Heat capacity studies in RPd2Al3 (R  Ce, Pr, Nd and Sm) systems

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Physica B 223&224 (1996) 354-358

Heat capacity studies in RPd2A13 (R = Ce, Pr, Nd and Sm) systems K. Ghosh, S. Ramakrishnan*, Aravind D. Chinchure, V.R. Marathe, Girish Chandra Tara Institute of Fundamental Research, Bombay 400 005, India

Abstract

In this paper we report the results of low-temperature (from 1.7 to 40 K) heat-capacity studies in RPd2A13 (R = Ce, Pr, Nd and Sm). Our heat-capacity data on CePd2A13 and SmPd2A13 agree with those of the previous studies. A ~,-type anomaly is observed at 6.0 K for NdPdEAl3. A large jump (11 J/mol K) in heat-capacity data near the magnetic transition temperature confirms the bulk magnetic ordering in NdPd2Ala. The data for PrPd2Ala do not show any magnetic ordering down to 1.7 K. We have calculated the magnetic contribution of entropy due to 4f electrons of all the samples. The temperature dependence of entropy in PrPd2A13 indicates a nonmagnetic singlet ground state. The magnetic contribution to the entropy for all samples at 40 K is less than the corresponding free ion entropy values (R ln(2J + 1)). This suggests that there is a large crystal-field contribution to the heat-capacity in this series.

The discovery of two new heavy-fermion superconductors UNi2A13 [1] and UPd2A13 [2] has given fresh impetus to study superconductivity and magnetism in heavy-fermion compounds. This discovery has revived the search for similar systems in lanthanide compounds. Kitazawa and coworkers [3] have succeeded in finding CePd2A13 as the new antiferromagnetic (Tn = 2.8 K) heavy-fermion (y = 380 mJ/molK 2) compound in the equivalent lanthanide series. During last few years extensive experimental studies have been made on UPd2A13 (a heavy-fermion superconductor) and CePdzA13 (an antiferromagnetic heavy-fermion) compounds, whereas very few studies have been made of other rare-earth-based compounds belonging to the same structure as CePd2AI3 [4]. Our previous studies [-5] of resistivity and susceptibility show that NdPdzA13 undergoes an antiferromag-

* Corresponding author.

netic transition at 6.5 K and PrPdEA13 does not show any ordering down to 1.5 K. In order to understand the magnetism in RPd2A13, we have measured the heat-capacity of these compounds. Our heat-capacity data of CePd2A13 and SmPdzA13 agree with the previous data. The heat-capacity study on PrPd/A13 does not show any magnetic ordering down to 1.7 K but a Schottky anomaly around 25 K is observed. A large jump in the magnetic heat-capacity data of NdPdzAI3 confirms the bulk nature of antiferromagnetism in this compound. The value of the magnetic contribution to the entropy is much lower than the expected free ion value (R ln(2J + 1)). This indicates the presence of strong crystal-field effect in these compounds. All the samples of RPd2A13 system were made by melting the individual constitutents in an arc furnace under high-purity argon atmosphere. The samples were annealed at 900°C for two weeks in a sealed quartz tube with He atmosphere. The samples were found to have the hexagonal P6/mmm (i.e. UPd2A13) structure [2] and the

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K. Ghosh et al. / Physica B 223&224 (1996) 354-358

lattice constants a and c agree with the previously published values [3-7, 9]. The heat-capacity in zero field between 1.7 and 40 K was measured using home-built adiabatic calorimeter [8]. The temperature dependence of magnetic contribution to the heat capacity Cm (which is obtained after subtracting from the measured Cp data those of LaPd2A13) from 2 to 40 K of CePd2AI3 is shown in Fig. 1. Our data are in agreement with the previous published data [3, 5]. A large jump at 2.8 K ( ~ 1.8 J/mol K) clearly shows bulk magnetic ordering in this sample. A Schottky anomaly is also observed around 15 K in this compound. The calculated entropy is also shown in the same figure. The fact

