Comparisons of electrical, magnetic and low temperature specific heat properties in group 13 and group 14 Ce8Pd24M compounds (M=B, Al, Ga, In and Si, Ge, Sn, Pb)

Physica B 262 (1999) 284—295

Comparisons of electrical, magnetic and low temperature specific heat properties in group 13 and group 14 Ce Pd M   compounds (M"B, Al, Ga, In and Si, Ge, Sn, Pb) C.D.W. Jones , R.A. Gordon , B.K. Cho , F.J. DiSalvo *, J.S. Kim
, G.R. Stewart
 Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, NY 14853, USA
Department of Physics, University of Florida, Gainesville, FL 32611-8440, USA Received 4 June 1998; received in revised form 28 September 1998

Abstract The Ce Pd M compounds, with an ordered structure derived from CePd , have been synthesized for various group    13 and 14 p-block elements. The electrical resistivities of the group 13 compounds have features that can be attributed to strong Kondo interactions while the group 14 compounds show typical metallic behavior. The thermopowers, or Seebeck coefficients, for each compound were measured at both 297 and 78 K, and are much smaller ("S"(25 lV/K) than the relatively large values for CePd (S+90 lV/K). All the compounds show Curie—Weiss behavior in the magnetic  susceptibility confirming the formation of nearly trivalent moments and the loss of the intermediate valence state found in CePd . The low-temperature magnetic susceptibility also indicates antiferromagnetic ordering for all the compounds  except M"B. The low-temperature specific heat was measured to investigate features of the antiferromagnetic ordering and potential heavy fermion behavior. While all the Ce Pd M compounds showed enhanced electronic specific heat   (C ) above the magnetic transitions, the group 13 compounds had larger values than the group 14 compounds. The  group 13 compounds also showed anomalies in the magnetic ordering behavior. While these properties in the group 13 materials could be due to heavy fermion behavior, there are several other possible explanations for the high C values  and heavy fermion behavior cannot be definitively proved.  1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetic susceptibility; Resistivity; Thermopower; Specific heat; Cerium intermetallic

1. Introduction * Corresponding author. Tel.: #1-607-255-7238; fax: #1607-255-4137; e-mail: [email protected].  Present address: Department of Physics, Simon Fraser University, Burnaby, BC Canada, V5A 1S6.  Present address: Pohang Center for Superconductivity, Department of Physics, Pohang University of Science and Technology, San 31, Hyoja-dong, Nam-gu, Pohang, Kyungbuk, 790-84, South Korea.  Also at Universita¨t Augsburg, Augsburg, Germany.

Cerium intermetallic compounds that have interactions between the 4f electrons and the conduction electrons often have interesting physical properties [1,2]. In these compounds, the RKKY interactions that lead to magnetic order are often in competition with Kondo interactions that lead to heavy fermion behavior. However, in some compounds it

0921-4526/99/$ — see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 8 0 - 6

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is possible to have both magnetic ordering and heavy fermion behavior [3—6]. The nature of the compound is determined by both the strength of the interaction between f electron states and conduction states near the Fermi level, and by the energy difference between the f level and the Fermi level. The physical properties are a delicate function of the chemical bonding environments of the cerium atoms. One of the most interesting cerium intermetallics is the intermediate valence material CePd [3,7—9].  In this compound, the presence of a strong Kondo interaction results in a broad peak in the resistivity at approximately 120 K [8,10—12] and a reduced high-temperature cerium moment which is screened out in the same temperature region [8,9,12]. Recently, we have reported some of the properties of a series of related compounds: Ce Pd M where M"(Ga, In, Sn, Pb, Sb, Bi)   [13,14]. The structure of these Ce Pd M mater  ials, shown in Fig. 1, has been discussed in detail

Fig. 1. The structure of Ce Pd M. The cerium atoms are in   black and the Pd atoms (not shown) sit at the vertices of the octahedra. The M atom sits in the center of a palladium octahedra (dark gray), causing its expansion and the distortion of neighboring empty octahedra (light gray). The magnitude of the distortions depends on the size of the M atom. The centered octahedra are arranged in an ordered fashion so as to preserve the overall cubic symmetry leading to a unit cell with an a-axis slightly larger than twice that of CePd . 

