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Searching for new heavy fermion materials in cerium intermetallic compounds
JournaE of the Less-Common
Metals, 149 (1989)
SEARCHING FOR NEW HEAVY FERMION INTERMETALLIC COMPOUNDS* J. TANG and K. A. GSCHNEIDNER,
341
341 - 347
MATERIALS
IN CERIUM
JR.
Ames Laboratory, Departments of Physics and Materials Science and Engineering, Iowa State University, Ames, IA 50011 (U.S.A.) (Received June 10,1988)
Summary We report the results of the heat capacity and magnetic susceptibility measurements on several cerium inter-metallic compounds. No heavy fermion behavior was found in these compounds. CePtGas has a broad anomaly in its heat capacity near 2 K and whose origin is not well understood. CeCdii has an unusual temperature dependence of its heat capacity which is due to a crystalline electric field effect which occurs at an exceptionally low temperature. CeCdz and CeCds are magnetically ordered at low temperatures. The low-temperat~e behavior of these and other cerium in~~et~lic compounds are discussed.
1. Introduction Most of the heavy fermion materials are found in cerium and actinide compounds in which hybridization of f electrons with conduction electrons of neighboring non-f-electron atoms is thought to play an important role. Examples of these are CeCu&, * CeAls, CeCu,, UPts, U,Zn,, and UCd,,. In compounds that have moderately large Ce-Ce, or U-U, separations the f-electrons are essentially localized [l], provided the hybridization with conduction electrons of the neighboring atoms is negligible. If that is not the case, depending on the strength of hybridization the f electrons will form a band and overlap the Fermi energy, and a large density of states at the Fermi energy can develop ]2], which is considered to be one of the hallmarks of the heavy fermion state. It is of interest to study the behavior of those compounds that have similar Ce-Ce spacing as those of the already discovered heavy fermions. It is for this reason that we have studied the lowtemperature properties of the following cerium intermetallic compounds.
*Paper presented at the 18th Rare Earth Research Conference, September 12 - 16,1988. 0022-5088/89/$3.50
@ Elsevier Sequoia/Printed
Lake Geneva, WI,
in The Netherlands
342
2. Experimental
details
Samples of CeCdz, CeCds, Ce&dss, CeCd,, CeCdll, LaCd,, and CezZn,-, were prepared by melting stoichiometric amounts of the constituents in sealed tantalum crucibles in a helium atmosphere using a resistance furnace. Samples of CePt,, CeGaz, LaGaz, CePtGaa and LaPtGa, were prepared by arc melting. The samples were heat treated to obtain homogenneous single-phase alloys. Metallography results confirmed that the samples were single phase. X-ray patterns of these compounds confirmed the previously reported crystal structures. The crystal structures of these compounds and their lattice parameters are listed in Table 1. Low-temperature (1.3 to 70 K) heat capacity measurements were carried out on these compounds using an adiabatic heat pulse type calorimeter [3]. Magnetic susceptibility measurements were made from 1.3 to - 298 K using a Faraday magnetometer [4]. 3. Results and discussion 3.1. CePtGa3 Figure 1 shows the heat capacities of CePtGa, as a function of temperature under different magnetic fields. The heat capacity at zero field starts to increase with decreasing temperature when the temperature passes through 7 K, and then it reaches a maximum at - 1.8 K. The maximum shifts to a higher temperature if a magnetic field is applied. This suggests that the maximum is due to a magnetic phase change. The significant feature for this compound is that the maximum in zero-field heat capacity is not a sharp but a broad one, but for an ordinary magnetic phase transition a sharp peak is normally found. This implies the existence of an unusual magnetic phase transition near 1.8 K. One possible explanation for this rounded peak is that the system undergoes a spin-glass transition since a rounded peak is one of the characteristics of a spin-glass system. The a.c. magnetic susceptibility measurement on CePtGas was also conducted from 1.3 to 20 K. The absence of a peak between 1.3 K and 1.8 K in a.c. susceptibility (Fig. 2) suggests that measurement below 1.3 K might be necessary before we can draw the conclusion that spin-glass ordering is responsible for the anomaly in heat capacity. Surprisingly, a peak was found in a.c. susceptibility near 8 K (Fig. 2). On the contrary, there is no indication of a peak near 8 K in heat capacity. This feature is not well understood yet. More detailed studies on CePtGas are being carried out. The electronic specific-heat coefficient and the Debye temperature of this compound were determined from the linear region of the C/T versus T2 plot from 10 to 20 K and are listed in Table 1. 3.2. The crystalline electric field in CeCdI1 and CeGa2 The low-temperature heat capacity of CeCd,i is quite unusual. It has a nearly constant heat capacity of -3 J mol-’ K-’ from 15 to 30 K, and rises
6.113
4.383 13.28 4.320 10.513
15.57
3.448
(4
th
5.075 7.223 15.77 15.732 9.319 9.334 5.368 9.071 4.321 6.100
C
a
aIn units of mJ (mol Ce atom)-’ KM2. bSimilar results have been reported by others. ‘Data estimated from LaCdii and LaGaz. “After ref. 12.
