found [15] to match well with the reported values. The residual resistivity ratio ( R R R = p(300 K)/p( >f T~)) has also been measured (Table 1). Our measured R R R values of 10 and 17 for La3Co and La3In respectively are better than the corresponding reported values [13, 16]. No p measurements exist for the other compounds. The theoretical equations, given later in the text, are fitted to the experimental p data by minimizing X2.

....

(73 . . . .

where ps(T)nt and ps(T) . . . . are the fitted and measured values of p at temperature T respectively.

3. Results 3.1. Resistivity data

The p(T) data of the La3X (X-A1, Sn, In, Ga, Ge, Co, Ni, Ru, Ir) and La3_xGdxln (x=0, 0.02, 0.06, 0.14, 0.20) systems are shown in Figs. 1 and 2 respectively. The striking feature of the p curves for most of these systems is the presence of a strong negative curvature at intermediate temperatures (T=150 K = 0D) and a tendency towards saturation at higher temperatures (T> 150 K), an aspect not predicted by the traditional theories. The parameter 0D is the Debye temperature, which typically has values [17] of 125 and 175 K for La and La3In respectively. The saturation effects are more pronounced (Fig. 1) for the La3X compounds with X--3d, 4d and 5d elements. Further, all the compounds except La3Ir and La3_xGdxln (with x = 0.14 and 0.20) undergo (Fig. 3) a superconducting transition with To< 10 K. The values (Table 1) of Tc for Laaln , La3Sn, La3AI, La3Ga and La3Co are in agreement [14] with the previously reported values. Another interesting feature is the presence of a T 2 term at low temperatures (T<30 K) in the p(T) behaviour of some of the La3X ( X - I n , Sn, Ir, Ru, Co) compounds and d.h.c.p. La. The values (Table 2) of the coefficient A of the T 2 term lie in the range (9-13) × 10 -3 ~ cm K -2, except for La3Co which has a larger value of A = 33 × 10 -3 /zl'~ cm K -2. The temperature range (Table 2) over which this T 2 behaviour holds good is given by Tt < T< T,, where T~ is found to be above Tc by at least 2 K and T, lies between 18 and 28 K. The values (Table 2) of A for the La3X compounds compare well with those for A15 compounds such as Nb3Sn (A = 7 × 10 -3) and V3Si (A = 1.7 × 10-3). Further, the presence of a low temperature anomaly in the form of a sudden change in slope of the p(T) curve at Ts has also been observed (Fig. 4) in La3AI, La3In and some of the La3_~Gd~In (x=0.02, 0.06, 0.14) systems. The derivative Op/OT shows (Fig. 4) a maximum at T,. The values of Ts deduced from the position of the maximum in Op/~T are around 26-27 K for both

C. S. Garde et aL / Resistivity and thermopower of La3X

167

TABLE 1. Crystal structure, lattice parameters and R R R = p(300 K)/p(>/T~) value for annealed La3X compounds. T~ is the superconducting transition temperature Compound

Structure

T, (K)

a (X)

b (A)

c (A)

RRR

La3Ga La3In La3Sn La3A1 La3Ge La3Ni La3Ir La3Ru

Cubic Cubic Cubic Hexagonal Orthorhombic Orthorhombic Orthorhombic Orthorhombic Orthorhombic

5.610 + 0.010 5.070 + 0.005 5.102 + 0.006 7.192 + 0.020 7.416 _+0.020 7.189 _ 0.034 7.458 + 0.034 7.465 + 0.004 7.282 + 0.007

-

La3Co

5.8 9.7 6.2 5.8 3.7 6.2 4.2 4.0

5.528 6.497 6.650 6.662 6.570 6.594

15.0 17.0 5.6 4.2 13.5 2.14 1.9 3.7 10.0

i

i

i

80

............"~" .

40

20 0 50

::L Q.

I

b

La3Sn .

................

~....,'"

......

8C

Lo

~'o ,~o ,;o ~,~o ~oo T(K)

..'" . . . . ""

,,,,,,,,,,,,,,.,,,.,."".... X=0.06

Lo3N;

E U

o°•.