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[15] and is easily derived from that of CePd  (Cu Au structure type). It is important to note that  the cerium occupies only one crystallographic site and is coordinated by 12 nearly equidistant Pd atoms, with a single next-near neighbor M atom some 20% farther away. Changing M allows us to vary the electron count, and thus the separation between the Fermi and 4f energy levels. However, the cerium coordination environment is also perturbed slightly as we change the size of the interstitial M atoms. The size of M affects both the magnitude of the distortions in the crystal lattice as well as the overall size of the Ce Pd M unit cell.   Initial investigations into the properties of these compounds have shown large changes from those of the parent compound CePd [13,14].  The magnetic susceptibility (s) data indicated that doping the p-block elements into CePd  causes the loss of the intermediate valence state and the formation of nearly trivalent Ce moments, which undergo antiferromagnetic ordering at low temperatures [13]. The electrical resistivity (o) data for M"Sn, Sb, Pb, and Bi showed metallic behavior associated with the conduction electrons weakly interacting with the local moments in the presence of the crystal field [13]. Changes in the o(T) near the magnetic ordering temperature were also observed [13]. For M"Ga and In, however, features were present that were attributed to strong Kondo interactions [13]. It was not clear whether it was the atomic size of M, where the different M were simply causing different amounts of lattice expansion and distortion, or if it was the specific “chemical nature” of M that had the most effect on the observed properties. Here the term “chemical nature” refers to the electron contribution to the conduction band, bonding interactions and other factors that may induce changes in the interactions between the cerium f electrons and the conduction electrons. To investigate these issues further, this paper explores the properties of the Ce Pd M compounds with   the smaller M"B, Si, Al, and Ge. Extensive doping studies of CePd M where  V M"B [16—28] and Si [19,21,24,25,29,30] have already been reported, as well as a brief mention of doping with Ge [21]. Again, magnetic susceptibility shows the loss of the intermediate valence state and the formation of almost trivalent Ce with

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antiferromagnetic ordering for CePd Si  V [19,21,25,29] and CePd Ge [21], but with no  V magnetic ordering for the CePd B series  V [17,19,20]. The advantage of studying the structurally ordered Ce Pd M compounds (if they   form) over the randomly doped alloys, is that all the cerium atoms occupy a single crystallographic position. This eliminates the possibility of multiple or randomly distorted cerium environments. In this paper, we continue our study of the Ce Pd M   series by extending to new M and emphasizing the marked differences in magnetic, electrical and specific heat properties of the group 13 and group 14 compounds. To make these comparisons, some data from our previous papers [13,14] have been included for further and revised analysis with these new compounds. In addition, the thermopowers (Seebeck coefficients) of the Ce Pd M compounds   have been measured and can be compared to the values for CePd .  2. Experimental The samples were prepared by arc-melting the elements ('99.9% purity) in the desired ratios on a tantalum-coated, water-cooled copper hearth under flowing titanium-gettered argon. Each sample was turned over several times and re-melted to help ensure homogeneous ingots. The mass loss after arc melting was always less than 1% of the desired stoichiometric value. The samples were then placed in sections of tantalum tubing, sealed in evacuated silica tubes and annealed at 900°C for two weeks. The single-phase nature of each sample was confirmed using powder diffraction on a SCINTAG h—2h diffractometer with Cu K radi? ation. The lattice parameters are listed in Table 1. Resistivity measurements to 4 K were done in the typical four-probe manner on rectangular sections cut by string saw from the annealed samples. The absolute scale of the resistivity for each sample could vary by $20% due to uncertainties in the measurement of the area factors and voltage probe positioning. Some of the resistivities included in this paper have been scaled from our previous report [13] to reflect more accurate room-temperature values. The thermopower measurements were

Table 1 Unit cell data as derived from powder X-ray diffraction. The values for La Pd In and CePd are from Ref. [13] and [35],    respectively Compound

a (As )

CePd B    Ce Pd Al   Ce Pd Ga   Ce Pd In   CePd Si    Ce Pd Ge   Ce Pd Sn   Ce Pd Pb   La Pd In   CePd 