CeGa2 CePtGas
hR19 hP3 oF20
hP3 cF16 hP142 ~I168 cP36 cP3 6 hP6
CeCdz CeCd:, Ce&d% CeCds CeCd rr LaCdrr CePts
Ce2Zn17
structure
Compound
Selected properties of cerium compounds
TABLE 1
4.38 4.43 4.32 4.31
5.11 4.37 5.83 6.59
3.44
dce-ce (‘f9 @D WI
173 150 146 138 - 28OC 280 200 253 - 326c 197
7"
12.0 49.4 23.1 51.6 26 17.2 15.0 15.0 32.6 37.8
(K)
-56b -52b -12 -9.5 -7.8 -34d
@B)
2.65b 2.60b 2.60 2.53 2.57 2.45d 2.91b 2.85 2.89
-1.7b +1.8 -14
8,
Peff
1.0 (N) 1.7 (N) -10 (C)
-20(N) 2.0(N)
W)
Tc> TN
344 I
0
I CePtGa,
I
10 TEMPERATURE
5
15
20
(K)
Fig. 1. Heat capacities of CePtGas under fields of H = 0, 5.3 and 9.8 T.
4r4 3-
4
:+ +#
2
+
$
44++ +A
0
CePtGo,
a +*
*
5
++ $*,
+ +
IO TEMPERATURE
15
20
(K)
Fig. 2. a.c. susceptibility of CePtGaj (H = 1 Oe, f= 100 Hz).
to a maximum of -3.8 J mol-1 K-’ at -7 K before it drops rapidly to about 0.4 J mol-’ K-’ at -2 K. An analysis of the heat capacity as a function of temperature indicated that this unusual temperature dependence of the heat capacity was due to crystalline electric field (CEF) splitting of the six-fold degenerate ground state of the Ce3+ ion, 2Fg,2, into three doublets [5]. Although CeCd 11 is cubic, the Ce3+ ion sits on a site of tetragonal symmetry which accounts for the three-doublet splitting. After subtraction of heat capacity of isostructural LaCd, 1 ‘to approximate the electronic and lattice contributions to the heat capacity the resultant data were fitted to a three-level CEF scheme with energies of 0, 17.5 and 80.2 K. The small energy between the CEF ground state and first excited state, 17.5 K, is one of the lowest CEF splittings found in cerium compounds. Full details are being published elsewhere [ 51. CeGa2 has been studied by several research groups [6 - 91. The study on single crystalline CeGa2 indicates [ 91 that this material undergoes a series of antiferromagnetic phase transitions between 8.4 and 11.4 K before it finally reaches a ferromagnetic ground state at 8.4 K. The heat capacity C
345
0
200
100
300
400
500
700
600
T2(K2)
Fig. 3. CIT vs. T2 of CceGa,tCCEpt 2
1
-
%aGa,h
1.5 *
++ +
L 5 s
C CEF
CeGop
5 % 9
(CC~G~,-
I
I
z Y p
CbGa,and
l-
+ + + + +
0.5 -
0
0
I 5
+ ++
+++ ++ I 15 10 TEMPERATURE (K)
Fig. 4. Low-temperature
+++ 20
magnetic moment of CeGaz.