..."

. ......... ~ °0.02

° * oe

. , ~ ' " ' " ' " .....

LO31 r

80

40 2O 0

c~

::L. ..,. ...... ,..-

40

~ .,.,.~,'"'" 0

.......,.,..... ........ ~..

,-

:o.]4

La3.xGdxlr

0

4O

o

I

0.20

..,"

30' "...'""

Lo]GI .. ...... - . . . . . . . . . . . . . . . .

""

0 r

// bosGa

2C

4C 20

-

....,.

• I o°°"

'"

0.013 0.031 0.033 0,027 0.006 0.009

I ,,.,."

. ...............~

,''"'"

./

0 /s

- ..

...., ..."" Lo3C o ...."

80

0

0

°,...°.o.'°"'"

+ + + + + +

• ..'""'"'X

°°~'°

30

I:S::'"

j,.."

,t

l

I

6O

Laaln

......~.- i,.a 3 A I

"'"

40

0.025 0.033 0.044 0.006 0.015

...... '" "'" ....

..

o

i

+ + + + +

.,. • • " "" " '

CI

0 , 40

9.954 10.160 10.096 10.016 10.020

La3R u

.#,s~ J ,..."

610

....

20 0

I;0

I 300 240

T(K)

Fig. 1. Curves of p vs. T for La3X with X =- (a) sp and (b) 3d, 4d and 5d elements. The fitted curves match well with the data points. The resolution of the numerical fitting based on the expon'ential model is about the size of the experimental points (dots) and therefore the fitted curves are not shown in all cases for the sake of visual clarity.

La3In and La3A1. It is interesting to note that Ts is found to be quite insensitive to Gd substitution in Laa_xGdxln, indicating that this anomaly is not dependent on magnetic impurity substitutions. Also, the p anomaly at Ts observed in the La3AI and La3_xGd~In systems is quite similar to that seen in A15 compounds [18, 19] such as V3Si and Nb3Sn. In these A15 compounds the p anomaly at Ts has been related [20] to the onset of the martensitic transformation. Although the presence of the p anomaly in our data cannot be treated as conclusive evidence for the above-mentioned effect, future experiments on elastic constants and the precise determination of lattice constants (across Ts) for compounds such as La3A1 and La3In would be of interest to check the origin of this anomaly. With the increased replacement of La by Gd in La3_/Gdxln, T¢ is found to drop from 9.7 to 7.9 and

,

.""

."

j

•

°.,"

x ooo

La 3 In

• ""

/ 2~ 02

1 2I 0

, • •

°° oi°

.... .Y

2(:/'- t" I~ r~ - / "

I

t

I

;

I

0

60

120

180

240

500

T(K)

Fig. 2. Curves of p vs. T for La3_xGdxln.

4.1 K for the x = 0, 0.02 and 0.06 systems respectively. On further increasing the Gd concentration (x>~0.14), magnetic order evolved in agreement [21] with earlier studies. From our p studies the magnetic transition temperature TM is found (Fig. 5) to be 4.7 and 7.1 K for the x=0.14 and 0.20 systems respectively.

3.2. Thermopower data The S curves of the LaaX compounds are found to show (Fig. 6) an extremely complex behaviour. Various extremum features are found to occur for these compounds. La, La3Sn, La3Ge, La3Ru and La3Co exhibit (Fig. 6) two extrema (minimum or maximum) at temperatures designated by T1 (T> 60 K) and 7"2 (T<60 K). On the other hand, La3Ga, La3In, La3A1, La3Ir and La3Ni are found to exhibit only one extremum (minimum or maximum). It is interesting to note that the high temperature extremum (at T1) is a positive maximum for all the compounds with X = sp metals

C. S. Garde et aL / Resistivity and thermopower of LasX

168 X= s p elements

X= 3d,4d,Sd

elements

La

2.5

•"

0

i

~,....,.ooo

I

•***

i

35

Lo31r

25

La 3 Ga ~,,°

•°

5 u

j

LO31 n

0

I

E s

15

I L% Ru 3o

o k.o 3 Sn 25

I,o

i

2([ 0

--•~

21 I

Lo3Ge

,,

l

i

00

I

10

I

,

,

30

20

q 40

50

T (K)

I

,o,

I

I 45

Lo3A{

20I

20 t

La3Co

~ ° . . * * o*'°

Fig. 4. Anomaly in the p(T) curve at T, for La3In and La3A1. The continuous curves denote the behaviour of ap/aT in arbitrary units.