4.166(2) 8.404(2) 8.406(2) 8.445(2) 4.194(2) 8.405(2) 8.445(2) 8.460(2) 8.494(2) 4.131(2)

measured on a home-built apparatus in which a series of small, steady temperature gradients (max+ 1.5 K) were placed across the sample while under vacuum ((4;10\ Torr), and the resulting voltages across the sample measured. A plot of these voltages versus the temperature gradients gave a slope equal to the thermopower of the sample plus a contribution from the thermopower of the wires used to read the voltages. This contribution was easily corrected for and the overall accuracy of the thermopower measurement is approximately $10%. The magnetic susceptibility data were collected by the Faraday technique in an average magnetic field of 11.6 kG (H dH/dz" 10.97 kG/cm). The high-temperature data were fit to a Curie—Weiss expression as described elsewhere [31]. Some low-temperature susceptibility curves were also collected using an MPMS SQUID magnetometer (Quantum Design Inc.) at a field of 11.6 kG. The specific heat measurements down to 1.2 K were made on small mass (+5 mg) pieces of each sample using a time constant method described in detail elsewhere [32].

3. Results and discussion 3.1. Crystal chemistry Table 1 shows that the ordering of the Ce Pd M structure roughly doubles the unit cell  

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parameter over that of pure CePd . For the  M"B and Si cases, however, the Ce Pd M   structure could not be confirmed because of the absence of visible ordering peaks in the powder X-ray diffraction pattern. Although the ordering peaks, primarily caused by the ordered distortion of the Pd lattice, are expected to be weak for the smaller M elements, they are present for M"Al, Ga, and Ge. Therefore, without confirmation of the Ce Pd M structure, the M"B and Si   compounds will be referred to as CePd M    alloys with non-doubled unit cells. For these two compounds, it may be possible to have inequivalent Ce environments. Table 1 does show that the unit cell sizes increase as one moves down a column of the periodic table. It should be noted, however, that the metallic radius of Ga is actually slightly smaller than that of Al because of d-block contraction [33]. This helps explain why the M"Al and Ga compounds have such similar unit cell sizes. Attempts were also made to form the Ce Pd M   structure with M"Zn and Te, however, due to the high vapor pressure of these elements, the arc-melting synthesis could not be used. Sealed tube reactions with these metals and powdered CePd yielded products that did show the re quired ordering peaks and gave the following cell constants: a "8.39(1) As and a "8.44(1) As . 8 2 However, the nature of the reaction conditions left some question to the homogeneity and the exact stoichiometry of these samples. Thus extensive property measurements on these samples were not performed. For similar reasons, the group 15 compounds in our original papers [13,14] were not studied further here. The high vapor pressure of particularly Sb, and possibly Bi, in the arc-melting synthesis yielded compounds whose compositions were likely to vary. This was shown to affect the low-temperature magnetic properties of Ce Pd Sb [13]. The possibility   of a small variability in composition, even in a single sample of any of the Ce Pd M   compounds, cannot be ruled out by our diffraction, or even by chemical analysis, due to the low concentration of M.

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Fig. 2. The resistivity as a function of temperature for the group 13 (a) and group 14 (b) compounds. Note the different vertical scales for these two plots. The reference compounds CePd and  La Pd In are also included in (a). The relative resistivity ratios   of some of the compounds are also shown (c).

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3.2. Electrical resistivity Figs. 2a and 2b show the dramatic differences in the electrical properties of the group 13 and group 14 compounds. All the group 13 compounds show some feature that can be attributed to a Kondo-like behavior. The CePd B compound shows a con   stant increase in resistivity as the temperature decreases (from 300 to 4 K) which matches well with previously measured compounds in the CePd B  V series, and is attributed to Kondo interactions [28]. For M"Al there is a large region of negative do/dT from 300 K down to the peak at 14 K. The M"Ga resistivity is similar with a peak at 15 K; however, the region of negative do/dT only extends from 15 K up to approximately 200 K. These lowtemperature peaks for M"Al and Ga indicate strong Kondo interactions and the sharp drops in resistivity at temperatures less than the peak temperature suggest that the “coherence” effects seen in CePd [34] are still present here. For M"In  there is no area where do/dT is negative, however, the rapid drop in resistivity below 50 K is probably indicative of coherence in a Kondo system [13]. The room-temperature resistivity values of the group 13 compounds also follow a trend: increasing o(300 K) with decreasing unit cell size. The resistivity data for the reference compound La Pd In [13] and the parent compound CePd    [35] have also been included for comparison in Fig. 2a. As expected the La compound has a much lower o(290 K) value and shows simple metallic behavior. The resistivity of the CePd compound  shows a dramatic Kondo peak at 120 K and agrees well with previous measurements on this compound [10—12]. Although the group 14 compounds show a similar range in unit cell sizes as their group 13 counterparts, all the group 14 compounds in Fig. 2b show simpler metallic behavior. The roomtemperature resistivity values, however, do not follow the clean trend with unit cell size as observed with the group 13 compounds. The weak and broad roll-overs on cooling can be attributed to the thermal depopulation of the cerium 4f crystal field levels [13]. The clear decreases in the resistivities below 7 K are consistent with the magnetic ordering temperatures detailed later. The resistivity