in the C/T versus T2 plot of our polycrystalline CeGaz sample (Fig. 3) shows one large combined peak at -10 K, which is the result of these conjunctive magnetic phase transitions. Clear indication of a ground state which is ferromagnetic in nature can be seen from the saturation behavior in the lowtemperature magnetic moment M versus T plot (Fig. 4). The Curie temperature and the effective magnetic moment determined from the Curie-Weisstype susceptibility of our sample are given in Table 1. Above the peak temperature, the C/T versus T2 curve follows nearly a straight line. The extrapolation of the straight line to 0 K is moderately large (-340 mJ mol-’ K-l), which might suggest heavy fermion behavior. The origin of this moderately large heat capacity is now believed to be due to the CEF effect in CeGaz. According to the inelastic neutron scattering experiment on CeGa, by Burlet et at. [S], the CEF splits the six-fold degenerate multiplet of Ce3+ into three doublets with energies of 0, 62.5 and 310 K. The contribution to the heat capacity from this three-level CEF system is significant and is shown in Fig. 3. As can be seen, after subtraction of the heat capacity of the CEF and that of isostructural LaGa from the
346
heat capacity of CeGa, the difference is only a magnetic peak, in C/T values, from 0 to about 20 K (i.e. about 400 K*). This suggests that the moderately large heat capacity is due to the CEF effect in CeGa2. The electronic specific heat coefficient of CeGa, is 32.6 mJ mol-’ K-*, which was estimated by adding the C/T value of CCeGa,- CCEF- CLaGa,, which is a constant from 25 to 40 K (500 to 1600 K*; only part of this range is shown in Fig. 3), to the electronic specific heat coefficient of LaGa*. 3.3. Other cerium compounds Six other cerium compounds were studied. Four of them order magnetically (antiferromagnets) CeCd, at 20 K, CeCda at 2.0 K, CePt, at 1.0 K and Ce2Zn,, at 1.7 K, and the two other CeCd, and CelsCdss do not order above 1.3 K, although their heat capacities suggest that they may order below 1.3 K. Two peaks were found in the heat capacity of CeCd, at T1 = 18.5 K and T2 = 22 K. Magnetic susceptibility data show only one peak at -20 K. Above the ordering temperature, it follows a Curie-Weiss behavior with ep = -56 K and peff = 2.65 cc,. These data are listed in Table 1 together with those of other cerium compounds. Further studies are needed to determine the nature of these two magnetic transitions. CeCds is an antiferromagnetic material, ordering at TN = 2.0 K. The peaks in heat capacity and susceptibility were obvious. Some results on Ce2 Zn,, and CePt, have been reported by Olivier et al. [lo] and Schroder et al. [ll], respectively. The antiferromagnetic phase transition in Ce2Zni7 at TN = 1.7 K is confirmed by a peak in our heat capacity us. temperature curve. CePts is reported to have an antiferromagnetic phase transition at TN = 1.0 K [ 111. The tendency towards magnetic ordering is clearly seen from the upturn in our heat capacity data at -4 K. Because of the low-temperature limit of our apparatus we are unable to make measurements below 1.3 K. The heat capacities show that there is no phase transition above 1.3 K in CeCd6 and Ce13Cd5s. In common with CePt,, however, both compounds show tendencies towards magnetic ordering at a lower temperature as indicated by the upturns in their heat capacities at -3 K. The paramagnetic Curie temperatures (Table 1) of the two compounds are negative, inferring that the orderings are antiferromagnetic in nature.
