MAGNETIC SUSCEPTIBILITY, ELECTRONIC SPECIFIC HEAT AND TRANSPORT PROPERTIES OF SOME INTERMETALLIC COMPOUNDS OF CERIUM

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MAGNETIC SUSCEPTIBILITY, ELECTRONIC SPECIFIC HEAT AND TRANSPORT PROPERTIES

OF SOME INTERMETALLIC COMPOUNDS OF CERIUM

J. Cooper, C. Rizzuto, G. Olcese

To cite this version:

J. Cooper, C. Rizzuto, G. Olcese. MAGNETIC SUSCEPTIBILITY, ELECTRONIC SPE- CIFIC HEAT AND TRANSPORT PROPERTIES OF SOME INTERMETALLIC COM- POUNDS OF CERIUM. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-1136-C1-1138.

�10.1051/jphyscol:19711406�. �jpa-00214447�

JOURNAL DE PHYSIQUE Colloque C I, supplt!ment a u no 2-3, Tome 32, FPurier-Mars 1971, page C 1 - 1136

MAGNETIC SU SCEPTLBEITY, ELECTRONIC SPECIFIC HEAT AND TRANSPORT PROPERTIES

OF SOME INTERMETALLIC COMPOUNDS OF CERIUM

J. R. COOPER and C. RIZZUTO

Gruppo Nazionale di Struttura della Materia del C. N. R.

Istituto di Scienze Fisiche dell'Universit8, 16132, Genova, Italy G . OLCESE

Istituto di Chimica Fisica dell'universiti, 16132, Genova, Italy

Resum&. - Des mesures de resistance ilectrique, de chaleur specifique electronique et de pouvoir thermoelectrique ont ete faites sur les composCs CeSn3 et CeBel3 pour lesquels des mesures antkrieures montraient un aplatissement de la courbe l l x ( T ) vers 200 OK. La chaleur specifique est grande (- 50 et ¹⁰⁰ mj/mole/OK2) et peut s'expliquer par un etat lie non magnetique 4f virtue1 avec une largeur de bande habituelle (- 0,01 eV) et un taux d'occupation 4 f egal A 1. On trouve de larges raies dans les courbes de pouvoir thermoelectrique. Les mesures de chaleur spkifique (CeAl? et CePb3) et de pouvoir thermdlectrique (CePb3) sont egalement indiquks par ces deux composes dont la susceptibilitk suit une loi de Curie-Weiss.

Abstract. - Measurements of therrnopower, electrical resistance and electronic specific heat have been made on the compounds CeSn3 and CeBe ^{1 3} for which previous measurements showed a flattening off in l/x versus Taround 200 ^OK.

The ^y values are large (- 50 and 100 mj/mole/OK2) and could arise from a non-magnetic 4f virtual bound state with the usual energy width (-- 0.01 eV) and a 4 f occupation number of unity. There are broad positive peaks in the thermopower curves various features of which scale as the ⁷ values (or the energy widths). Specific heat (CeA12 and CePb,) and ther- mopower (CePb3) results for two compounds for which x is essentially Curie-Wiess are also reported.

In the study of dilute magnetic alloys cerium forms an interesting link between the other rare earth impu- rities and the 3 d transition elements. Because the 4 f wave functions are well-localized, intermetallic com- pounds of Ce and the pure metal itself have several features in common with dilute alloys. For example Kondo type resistance minima have been observed in the compounds CeAI, [ I ] and CeB, [2] and, on the theoretical side, the Friedel-Anderson model, originally developed for isolated 3 d impurities, has been used as the starting point for a successful treat- ment of the anomalous phase diagram and magnetic properties of cerium metal [3].