curve for the CePd Si sample is similar to that    of the ordered Ce Pd Ge compound, but differs   slightly from that of the previously reported CePd Si compound [30]. The curve in Fig. 2b    has stronger temperature dependence and a o(290 K) value that is about three times smaller than that reported. These inconsistencies could arise from differing amounts of disorder in the samples due to slightly different stoichiometries and to different annealing conditions. Other sources of error, in either study, could also explain these differences: errors in determining accurate area factors, internal sample cracks and the poor placement of the four contacts (i.e. non-uniform current distribution between the voltage contacts). Overall, however, the resistivity data of all the Ce Pd M compounds offer clear evidence of the   major differences between the group 13 and group 14 compounds. To emphasize the differences, the scaled resistivities are shown in Fig. 2c. When scaled to the same value at 290 K the Group 14 compounds are all very similar and thus only the Ce Pd Ge compound is shown in Fig. 2c.   3.3. Thermopower (Seebeck coefficient) The thermopower (S) for CePd is known to be  much larger than a typical metal due to the intermediate valence nature of the cerium in this compound [36]. Fig. 3 shows the temperature

Fig. 3. The thermopower (Seebeck coefficient) of CePd and  several of the group 13 Ce Pd M compounds. The lines are   shown only as a guide to the eye.

C.D.W. Jones et al. / Physica B 262 (1999) 284—295 Table 2 Thermopower (Seebeck coefficient) data Compound

S at 297 K (lV/K)

CePd B    Ce Pd Al   Ce Pd Ga   Ce Pd In   CePd Si    Ce Pd Ge   Ce Pd Sn   Ce Pd Pb   CePd 

12.9 !5.7 !8.0 !21.8 !6.3 0.5 !2.9 8.5 86.5

S at 78 K (lV/K) 24.7 !0.1 !0.3 !6.5 !1.0 !0.1 !0.4 4.0 90.9

dependence of the thermopower of CePd , reaching  a maximum of approximately 115 lV/K at 120 K. This data agrees well with that previously reported [28,36—38], although the reported maximum ranges from approximately 100 lV/K [28,37,38] to 125 lV/K [36]. These differences are most likely due to small differences in annealing conditions and sample preparation. Also shown in Fig. 3 are the thermopower curves for some of the group 13 compounds (M"B, Al, In). The thermopower values for CePd B are a little larger than ex   pected for a typical metal and agree very well with previously published data for CePd B [28].    These values suggest that the interactions resulting in intermediate valence behavior in CePd have  not been fully eliminated in CePd B . The ther   mopower of Ce Pd Al, however, shows typical   metallic behavior. The somewhat large, negative values for Ce Pd In do not suggest intermediate   valence character, but instead probably result from simple band effects. Cerium intermediate valence compounds show thermopowers with positive sign at room temperature [38]. Listed in Table 2 are the thermopower values for all the Ce Pd M compounds at 297 and 78 K. The   Ce Pd Ga compound has thermopower values   similar to Ce Pd Al and all the group 14 com  pounds show typical metallic behavior as well. There is a clear trend in decreasing thermopower as a function of increasing unit cell size for the group 13 compounds. As with the resistivity, the M"Sn compound does not seem to fit the trend in the group 14 compounds. Overall, except for perhaps