4. Conclusions It was suggested by Meisner et al. [2] that the correlation between heavy fermion materials and Ce-Ce spacing in the lattice is important. For a sufficiently short Ce-Ce spacing, the overlap of neighboring f wavefunctions leads to the formation of a broad f band which favors a non-magnetic ground state. For a large Ce-Ce spacing, the f electrons are essentially localized and hence the magnetically ordered state occurs. An appropriate
347
hybridization of the f electrons with band electrons on neighboring non-felectron atoms gives rise to an itinerant hybridized narrow f band even when no direct overlap between the f electrons is present. This happens when the Ce-Ce spacing is moderate (just above the Hill limit = 3.41 8) [ 11, and the heavy fermion behavior develops in this regime. The electronic specific heat coefficients y, and the Ce-Ce spacings dCewCeof the compounds discussed above, are listed in Table 1. As can be seen, most of the compounds fall into the intermediate Ce-Ce spacing regime, while none of them shows heavy fermion behavior. This means that a favorable Ce-Ce spacing for heavy fermion behavior is a necessary condition but not a sufficient one. In summary, we report the low-temperat~e heat capacity and magnetic susceptibility data of a series of cerium intermetallic compounds. No heavy fermion behavior is found. CEF plays an important role in the heat capacities of CeGaz and CeCd,,. To our knowledge, the antiferromagnetic phase transitions in CeCd, and CeCds are reported for the first time. The origin of the interesting behavior of CePtGas needs to be determined.
Acknowledgments The authors would like to thank B. J. Beaudry, N. M. Beymer and K. Funke for their assistance in preparing the samples. This work was supported by USDOE, Office of Basic Energy Sciences, Division of Materials Sciences, under Contract No. W-7405-ENG-82.
References 1 H. H. Hill, in W. N. Miner (Ed.), Plutonium 1970, The Metallurgical Society of the AIME, New York, 1970, p. 2. 2 G. P. Meisner, A. L. Giorgi, A. C. Lawson, G. R. Stewart, J. 0. Willis, M. S. Wire and J. L. Smith, Phys. Reu. Left., 53 (1984) 1829. 3 K. Ikeda, K. A. Gschneidner, Jr., B. J. Beaudry and U. Atzmony, Phys. Rev. B, 25 (1982) 4604. 4 R. J. Stierman, K. A. Gschneidner, Jr, T.-W. E. Tsang, F. A. Schmidt, P. Klavins, R. N. Shelton, J. Queen and S. Legvold, J. Magn. Magn. Muter., 36 (1983) 249. 5 J. Tang and K. A. Gschneidner, Jr., J. Magn. Magn. Mater., 75 (1988) 355. 6 T. H. Tsai, J. A. Gerber, J. W. Weymouth and D. J. Sellmyer, J. Appl. Phys., 49 (1978) 1507. 7 W. H. Dijkman, Thesis, University of Amsterdam, 1982. 8 P. Burlet, M. A. Fremy, D. Gignoux, G. Lapertot, S. Quezel, L. P. Regnault, J. RossatMignod and E. Roudaut, J. Magn. Magn. Mater., 63 & 64 (1987) 34. 9 M. Takahashi, H. Tanaka, T. Satoh, M. Kohgi, Y. Ishikawa, T. Miura and H. Takei, J. Phys. Sot. Jpn., 57 (1988) 1377. 10 M. Olivier, T. Siegrist and S. P. McAlister, J. Magn. Magn. Mater., 66 (1987) 281. 11 A. Schroder, R. van den Berg and H. v. Lohneysen, Solid State Commun., 65 (1988) 99. 12 K. H. J. Buschow, Rep. Prog. Phys., 42 f19’79) 1641.
Metals, 149 (1989)
SEARCHING FOR NEW HEAVY FERMION INTERMETALLIC COMPOUNDS* J. TANG and K. A. GSCHNEIDNER,
341
341 - 347
MATERIALS
IN CERIUM
JR.