Previous measurements [4] of magnetic susceptibi- lity down to 770K on the compounds CeSn, and CeBe,, showed a levelling off in the l / ~ vs. T curves around 200 OK. This was associated with a magnetic- nonmagnetic transition of the 4 f virtual bound state of cerium. The lattice parameter of CeSn, is also strongly temperature dependent [5]. To investigate this transition further we have measured the thermo- power, susceptibility, electrical resistance and low temperature specific heat for these two compounds, for CePb, (which is iso-structural with CeSn, but has a different x ( D dependence) and for CeAI, (for which susceptibility [6], resistance [I], and specific heat [I I], data have already been published).

The susceptibility results are shown in figure 1 : the three curves are approximately parallel at high temperatures, the slope corresponding approxima- tely to the magnetic moment of the free Ce3+ ion.

The points with the large error bars below 77 OK were taken using a zero field AC method and so the low temperature rise in 1 / ~ may not be significant.

In contrast to CeSn,, for CePb,, 1/x continues to fall approximately linearly with T down to loOK, in agreement with previous work [7].

1% MOLE /EMU

.I

FIG. 1. - 11% (molele. m. u.) versus T P K ) ( ~ a t a above 77 OK, for CeSn3 ^and CeBera from ref. ^{[4] and} for CePb3 from ref. [7]).

CeSn3 ^e, CeBel3 H, CePb, ^A.

The specific heat results are plotted in figure 2.

In all cases the coefficient of the electronic specific heat, y , is extremely large, ranging from 53 mJ/mole/

0K2 for CeSn, to 225 mJ/molel0K2 for CePb,. Compar- ing with the value of y, approximately 10 mJ/mole/OK2, for LaSn,, LaPb, [8] and non-magnetic a-Ce [9] it can be seen that, in our compounds, the main contri- bution to y comes from a 4 f level near the Fermi level.

For the two specimens, CeAI, and CePb,, for which a localized magnetic moment exists at low tempera- tures, there is a marked increase in C/T. Our results for CeAl, are in agreement with the more accurate ones of ref. [ l l ] taken between 0.5 and 14 OK, which show a ⁽⁽ I anomaly ^)> associated with magnetic order- ing at 3.40K. However our interpretation is diffe- rent in that we have used the linear part of the C / T versus T Z curve between 10 OK and 20OK to obtain values of y and the Debye temperature 8,(0).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711406

MAGNETIC SUSCEPTIBILITY, ELECTRONIC SPECIFIC HEAT A N D TRANSPORT PROPERTIES C 1

-

Compound CeSn, CeBe,

,

CePb, CeAI,

The absolute thermopowers S of the three speci- mens are shown in figure 3. We note that :

a) In all cases there is a large positive peak (40 pV/OK) in S.

b) For CeSn, and CeBe,3 (and approximately for CePb,) S is proportional to T at low temperatures, as expected for the ⁽⁽ electron diffusion thermopower D

when phonon effects can be neglected, but the initial slopes of the two curves differ by a factor of 8.

c) At high temperatures the two curves merge and show the same linear dependence above 1500K for CeBe,, and 300 OK for CeSn,.

The resistance results for CeBe,, and CeSn, show no sharp anomalies but at room temperature (cer- tainly well above 0, for CeSn,) the resistance rise is slower than the T1 dependence expected. This effect, when observed in transition metals, has been attri- buted to narrow electron energy bands whose width is comparable with kT [lo].

We have tried to analyse the specific heat and ther- mopower results for CeSn, and CeBe,, using the 4 f density of states p,(E) from the Friedel-Anderson model referred to previously [3]. Application of the standard formula for the thermopower [lo] for scat- tering into a narrow f-band gives the observed sign and low temperature dependence. Unfortunately the analysis is not self-consistent since the numerical values of the half-width A and the position of the

900

Enhanc. factor n,(Edln,(E,)

&

M J/MOLE/ 'K2 '

- T

FIG. 2. - Spccific heat data, C/T(rnJ/rnole/uK2) versus TqoK2). resonance, E, - EF, give a 4 f occupation number,

CeSn3 0 , CeBe13 m, CePb3 A, CeA12 O. n,, greater than unity.