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Table 3 Magnetic susceptibility data. The k and h values were obtained by a high-temperature fit (see range) of the data to the Curie—Weiss law (values for M"Ga, In, Sn and Pb are from Ref. [13]). The ¹ values were taken as the maximum in s unless , otherwise noted Compound

k (k )

h (K)

Fit range (K)

¹ (K) ,

CePd B    Ce Pd Al   Ce Pd Ga   Ce Pd In   CePd Si    Ce Pd Ge   Ce Pd Sn   Ce Pd Pb  

2.32(2) 2.42(2) 2.42(2) 2.43(2) 2.44(3) 2.46(3) 2.49(4) 2.48(3)

!33(2) !11(2) !13(2) !7(2) !9(3) !10(2) !11(3) !10(2)

80—300 100—335 100—320 100—310 100—320 100—300 100—320 80—310

— 3.8(2) 3.6(2), 3.1(2) 4.8(2), 2.5(2) 4.3(3) 5.3(3) 7(1) 6.3(5)

The temperature of maximum rate of change in s below ¹ . ,

the M"B compound, there is no evidence for intermediate valence behavior from the thermopower of these compounds. 3.4. Magnetic susceptibility The Curie—Weiss parameters from high-temperature fits of the susceptibility data are listed in Table 3. All show effective moments slightly reduced but consistent with the Ce> free ion value of 2.54k . Also listed in Table 3 are the Ne´el temperatures (¹ ) observed in the low-temperature , susceptibilities as seen in detail in Fig. 4a and b. The CePd B compound has the smallest mo   ment, largest h value, and does not show any magnetic ordering; consistent with data for previously reported CePd B compounds [17,19,20]. The low  V effective moment and exceptionally large h value suggests that this compound still exhibits some of the intermediate valence behavior of CePd .  The rest of the Ce Pd M compounds also show   negative but much smaller h values, consistent with the observed antiferromagnetic interactions. In all cases the "h" values are larger than the observed ¹ temperatures. However, the h values cannot be , interpreted as resulting from antiferromagnetic exchange alone, since they most certainly include a contribution from crystal field splitting energies. The small deviations in s from the Curie—Weiss law

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order or sample inhomogeneity (slight local variations in stoichiometry, defects or non-magnetic atom disorder). These issues will be explored further when looking at the low-temperature specific heat data. The group 14 compounds have sharp antiferromagnetic transitions, but again there is no simple correlation between unit cell size and the Ne´el temperatures. 3.5. Specific heat data

Fig. 4. The high-field (11.6 kG) low-temperature magnetic susceptibility as a function of temperature for the group 13 (a) and group 14 (b) compounds.

(below approximately 100 K) can also be attributed to a crystal field split 4f state. Figs. 4a and 4b show the low-temperature magnetic susceptibilities for the group 13 and group 14 compounds. All the compounds have approximately the same magnetic susceptibility at 15 K (0.022 emu/mol Ce) except for the distinctly low moment of the CePd B compound. The group    14 compounds all show classical antiferromagnetic ordering peaks while the M"Al, Ga, and In compounds show less sharp, more complex antiferromagnetic ordering transitions. As mentioned, the M"B compound shows no magnetic ordering. The data in Fig. 4a were collected on an MPMS SQUID magnetometer and supplement our previous work [13] using the Faraday balance. In particular, this data shows an antiferromagnetic transition for Ce Pd Ga and extends the range of   data for Ce Pd In to lower temperature. The   anomalous shapes of s for these group 13 antiferromagnetic transitions may result from possible Kondo interactions, as suggested by the low-temperature behavior of the electrical resistivities. Other possibilities include short-range magnetic

The specific heats (C) of selected group 13 and group 14 compounds are shown in various forms in Figs. 5—9. As expected, those compounds that showed antiferromagnetic ordering in the susceptibility also show a corresponding peak in the specific heat. Again, the group 13 and 14 specific heat features near ¹ differ in shape, but the transition , temperatures (as determined from the peaks in C, see Table 4) match well with features in the susceptibility. The specific heat of the La Pd In reference   compound (Fig. 5) is featureless, as expected, and is reasonably described over the entire temperature region in the usual way: C"c¹#b¹.