Ames Laboratory, Departments of Physics and Materials Science and Engineering, Iowa State University, Ames, IA 50011 (U.S.A.) (Received June 10,1988)
Summary We report the results of the heat capacity and magnetic susceptibility measurements on several cerium inter-metallic compounds. No heavy fermion behavior was found in these compounds. CePtGas has a broad anomaly in its heat capacity near 2 K and whose origin is not well understood. CeCdii has an unusual temperature dependence of its heat capacity which is due to a crystalline electric field effect which occurs at an exceptionally low temperature. CeCdz and CeCds are magnetically ordered at low temperatures. The low-temperat~e behavior of these and other cerium in~~et~lic compounds are discussed.
1. Introduction Most of the heavy fermion materials are found in cerium and actinide compounds in which hybridization of f electrons with conduction electrons of neighboring non-f-electron atoms is thought to play an important role. Examples of these are CeCu&, * CeAls, CeCu,, UPts, U,Zn,, and UCd,,. In compounds that have moderately large Ce-Ce, or U-U, separations the f-electrons are essentially localized [l], provided the hybridization with conduction electrons of the neighboring atoms is negligible. If that is not the case, depending on the strength of hybridization the f electrons will form a band and overlap the Fermi energy, and a large density of states at the Fermi energy can develop ]2], which is considered to be one of the hallmarks of the heavy fermion state. It is of interest to study the behavior of those compounds that have similar Ce-Ce spacing as those of the already discovered heavy fermions. It is for this reason that we have studied the lowtemperature properties of the following cerium intermetallic compounds.
*Paper presented at the 18th Rare Earth Research Conference, September 12 - 16,1988. 0022-5088/89/$3.50
@ Elsevier Sequoia/Printed
Lake Geneva, WI,
in The Netherlands
342
2. Experimental
details
Samples of CeCdz, CeCds, Ce&dss, CeCd,, CeCdll, LaCd,, and CezZn,-, were prepared by melting stoichiometric amounts of the constituents in sealed tantalum crucibles in a helium atmosphere using a resistance furnace. Samples of CePt,, CeGaz, LaGaz, CePtGaa and LaPtGa, were prepared by arc melting. The samples were heat treated to obtain homogenneous single-phase alloys. Metallography results confirmed that the samples were single phase. X-ray patterns of these compounds confirmed the previously reported crystal structures. The crystal structures of these compounds and their lattice parameters are listed in Table 1. Low-temperature (1.3 to 70 K) heat capacity measurements were carried out on these compounds using an adiabatic heat pulse type calorimeter [3]. Magnetic susceptibility measurements were made from 1.3 to - 298 K using a Faraday magnetometer [4]. 3. Results and discussion 3.1. CePtGa3 Figure 1 shows the heat capacities of CePtGa, as a function of temperature under different magnetic fields. The heat capacity at zero field starts to increase with decreasing temperature when the temperature passes through 7 K, and then it reaches a maximum at - 1.8 K. The maximum shifts to a higher temperature if a magnetic field is applied. This suggests that the maximum is due to a magnetic phase change. The significant feature for this compound is that the maximum in zero-field heat capacity is not a sharp but a broad one, but for an ordinary magnetic phase transition a sharp peak is normally found. This implies the existence of an unusual magnetic phase transition near 1.8 K. One possible explanation for this rounded peak is that the system undergoes a spin-glass transition since a rounded peak is one of the characteristics of a spin-glass system. The a.c. magnetic susceptibility measurement on CePtGas was also conducted from 1.3 to 20 K. The absence of a peak between 1.3 K and 1.8 K in a.c. susceptibility (Fig. 2) suggests that measurement below 1.3 K might be necessary before we can draw the conclusion that spin-glass ordering is responsible for the anomaly in heat capacity. Surprisingly, a peak was found in a.c. susceptibility near 8 K (Fig. 2). On the contrary, there is no indication of a peak near 8 K in heat capacity. This feature is not well understood yet. More detailed studies on CePtGas are being carried out. The electronic specific-heat coefficient and the Debye temperature of this compound were determined from the linear region of the C/T versus T2 plot from 10 to 20 K and are listed in Table 1. 3.2. The crystalline electric field in CeCdI1 and CeGa2 The low-temperature heat capacity of CeCd,i is quite unusual. It has a nearly constant heat capacity of -3 J mol-’ K-’ from 15 to 30 K, and rises
6.113
4.383 13.28 4.320 10.513
15.57
3.448
(4
th
5.075 7.223 15.77 15.732 9.319 9.334 5.368 9.071 4.321 6.100
C
a
aIn units of mJ (mol Ce atom)-’ KM2. bSimilar results have been reported by others. ‘Data estimated from LaCdii and LaGaz. “After ref. 12.