I

Lo3N i

°•*" ° • l

,.,; i 6 T(K)

0

112

0

-

6 T(K)

112 30 I I

Fig. 3. Curves of p vs. T for La3X near the superconducting transition temperature To. We note that only La3Ir does not undergo this transition down to 1.7 K.

Q

@

× = 0.20

I

u 15

I

6

T A B L E 2. Coefficient A of the T 2 term in the p(T) data and range of temperature TI ~
A (10 -3 /zl') cm K -2)

La3Ru La3Co Unannealed d.h.c.p. La Annealed d.h.c.p. La NbaSn V3Si

Q

0

I

•

•

X'O.14

I

I

L%_xGd x In

(K) 13.0 8.8 12.0 12.0 33.0 10.0 11.0 7.0 a 1.7"

I

I

Range TI

La3In LaaSn Laalr

8

z.-~r i

Compound

I

T(K)

12.4 11.2 8.3 8.3 11.5 13.2 12.2 18.4 a 17.0"

T~

(K) 17.3 24.0 27.0

28.5 25.3 23.6 26.4 30.0 a 28.0 a

aFrom ref. 33.

and a negative minimum for those with X = 3d, 4d and 5d elements (except LaaRu which exhibits a negative maximum). It is further noted that pure (d.h.c.p.) La metal shows a negative minimum at T1, similar to the compounds with X = 3d, 4d and 5d elements. The nature of the low temperature extremum (at T2), on the other hand, does not show any correlation with the type of X atoms.

'5r 2

I

4

T(K)

r

6

Fig. 5. Low temperature magnetic transition for La 3_xGdxln with x = 0 . 1 4 and 0.20. Note that the temperature scales for t h e x = 0 . 1 4 and 0.20 systems are different. The dashed lines highlight the change in slope at the transition temperature TM.

The S behaviour (Fig. 7) of the La3_xGdxln systems also exhibits several interesting features. Although the pure La3In compound shows (Fig. 6) a broad maximum at T= 80 K, substitution of small amounts (x>~0.06) of magnetic atoms such as Gd at the La site leads (Fig. 7) to the formation of a sharp minimum irrespective of whether the system exhibits magnetic order or not. This negative minimum occurs at T,,i, = 8 K, which is close to the magnetic transition temperature TM for the x=0.14 (TM=4.7 K) and x=0.20 (TM=7.1 K) systems. No other distinguishing feature at TM was observed from the S data. At higher temperatures the La3_xGdxln systems do not show any characteristic feature at Ts in the S data. However, La3AI shows (Fig.

C. S. Garde et aL I Resistivity and thermopower of La~X

(3

169

ature-independent residual resistivity and pi(T) is the temperature-dependent part. If the temperature-dependent part arises from the electron-phonon interaction, it is then given [3, 22, 23] by the expression

La

-1.2 e ~ e w°'O~e

~...--"'""L% G?... -5,6 2.0

0

-2.0

~- e* e e ° % l o j

2

~>

1.6: -[ 0,8 _

~"

1.0 ~ r

-4.0

L• ~ In ee

b

-

• ,,:,

L • 3 Ir

il.

-8.0

..." ......... I.l~eIo*

oe, l i o e eo ~.La 3 Sn

-I.5

•

LesRu

"'.,

L• 3 Co

im* "* i *loeoojoe,o,%

-2.5

0

-5.5 1.6 ,%

-1,0. 4,0 ~ ,

La3Ge

0 -1.6

0 1.5 "

II°,ol °**lj

0.3 -0,3

0,5 "l°

-0.5

L

80

0

I

I

%

.,,'"

,,,

I

160 240 320

80

T(K)

La3Ni

I .160 240 320 I

T(K)

Fig. 6. C u r v e s o f S vs. T for La3X with X - = ( a ) sp a n d (b) 3d, 4d a n d 5d e l e m e n t s .