(1)

The value for the electronic specific heat (c) is small at 0.5(6) mJ/K mol Ce. The lattice contribution to the specific heat (b"0.69(1) mJ/K mol Ce)

Fig. 5. The specific heat of Ce Pd In and the reference com  pound La Pd In plotted as C/T versus T. The standard   C/¹"c#b¹ fit Eq. (1) is shown, although the relevance of the fit for the cerium compound is discussed in the text.

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Fig. 6. The specific heat (䢇), electronic specific heat (;) and electronic entropy (scaled by the gas constant R) are shown as a function of temperature for the group 13 compounds.

Fig. 7. The specific heat (䢇), electronic specific heat (;) and electronic entropy (scaled by the gas constant R) are shown as a function of temperature for the group 14 compounds.

yields a Debye temperature (h ) of 227(1) K. This " h agrees reasonably well with the reported value of " 241 K for LaPd B [22].  As a first approximation, the specific heat data above the magnetic transitions for the cerium compounds could also be fit to Eq. (1). An example is shown graphically for Ce Pd In in Fig. 5 (c"   230(4) mJ/K mol Ce, b"0.63(2) mJ/K mol Ce, h "234(3) K). If the extrapolated c value is large " (greater than, say, 400 mJ/K mol Ce [1]) then this may be an indication of heavy fermion behavior. The c value for the intermediate valence compound CePd has been measured to be only 37(2)  mJ/K mol Ce [39,40]. However, it is important to emphasize that this simple expression, Eq. (1), does not adequately model all specific heat data and can often lead to indications of false heavy fermion behavior [41]. This is especially true when the

Fig. 8. The electronic specific heat divided by temperature (C /¹) shown as a function of T for the group 13 compounds.  Both the high-temperature and low-temperature behavior are shown. Note the different vertical scales for the two low-temperature plots.

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mol Ce. However, the specific heat data for the M"B, Al and Ga compounds fit Eq. (1) with anomalous values for b. In order to explore both the possibility of heavy fermion behavior, and the nature of the antiferromagnetic transitions at low temperatures, the electronic (and magnetic) components of the specific heat (C ) were calculated (Figs. 6 and 7). This was  accomplished by subtracting out the lattice contribution from the La reference compound using the following formula:

Fig. 9. The electronic specific heat divided by temperature (C /¹) shown as a function of ¹ for the group 14 compounds.  Both the high-temperature and low-temperature behavior are shown.

Table 4 Specific heat data. The ¹ values were taken as the temperatures , of the peaks in the specific heat. The c values are the C /¹ &  values at 10 K Compound

¹ (K) ,

c (mJ/K mol Ce) &

CePd B    Ce Pd Al   Ce Pd Ga   Ce Pd In   CePd Si    Ce Pd Ge   Ce Pd Sn   Ce Pd Pb  

— 3.8(2) 3.1(2) 2.6(1), 3.9(2) 4.2(2) 5.2(1) 5.7(2) 5.8(2)

275 350 350 215 130 145 130 140

The c values are changing rapidly near 10 K (see Figs. 7 and 8). &

compounds either (1) order magnetically at low temperature and have low-lying crystal field levels or (2) exhibit a variable composition range that permits non-magnetic atom disorder (NMAD) [41]. As will be discussed, both of these conditions may apply for these compounds and thus the interpretation of the specific heat data is difficult. Nevertheless, for the group 14 compounds (M"Si, Ge, Sn, Pb) the specific heat data between 10 and 19 K fits Eq. (1) reasonably, with b values yielding Debye temperatures of 210—240 K. The corresponding c values are high, ranging from 100 to 140 mJ/K

C "*C"C!b   ¹. (2)  * . ' It has been assumed that the lattice contribution for the La Pd In sample will be representative of   all the Ce Pd M compounds. This will introduce   some small amount of error, especially in the higher temperature ('10 K) values of C , due to the ex pected small differences in h between compounds. " The entropy associated with C can also be cal culated:



2 C /¹ d¹. (3)  2 The dimensionless ratio *S/R (where R is the gas constant, 8.314 J/K mol) is shown as a function of temperature in Figs. 6 and 7 (¹ is taken as the  lowest temperature measured, approximately 1.2 K).