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that the total entropy due to magnetic contribution at 40 K is less than the free ion value (R In (2J + 1)) and the Schottky anomaly in the magnetic heat-capacity signify the contribution from crystal-field effects in this sample. The total entropy below T, is found to be 2 J/mol K, which is much smaller than the value expected from the magnetic doublet ground state ( = R ln2). The presence of the Kondo effect could also be playing a part for this behavior. The temperature dependence of magnetic contribution to the heat capacity Cm from 2 to 40 K of PrPd2A13 is shown in Fig. 2. We do not observe any bulk magnetic ordering down to 1.7 K in this compound. There is

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356

K. Ghosh et al. /Physica B 223&224 (1996) 354-358

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a small anomaly in the heat-capacity data around 5 K in this compound. This anomaly could be due to the presence of small impurity ( < 5%), which could not be detected from the X-ray diffraction studies. We have also observed a Schottky anomaly around 25 K in this compound. As in the case of CePd2A13, the magnetic contribution to the entropy does not reach its saturation value at 40 K, which again emphasizes the importance of crystal-field effects in this sample. The temperature dependence of magnetic contribution to the heat capacity Cm from 2 to 40 K of NdPd2AI3 is shown in Fig. 3. A large jump at 6.0 K (11 J/molK) clearly shows bulk magnetic ordering in this sample, It

confirms the observed antiferromagnetic ordering below 6.5 K by our earlier resistivity and susceptibility studies. The total entropy below T, is found to be 7 J/mol K, which is of the order of R In 2, and this shows that the ground state is probably a doublet. From the entropy curve one can note that the crystal-field contributions are significant in this sample as well. The temperature dependence of magnetic contribution to the heat capacity Cm from 2 to 40 K of SmPd2AI3 is shown in Fig. 4. Three anomalies have been observed in the heat-capacity data at 3, 6 and 12 K which agree with the previously published data I-9]. The calculated entropy is also shown in the same figure and from this curve

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K. Ghosh et al. / Physica B 223&224 (1996) 354-358

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we conclude that the crystal-field contributions are significant. The total entropy below Tn is found to be 5 J/mol K, which is close to R In 2, and this shows that the ground state is also a doublet. A preliminary analysis of the heat-capacity data of all the samples of RPd2A13 (R = Ce, Pr, Nd and Sm) has been carried out in terms of the crystal-field Hamiltonian using superposition model. We could analyze our heatcapacity data for these samples using almost the same set of parameters. The detailed analysis of susceptibility and heat-capacity data using this model will be published

elsewhere. Schottky anomaly and entropy data of CePd2AI3 fit well with the theoretical model. In this case the ground state is a doublet. Both first and second excited states are also doublets. The first excited state is at 32 K and the second one is at 800 K, which agree with the values obtained using the point charge model. Our analysis shows that PrPd2A13 has a spin singlet nonmagnetic ground state and the first excited state is a doublet at 40 K. NdPd2Ala has a doublet ground state and a quartet as the first excited state which is very close ( ~ 10K) to the ground state. The ground state is

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K. Ghosh et al. / Physica B 223&224 (1996) 354-358

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magnetic in SmPd2AI3, but excited states are not still clear in this case. Detailed analyses are in progress and will be published elsewhere.

References [1] C. Geibel et al., Z. Phys. B 83 (1991) 305. [2] C. Geibel et al., Z. Phys. B 84 (1991) 1.

[3] [4] [5] [6] [7] [8] [9]

H. Kitazawa et al., J. Phys. Soc. Japan 61 (1992) 1461. S.A.M. Mentink et al., Physica 186-188 (1993) 497. K. Ghosh et al., Phys. Rev. B 48 (1993) 6249. S. Mitsuda et al., J. Phys. Soc. Japan 61 (1992) 4667. S. Suga et al., Physica B 186-188 (1993) 661. S. Ramakrishnan et al., J. Phys. E 18 (1985) 650. H. Kitazawa et al., Physica B 186-188 (1993) 661.