CeGa2 CePtGas
hR19 hP3 oF20
hP3 cF16 hP142 ~I168 cP36 cP3 6 hP6
CeCdz CeCd:, Ce&d% CeCds CeCd rr LaCdrr CePts
Ce2Zn17
structure
Compound
Selected properties of cerium compounds
TABLE 1
4.38 4.43 4.32 4.31
5.11 4.37 5.83 6.59
3.44
dce-ce (‘f9 @D WI
173 150 146 138 - 28OC 280 200 253 - 326c 197
7"
12.0 49.4 23.1 51.6 26 17.2 15.0 15.0 32.6 37.8
(K)
-56b -52b -12 -9.5 -7.8 -34d
@B)
2.65b 2.60b 2.60 2.53 2.57 2.45d 2.91b 2.85 2.89
-1.7b +1.8 -14
8,
Peff
1.0 (N) 1.7 (N) -10 (C)
-20(N) 2.0(N)
W)
Tc> TN
344 I
0
I CePtGa,
I
10 TEMPERATURE
5
15
20
(K)
Fig. 1. Heat capacities of CePtGas under fields of H = 0, 5.3 and 9.8 T.
4r4 3-
4
:+ +#
2
+
$
44++ +A
0
CePtGo,
a +*
*
5
++ $*,
+ +
IO TEMPERATURE
15
20
(K)
Fig. 2. a.c. susceptibility of CePtGaj (H = 1 Oe, f= 100 Hz).
to a maximum of -3.8 J mol-1 K-’ at -7 K before it drops rapidly to about 0.4 J mol-’ K-’ at -2 K. An analysis of the heat capacity as a function of temperature indicated that this unusual temperature dependence of the heat capacity was due to crystalline electric field (CEF) splitting of the six-fold degenerate ground state of the Ce3+ ion, 2Fg,2, into three doublets [5]. Although CeCd 11 is cubic, the Ce3+ ion sits on a site of tetragonal symmetry which accounts for the three-doublet splitting. After subtraction of heat capacity of isostructural LaCd, 1 ‘to approximate the electronic and lattice contributions to the heat capacity the resultant data were fitted to a three-level CEF scheme with energies of 0, 17.5 and 80.2 K. The small energy between the CEF ground state and first excited state, 17.5 K, is one of the lowest CEF splittings found in cerium compounds. Full details are being published elsewhere [ 51. CeGa2 has been studied by several research groups [6 - 91. The study on single crystalline CeGa2 indicates [ 91 that this material undergoes a series of antiferromagnetic phase transitions between 8.4 and 11.4 K before it finally reaches a ferromagnetic ground state at 8.4 K. The heat capacity C
345
0
200
100
300
400
500
700
600
T2(K2)
Fig. 3. CIT vs. T2 of CceGa,tCCEpt 2
1
-
%aGa,h
1.5 *
++ +
L 5 s
C CEF
CeGop
5 % 9
(CC~G~,-
I
I
z Y p
CbGa,and
l-
+ + + + +
0.5 -
0
0
I 5
+ ++
+++ ++ I 15 10 TEMPERATURE (K)
Fig. 4. Low-temperature
+++ 20
magnetic moment of CeGaz.