L

I

[

I

°s m° • , , ~ , • * ° ~ , ° ,

0

I

I

°,~,°,

t

~-.,~...

•

• ..,%

X:0.2

-I ,I°# • °m°.N.• ,°.,,~N °

0 l

zLO (/3

• °°, .~,, ,~o , , , , . .

/

X : 0.14 :

.:

~: ~

"'-,. °,,~..

'"

.,-,.......,

t"

X=O,06 ""~"-,.,%

l -

.%,,~

•

°,°

0

x:o.o2 Lo3_ x Gdxln

0

I

I

60

120

I 180

I 240

I

300

T (K) F i g 7 Curves of S vs. T for La3_.Gd.In.

6) a bend in the S curve at a temperature of around 25 K, close to Ts where the p anomaly occurs.

4 Discussion

4.1. High temperature resistivity behaviour The p behaviour of a non-magnetic metal is usually expressed as p(T)= po + p~(T), where po is the temper-

The constants R and OR denote the strength of the electron-phonon scattering and the cut-off temperature for the Debye spectrum respectively. Js(OR/T) is the usual Debye integral [23]. For non-magnetic compounds pi(T)=RT at T>~ 04 and hence exhibits a linear temperature dependence at high temperatures. However, the p(T) behaviour of all our La3X compounds, showing a tendency towards saturation at high temperatures, cannot be explained by the above formulae. Such a deviation from the traditional theory could arise because R, which depends on the density of states N(EF) at the Fermi energy Ev, though approximately treated [24] as a constant, may not be so for the La3X compounds. In fact, band structure calculations predict [25] that La and its compounds could have the 5d band situated near Ev. At high temperatures the fine structure of this 5d band could lead to a temperature-dependent N(Ev), thus making R also temperature dependent. This aspect could then lead to a deviation from the conventional linear behaviour of p(T). Such temperature-dependent effects have been found [26, 27] to be important in YCo2, LaAI2 and YAI2, where saturation effects in the high temperature p(T) curve have been observed. In such systems the conduction electrons scatter predominantly from the spin fluctuations (arising from the proximity of the 5d band to Ev) and these effects in turn influence the p(T) and S(T) behaviour at high temperatures. Theoretical calculations [28] taking into account these spin fluctuations also predict a T 2 dependence at low temperatures in the p(T) curve. Such a temperature dependence has been observed (see Section 4.2) in some of the La3X compounds. We thus believe that spin fluctuation effects play an important role in these compounds. In view of the importance of the above-mentioned s-d scattering in these compounds, we have numerically fitted our p(T) data using an exponential model. The total resistivity in this model is given by the expression P=Pot-Pp JrPex, where Po is the residual resistivity and pp is the phonon term. For simplicity, the pp term is taken to vary [29] linearly with temperature and expressed as pp=alT, where al is a constant which represents the strength of this phonon term. We note that in the traditional theories this linear behaviour occurs only for T>~ 0D. The term pc×=C exp(-To~T) arises [30] from the scattering of conduction electrons between the s-like and d-like pockets of the Fermi

170

C. S. Garde et al. / Resistivity and thermopower of Za3X

surface separated by wavevector qo. Thus Pox could be treated as a parametrization of the spin-fluctuationscattering term as discussed earlier. To is the minimum temperature required to excite phonons with wavevector qo. C is a constant which denotes the strength of the Pox term. This excitation process becomes strong when there is a sharp peak [30] in the phonon density of states (PDS) at Tp = To. This equality condition is found to be roughly satisfied [30] for the A15 superconductor