*S"

3.5.1. Specific heat of the magnetic transitions As shown by the C versus T plots, all the group  14 compounds (Fig. 7) have strong, sharp, j-like antiferromagnetic peaks; the peak temperatures correspond well to those found in the magnetic susceptibility data. Applying mean field theory to an S" doublet ground state spin system, one  would expect a C discontinuity of 3R/2 (12.5 J/K  mol Ce) at the magnetic transition. The height of the specific heat peak, for each of the group 14 compounds, agrees reasonably well with this value. The entropy increases up to ¹ are also between , 60% and 75% of R ln2, the expected mean field value for a cerium 4f electron in a doublet ground state. The slower increase above ¹ to reach the , value of R ln2 is ascribed largely to magnetic fluctuations.

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As stated previously, the M"B compound is unique and does not show any antiferromagnetic transition (Fig. 6). The values of C between 1.1 and 10 K are very similar to the previously published data for CePd B [20,22], and the *S of this    system increases smoothly, only reaching R ln2 near 20 K. However, since C /¹ is increasing as  T is lowered (Fig. 8), there is a contribution to *S from below 1.1 K that is not included in Fig. 6 (this is unique for the M"B case). In fact, other work on CePd B compounds has shown that there are  V low-temperature features ((1 K for x"0.19) that do contribute to *S [22]. These features have most recently been attributed to spin glass behavior resulting from non-magnetic atom disorder (NMAD) [41]. One might have expected NMAD effects in the M"Si compound as well, where the structural ordering was not confirmed. However, nothing of this nature is seen in the specific heat of CePd Si in the temperature range investigated.    While a detailed description of the electronic state of CePd B is not available, it seems likely that    the properties differ from those of the other group 13 materials because of NMAD. The other group 13 compounds do show antiferromagnetic ordering (Fig. 6). For M"Al and Ga the antiferromagnetic peaks are weak and broad, not reaching half the predicted mean field value of 12.5 J/K mol Ce. Furthermore, C maintains a re latively high and constant value at temperatures just above the transition. Correspondingly, the entropy increase up to ¹ does not climb as sharp, ly as those for the group 14 compounds, and are only 45% and 30% of R ln2 respectively by ¹ . For , M"In there is a sharp peak in C at the same  temperature as the sharp drop in s, suggesting that this feature is antiferromagnetic in origin. However, as shown in Fig. 8, the temperature dependence below the transition is much different from the other compounds. There is also a second feature in the specific heat, a broad peak centered at around 4 K, which may complicate the behavior below the transition. As noted in the magnetic susceptibility, 4.8(2) K is the temperature where s first starts to decrease. The *S value has only reached 18% of R ln2 by 2.8 K and only 50% of R ln2 by 3.9 K. There are several possible explanations for the added feature in the M"In data. Because of the

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ordered nature of the M atoms in Ce Pd In,   NMAD should not be an issue in this compound. However, partial disorder or defects in this particular sample may affect the data. Also, because there is magnetic ordering in the M"In compound, short-range magnetic order before the onset of true antiferromagnetic order is also a strong possibility for this broad peak. There is also the possibility that the peak is a result of Kondo interactions. Although the physical origin of this shoulder to the M"In data is not yet clear, it is conceivable that a similar but broader peak, centered at perhaps a slightly higher temperature, is also responsible for the roughly constant C value in the M"Al and  Ga compounds just above ¹ . This might also help , explain the broad transitions seen in the magnetic susceptibility. Further experiments studying the field dependence of the magnetic susceptibility might help resolve the nature of these features that are present in the group 13 compounds but absent in the group 14 compounds. 3.5.2. Specific heat above the magnetic transitions As previously mentioned, extracting reliable c values from the data above the magnetic transitions is difficult due to the many possible contributions to the C term. The antiferromagnetic interactions  produce fluctuations that extend above ¹ , and , defects or partial disorder may produce magnetic contributions to C . As shown in Figs. 6 and 7, the  C curves have a somewhat unusual shape, increas ing from around 10 to 15 K then leveling off, even rolling over, by 20 K. The shape of the C curves is  particularly sensitive to the subtraction of the phonon contribution. It is important to remember that we have assumed that the phonon contribution is identical for each sample and that b remains constant over the 10—20 K temperature range. When plotted as C /¹ versus ¹ in Figs. 8 and 9,  the curves are linear with substantial negative slopes for M"B, Al, and Ga but near zero slope for M"In, Si, Ge, Sn, and Pb. This is to be expected from the initial agreement to Eq. (1). The C /¹ values at 10 K (c ) are listed in Table 4 and  & are higher for the group 13 compounds. An additional factor that could contribute to the enhanced C values, and possibly their un usual temperature dependence, are Schottky-type

