ELSEVIER Physica B 230-232 (1997) 186-188

Antiferromagnetic ordering in CeHg3 J. T a n g 1, K .A . G s c h n e i d n e r J r*

,4mes Laboratory and Department of Materials Science and Engineering, Iowa State University. Ames, 1,4 50011-3020, USA

Abstract

C e H g 3 was found to undergo an antiferromagnetic phase transition at 1.6 K. This was indicated by the 2-type peak in its heat capacity and a negative paramagnetic Curie temperature. The electronic specific heat coefficient 7 of 52 mJ/mol K 2 suggested that CeHg 3 is not a heavy fermion material contrary to its isostructural counterpart CeA13.

Keywords: CeHg3; Antiferromagnet order; Specific heat; Heat capacity

CeHg 3 crystallizes in a hexagonal NiaSn-type structure with lattice parameters a = 6.760A and c = 4.941 A, respectively [1]. The spacing between two nearest-neighboring Ce atoms in the CeHg3 lattice is 4.62 A, which is slightly larger than the Ce-Ce spacing in the isostructural CeA13 (4.43 ~, for CeA13) I-2]. Since CeA13 is a well-known heavy fermion [2], it is of interest to examine the low- temperature behaviors of CeHg 3 and study the similarities and differences between the two.

CeHg3 sample was prepared as follows. Small pieces of Ce metal and the corresponding amount of Hg were sealed under helium in a quartz tube, which was heated to --~650°C. At this temperature, which is below the melting temperature of CeHg3, the Hg slowly vaporizes and reacts with Ce to form CeHg3. Sample was heated for two months and the reaction between Ce and Hg occurred through dif- fusion. New compound, CeHg3 was formed after the completion of the reaction.

Low-temperature heat capacity measurement was carried out using an adiabatic heat pulse type calorimeter [3]. Since CeHg3 is an unstable c o m -

* Corresponding author. Present address: Department of Physics, University of

New Orleans, New Orleans, LA 70147, USA.

pound and reacts readily with air, its heat capacity was measured by sealing a CeHg3 sample in a small brass container under helium atmosphere, then the heat capacity of the sample together with the con- tainer was measured and finally the heat capacity of the container was subtracted off.

Fig. 1 shows the low-temperature portion of the heat capacity of CeHg3. The 2-type magnetic peak at ,-~1.6 K is obvious. The entropy associated with this peak was estimated to be ,-~2.4 J/mol K, which is --~40°,/o of the expected value R In 2 = 5.76 J/mol K.

The reason that the experimental entropy is so far away from the expected value, is perhaps due to the lack of experimental data below the ordering tem- perature. Actually, we were only able to reach a temperature just below the peak ( ~ 1.5 K). There- fore, the extrapolated values of the heat capacity used to calculate entropy below the peak temper- ature can be significantly different from the true entropy value. The possibility that the magnetic peak is an impurity effect can be ruled out because an entropy of 40% of R In 2 is a clear indication of the bulk property of CeHg3. The nature of the ordering will be discussed later together with the magnetic susceptibility data.

Above the peak temperature, the C / T versus T 2 plot (see Fig. 2) shows a straight line behavior. The

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Tang, K.A. Gschneidner Jr / Physica B 230-232 (1997) 186-188 187

o

"6

0

I I

/

0 i i i

1 1.5

I ' I ' I ' I '

I I I t I I I

2 2.5 3 3.5 4

T (K)

Fig. 1. Heat capacity of CeHg3 as a function of temperature (1.5-4 K). The peak at 1.6 K is due to the antiferromagnetic phase transition.

o, ~D - 6 2 E

0

\ i I

0 113

I ' I ' I ' I ' I ' I '

I i I I I i I j I

20 30 40 50 60

T 2 (K 2)

Fig. 2. C/T versus T 2 plot for CeHg3.

I

I i

70 80

Debye temperature 0D and electronic specific heat coefficient 7 were determined from the linear re- gion. The moderate 7-value of 52 mJ/mol K 2 sug- gests that CeHg3 is not a heavy fermion material.

Compared with CeA13, one may conclude that by o

increasing the Ce-Ce spacing from 4.43A for o

CeA13 to 4.62 A for CeHg3 the non-magnetic heavy fermion behavior is destroyed and a magnetically

188 J. Tang, K.A. Gschneidner Jr / Physica B 230-232 (1997) 186-188

500

400

A

E 300

0

200

100 ******

0 ' * ~ J ' 0 20 40 60

I I I I I I I I I I I

80 100 120 140 160 180 200220 240 260 280 300 T (K)

Fig. 3. The inverse magnetic susceptibility of CeHg3.

ordered state is created. Although a large Ce -Ce spacing is necessary for the formation of heavy fermion state [4], it is not a sufficient condition. Probably the 4f -p state interaction (hybridization) changes sufficiently between A1 and Hg in these CeM3 compounds to give these different behaviors. The Debye temperature 0D of CeHg3 was found to be l17K, which is one of the lowest 0D values reported for Ce compounds.

Magnetic susceptibility was measured using a SQUID magnetometer. The measurement was done from 2.5 to 300 K in an applied field of 100 G. The inverse of magnetic susceptibility of CeHg3 is shown in Fig.3. The Curie-Weiss behavior is in- dicated by the linear array of points in this plot. The effective moment and paramagnetic Curie tem- perature are Peff = 2.32#B and 0p = - 1 0 K, respec- tively. These values are different from the ones of Olcese [-1], who reported peff=2.93/~B and 0 p = - 5 3 K from susceptibility measurements taken from 100 to 350K. The long range of the extrapolation to obtain 0p might account for part of this difference in the 0p values. Furthermore, they did not show any values on their graphs, and so we are unable to compare their values with ours in the region of overlap, 100-300 K. One point that both experiments agree upon is the negative Curie tem- perature 0p. This implies that the peak in the heat capacity at 1.6 K is due to an antiferromagnetic phase transition.

In summary, CeHg3 is an antiferromagnet with TN = 1.6K. Both low-temperature heat capacity and susceptibility data supported this conclusion. The moderate electronic specific heat coefficient (~, = 52 m J (m o l /K 2) suggested that, unlike its iso- structural counterpart CeA13, CeHg 3 is not a heavy fermion. This difference is probably due to the difference in their Ce -Ce spacings, which probably changes the nature of the 4f -p state interactions.

A c k n o w l e d g e m e n t s

The authors wish to thank B.J. Beaudry and N.M. Beymer for preparing the sample. This work was supported by US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences. The Ames Laboratory is operated by Iowa State University under contract no. W-7405- ENG-82.

References

[1] G. Olcese, Atti Accad. Naz. Lincei C1. Sci. Fis. Mat. Nat. Rent. 35 (1963) 7. ~

[2] G.R. Stewart, Rev. Mod. Phys, 56 (1984) 779. [3] K. Ikeda, K.A. Gschneidner Jr., B.J. Beaudry and U. Atz-

mony, Phys. Rev. B 25,(1982) 4604. I-4] H.H. Hill, in: Plutonium 1970, ed. W.N. Miner (Metallurgi-

cal Society of the AIME, New York, 1970) p. 2.