v3si. The best fit to the experimental data is obtained by varying the four parameters po, al, C and To. This model fits the data (with X < 2%) for all the compounds except LarGe. For La3Ge a l < 0 , thus rendering the model inapplicable for this system. For all the other La3X systems the fitted curves match (within the size of the data points in Figs. 1 and 2) very well with the experimental curves for T > 3 0 K. The value o f To for our compounds is found to lie in the range 40-80 K. From the p data [31] of f.c.c. La it is estimated that To -- 88 K. The To value is found to be quite close to the value of Tp= 100 K deduced from the PDS data of La obtained from neutron studies [32]. However, PDS data are not available for any of the other La3X alloys to check the above correlation between Tp and the numerically deduced values of To from the exponential model. The value of Po (another fitting parameter in this model) is found to be close to p(0), which has been evaluated by extrapolating (to T= 0 K) the experimentally measured p(T) curves from T>~ T¢ and varies from 1 /z12 cm for annealed La to 70 ~ cm for La3Ir. We now comment on the relative strengths of the pp and p~x terms. The value of a~ is found to lie between 0.026 and 0.095 /z~ cm K -~ for LaaNi, La3Ir, La3Ru and La3Co (X---3d, 4d and 5d elements), which is about two to five times smaller than that for La3Ga, La3In, La3Sn, LaaA1 (X = sp elements) and La. The value of C is found to lie in the ranges 42-150 and 32-82 tzf~ cm for the compounds with X = 3d, 4d and 5d elements and X - s p elements respectively. Thus the latter compounds ( X - sp elements) are found to exhibit a phonondominated behaviour. For these systems our numerical studies reveal that pp = p~x at 300 K, whereas for the former compounds (X = 3d, 4d and 5d elements) our numerical estimates reveal that P~x dominates and we find p~x> pp at 300 K. This is reflected in a marked saturation of the p curve (Fig. 1) at high temperatures. This is further indication that the d-electrons at the Fermi level have an important influence on the p(T) behaviour and that spin fluctuations could play an important role in these systems.

4.2. Low temperature resistivity behaviour An important feature of the low temperature p behaviour of the La3X compounds is the presence of a T 2 term for some of our compounds ( X - I n , Sn, Ir, Ru, Co and d.h.c.p. La). We note that for the solitary case of La3Co the value (Table 2) of A is three times that for the other La3X compounds. This large value could possibly be related [16] to the Co atoms possessing a magnetic moment in La3Co. Again, this T 2 term could arise from spin fluctuation effects. The possibility of other electron-electron-scattering terms [33] is ruled out, since the value of t h e A / y 2 ratio (y is the coefficient of the linear term, pertaining to the electronic specific heat, in units of mJ (g atom) -1 K -2) for La3In (A/ 72--6.6×10 -5) and d.h.c.p. La (A/y2=10×10 -s) is much larger than that for the transition metals [34] where this mechanism is supposed to be operative, e.g. A / y 2 = 3 . 3 × 1 0 -7 for Pd. We thus believe that the T 2 term in p(T) arises [35] from spin fluctuation effects in these nearly magnetic systems. For some of the La compounds the presence of spin fluctuations can be deduced from resistivity [13], susceptibility [36] and Lin edge experiments [6]. The temperature-dependent susceptibility and Knight shift exhibited [17, 37] in La and La3In could also be understood in accordance with the above view. Further evidence for spin fluctuations in La is obtained from the pressure dependence of To. The value of Tc for La has been found [7] to increase with increasing external pressure up to 20 GPa. This enhancement of the value of Tc by external pressure has been attributed to the suppression of spin fluctuations by pressure. Hence the presence of spin fluctuation effects in our La3X compounds could explain the low temperature T 2 term in the p(T) data. In view of the above evidence, we believe that in the p(T) behaviour of the La3X compounds spin fluctuations play an important role in the manifestation of the high temperature saturation effects as well as the low temperature T 2 dependence. 4.3. Low temperature thermopower behaviour In the simplest approximation the total contribution to the S behaviour can be expressed as S=Sd+Sg, where Sd is the electron diffusion term and Sg is the phonon drag term. The value [17] of 0D for La and La3In obtained from specific heat experiments is found to be 125 and 170 K respectively. If we assume that the value of 0D for the other La3X compounds is not very different from these values, then (by assuming a common value of 0D = 150 K) it is observed that 7"2 lies between 0.10D and 0.300. Since the phonon drag effect is expected to exhibit an extremum feature (minimum or maximum) around T= 0.20D, this effect could be one of the possible mechanisms for the occurrence of the peak observed at T2 in our compounds. However,