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anomalies from the crystal field split 4f state. With the limited temperature range of data collected, the nature of the crystal field contribution in these compounds cannot be determined. It does seem reasonable, however, from the integrated entropies of the magnetic transitions, that the ground state is in fact a doublet. Previous work on the specific heat of CePd Si has shown a Schottky-type anomaly,    resulting from low-energy crystal field excitations, centered at 25 K [24,25]. The anomaly can be fit well to the expected C of a ground state doublet  with a quartet excited state (C —C ) using an energy   gap of 66 K [24]. The C of this C —C excitation    has the predicted maximum of 6.31 J/K mol Ce. In the Ce Pd M compounds, however, the magni  tudes of C in the 10—20 K region are too small  to be due to C —C excitations with a crystal   field splitting )70 K. The magnitude of the “rollovers” in C are (in most cases) also too weak to be  doublet—doublet excitations which have a predicted C maximum of 3.64 J/K mol Ce. Therefore, it is  unlikely that there is a large contribution to C at  &10 K from normal crystal field contributions. In previous work on CePd B a Schottky   type anomaly is also observed, but with a smaller maximum (C (max.)+(4.5 J/K mol Ce) [24,25].  To explain this reduced maximum, a recent report using inelastic neutron scattering suggests that the ground state of CePd B is actually the quartet    state [27]. The observed Schottky-type anomaly may then fit a superposition of a C —C transition   (which has a C (max.)"2.00 J/K mol Ce) and  a moderately large cT term [27]. For the CePd B  V compounds, there is no antiferromagnetic ordering from which one might infer a doublet ground state from the magnitude of the entropy change. However, if a doublet ground state is still assumed, the reduced Schottky-type anomaly can still be explained. It is known that if the crystal field splitting energy is comparable to an associated Kondo energy, the Schottky anomaly and a Kondo peak can merge to produce a broader combined peak [42,43]. The maximum of this combined peak can be smaller than the standard maximum C  values. This is an alternate explanation to the lower intensity Schottky-type anomaly observed for these CePd B compounds [42], and could also explain  V the high C values for the Ce Pd M compounds.   

However, further experimental support for this hypothesis is necessary before one can be sure. In summary, Figs. 6 and 7 indeed show that the C values above the magnetic transitions are much  larger than what would be expected from the cT contribution of a normal metal. It is apparent, however, that no unique interpretation of the large C is possible and therefore we cannot determine if  these compounds have heavy fermion character by the simple extrapolation of a high-temperature c value.

4. Conclusion The Ce Pd M compounds, with an ordered   structure derived from CePd , show enhanced elec tronic specific heat (C ) above an antiferromag netic transition. This may indicate heavy fermion behavior rather than the intermediate valence behavior of CePd . However, other reasons why  this C term might be larger include: low-lying  crystal field levels, short-range magnetic fluctuations, or magnetic disorder induced by partial non-magnetic atom disorder. The C terms for the  group 13 compounds are larger than the corresponding values for the group 14 compounds. There are also significant differences in the lowtemperature magnetic transitions (seen in magnetic susceptibility and in specific heat) and in the electronic resistivity between the group 13 and 14 compounds. Taken together, this data suggests that the group 13 compounds have Kondo interactions that are absent in the group 14 compounds even thought both sets of compounds span the same range of unit cell sizes (or Ce—Ce distances). This suggests that the chemical nature of M greatly affects the properties of Ce Pd M. Overall, how  ever, the properties of these compounds do not show the intermediate valence behavior found in the parent compound CePd .  Acknowledgements We thank R.C. Haushalter and G.R. Kowach for the use of and training on the MPMS SQUID magnetometer at the NEC Research Institute and

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Y. Ijiri for the resistivity data on CePd . This  work was supported by the Office of Naval Research and by the Natural Sciences and Engineering Council of Canada for a fellowship to C.D.W. Jones.

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