J - ^-

-

600 .A'/., -

400 300 200;

100

0 200

100

However we can obtain values of A and Eo - EF from the values of y alone, assuming n, = 1. The resulting values of A (table I) are of the expected magni- tude for Ce [3]. In figure 3 we see also that the height of the thermopower maximum scales as l/A, the limiting dependence of S is reached at T = 2.5 A and in the low temperature region the first deviations from S / T = constant occur at T = 412 in both cases.

The susceptibility enhancement factor determined from the (approximately) temperature independent susceptibility and y is surprisingly small in both compounds (see table 1). Possible phonon enhance- ment of the observed specific heat has been ignored.

In contrast, the susceptibility enhancement factor in non-magnetic a-Ce is believed to be about 20 131.

In conclusion the high electronic density of states observed in two compounds of Ce is consistent with reasonable values of A together with a 4 f occupation number of unity. Despite this there appears to be no localized moment at low temperatures and the sus- ceptibility enhancement factor is low.

4 0 -

3 0 - 20

40 30- 20- 10

-

Lgiq

-

!'A

\

- ^,/

/ O \ 4

- - - - O--O---p=~---O- O -

0

-

¤ +!C.-+

-

':

.

1 o o ~ ' ~ 0 ~ - '

-

' ' I

> I 1 ' ' -

-

*.-".* *

-,.,ed -

6

0 ~ ' 1 1 100 ' 1 1 1 " 200 1 1 1 1 1 300 1 1 1 'T'K 1 Various features of the observed thermopower

FIG. 3. -Absolute thermopower ( p ~ / ~ ~ ) . versus ^=(OK). curves scale as the ratio of these widths. In contrast CeSn, e, CeBel, m, CePb,A

.

TABLE I

Parameters determined from specific heat and susceptibility measurements s (,uv/'K )

%+

A

1

-

A A\

10;

.. .

/.4< -'Ay\

0-

d*

B \ ~ l l ' l ' l ' l

- _I

.

;I $'

\.

I

\,./' // ^,

O o l l l 50 l l l l l l100 l l l l l 150 l l lf? ',mK2)

C 1 - 1138 J. R. COOPER AND C. RIZZUTO

anomalies are seen in either the electrical resistance o r T h e two other compounds (CePb,, CeAI,) have the specific heat. This reinforces the view t h a t n o even larger y values and, between them, these two discontinuous change i n the 4 f occupation number show all the features commonly associated with the occurs for o u r compounds. K o n d o effect.

References

[I] VAN DAAL (H. J.) and BUSCHOW (K. H. J.), Solid [7] TSUCHIDA (T.) and WALLACE (W. E.), Jouril. of State Comm., 1969, 7, 217. Chem. Phys., 1965, 43, 3811.

[2] FISK (Z.), MATTHIAS (B. T.) and CORENZWIT (E.), [8] BUCHER (E.), ANDRES (K.), MAITA (J. P.) and HULL Proc. Nat. Acad. Sci., 1969, 6A, 1151. (G. W.), Helv. Phys. Acta, 1968, 41, 723.

[3] BLANDIN (A.) and COQBLIN (B.), Adv. in Phys., 1968, [9] PHILLIPS (N. E.), HO (J. C . ) and S M I ~ (T. F.), Phys.

27, 281 (in French). Lett., 1968, 27A, 49.

[4] OLCESE (G.) unpublished work. [lo] M o n (N. F.) and JONES (H.), ((The theory of the [5] HARRIS (I. R.) and RAYNOR (G. V.), J. Less Comm. Properties of Metals and Alloys ⁾⁾ (1936).

Metals, 1965, 9 , 7. [I I] HILL (R. W.) and MACHADO DA SILVA (J. M.), Phys.

[6] WHITE (J. A.), WILLIAMS (H. J.), WERNICK (J. H.) Letters, 1969, 30A, 13.

and SHERWOOD (R. C.), Phys. Rev., 1963, 131, 1039.