C. S. Garde et al. / Resistivity and thermopower of La3X

Sg generally follows [38] a T 3 and a 1/T behaviour for T,~OD and T~,OD respectively. Such a. temperature dependence is not observed for our compounds. Also, it is generally believed that the Sg contribution could be small compared with S~ in this class of compounds. Sg is known to be suppressed by phonon-impurity scattering in alloy systems. In Labased compounds [39-41] such as LaA12, LaSn 3 and LaNi no low temperature peak due to the phonon drag effect is observed. In the case of Lain3 the S curve [42] is found to show a very sharp maximum at low temperatures and a cross-over to negative values at higher temperatures. It has been argued [42] that this low temperature peak in Lain3 is too sharp to be attributed to the phonon drag effect. Rather, it is believed that it is associated with the anomalous p behaviour [1, 42] and the possible valence-fluctuating nature of La in Lain3. Even for a non-magnetic compound such as YAI2, the low temperature extrema have been analysed [43] on the basis of the diffusion thermopower alone, since it has been argued that the phonon drag is small. Only in the case of pure metals such as Rb, Au, Cu and Ag has a peak due to phonon drag been definitely established [38]. Thus it is possible that the phonon drag term in our compounds could be small compared with Sd. The main contribution to S could therefore arise from Sa alone. Such tow temperature extremum features observed [26, 44] in intermetallic systems such as YCo2 and Y 9 C o 7 have been attributed to the spin fluctuation contribution to Sd, though no well-understood theory exists so far to explain these S(T) data. In addition, as we have argued earlier (see Sections 4.1 and 4.2), the p(T) data of the La3X systems also exhibit clear evidence for the presence of spin fluctuations.

171

Section 4.1), the La3X compounds could have sharp features in the density of states near EF. At high temperatures the details of this density of states near EF could lead to deviation from the linear behaviour. Such a non-linear behaviour at high temperatures for YCo2, LaA12 and YA12 has been analysed [26, 27, 45] by taking into account spin fluctuations arising from the band structure effects. We feel that more studies would be useful to understand the complex behaviours associated with the high and low temperature scales T1 and T2 respectively and whether they could be explained conclusively on the basis of spin fluctuations in the La3X systems.

5. Conclusions All the La3X compounds except La3Ir become superconducting below 10 K. Both the low temperature T 2 dependence and the high temperature saturation effects in the p curve of these compounds could be attributed to spin fluctuation effects. The compounds with X = 3 d , 4d and 5d elements are found to exhibit the high temperature saturation effects in the p(T) data more conspicuously than those with X - s p elements. The S studies reveal that almost all the La3X compounds exhibit a low temperature (T= 10 K) as well as a high temperature (T= 80 K) extremum. It would be of interest to have more conclusive evidence that these complex p(T) and S(T) behaviours could arise due to spin fluctuations. La3In, La3A1 and some of the La 3_xGdxln systems exhibit a p anomaly at Ts which could possibly be related to a structural transformation. This speculation also needs verification by independent experiments.

4.4. High temperature thermopower behaviour The S behaviour at high temperature (T~> 0D) for the La3X compounds is also quite complex. All the compounds except La3Ga exhibit an extremum feature at T1. Since T1>0.40o for all these compounds, the extremum feature at T1 may not be associated with the phonon drag effect. Further, although 0D = 100-200 K for these compounds, the S behaviour is found (Fig. 6) to be non-linear even around 300 K (T> 0D) and does not extrapolate to zero (at T= 0) as expected [42] from the theory. This is in contrast to the behaviour shown by non-magnetic simple metals [38] such as A1, Cu and Ag and compounds [42] such as Luln3, where the high temperature linear behaviour is found to extrapolate to zero as T ~ 0 . The Mott formula [23, 38] for Sd of metals predicts a linear temperature dependence for T>~ 0D. However, this formula does not take into account the details of the conduction electron states near the Fermi level. As discussed earlier (see

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