in the C/T versus T2 plot of our polycrystalline CeGaz sample (Fig. 3) shows one large combined peak at -10 K, which is the result of these conjunctive magnetic phase transitions. Clear indication of a ground state which is ferromagnetic in nature can be seen from the saturation behavior in the lowtemperature magnetic moment M versus T plot (Fig. 4). The Curie temperature and the effective magnetic moment determined from the Curie-Weisstype susceptibility of our sample are given in Table 1. Above the peak temperature, the C/T versus T2 curve follows nearly a straight line. The extrapolation of the straight line to 0 K is moderately large (-340 mJ mol-’ K-l), which might suggest heavy fermion behavior. The origin of this moderately large heat capacity is now believed to be due to the CEF effect in CeGaz. According to the inelastic neutron scattering experiment on CeGa, by Burlet et at. [S], the CEF splits the six-fold degenerate multiplet of Ce3+ into three doublets with energies of 0, 62.5 and 310 K. The contribution to the heat capacity from this three-level CEF system is significant and is shown in Fig. 3. As can be seen, after subtraction of the heat capacity of the CEF and that of isostructural LaGa from the
346
heat capacity of CeGa, the difference is only a magnetic peak, in C/T values, from 0 to about 20 K (i.e. about 400 K*). This suggests that the moderately large heat capacity is due to the CEF effect in CeGa2. The electronic specific heat coefficient of CeGa, is 32.6 mJ mol-’ K-*, which was estimated by adding the C/T value of CCeGa,- CCEF- CLaGa,, which is a constant from 25 to 40 K (500 to 1600 K*; only part of this range is shown in Fig. 3), to the electronic specific heat coefficient of LaGa*. 3.3. Other cerium compounds Six other cerium compounds were studied. Four of them order magnetically (antiferromagnets) CeCd, at 20 K, CeCda at 2.0 K, CePt, at 1.0 K and Ce2Zn,, at 1.7 K, and the two other CeCd, and CelsCdss do not order above 1.3 K, although their heat capacities suggest that they may order below 1.3 K. Two peaks were found in the heat capacity of CeCd, at T1 = 18.5 K and T2 = 22 K. Magnetic susceptibility data show only one peak at -20 K. Above the ordering temperature, it follows a Curie-Weiss behavior with ep = -56 K and peff = 2.65 cc,. These data are listed in Table 1 together with those of other cerium compounds. Further studies are needed to determine the nature of these two magnetic transitions. CeCds is an antiferromagnetic material, ordering at TN = 2.0 K. The peaks in heat capacity and susceptibility were obvious. Some results on Ce2 Zn,, and CePt, have been reported by Olivier et al. [lo] and Schroder et al. [ll], respectively. The antiferromagnetic phase transition in Ce2Zni7 at TN = 1.7 K is confirmed by a peak in our heat capacity us. temperature curve. CePts is reported to have an antiferromagnetic phase transition at TN = 1.0 K [ 111. The tendency towards magnetic ordering is clearly seen from the upturn in our heat capacity data at -4 K. Because of the low-temperature limit of our apparatus we are unable to make measurements below 1.3 K. The heat capacities show that there is no phase transition above 1.3 K in CeCd6 and Ce13Cd5s. In common with CePt,, however, both compounds show tendencies towards magnetic ordering at a lower temperature as indicated by the upturns in their heat capacities at -3 K. The paramagnetic Curie temperatures (Table 1) of the two compounds are negative, inferring that the orderings are antiferromagnetic in nature.
4. Conclusions It was suggested by Meisner et al. [2] that the correlation between heavy fermion materials and Ce-Ce spacing in the lattice is important. For a sufficiently short Ce-Ce spacing, the overlap of neighboring f wavefunctions leads to the formation of a broad f band which favors a non-magnetic ground state. For a large Ce-Ce spacing, the f electrons are essentially localized and hence the magnetically ordered state occurs. An appropriate
347
hybridization of the f electrons with band electrons on neighboring non-felectron atoms gives rise to an itinerant hybridized narrow f band even when no direct overlap between the f electrons is present. This happens when the Ce-Ce spacing is moderate (just above the Hill limit = 3.41 8) [ 11, and the heavy fermion behavior develops in this regime. The electronic specific heat coefficients y, and the Ce-Ce spacings dCewCeof the compounds discussed above, are listed in Table 1. As can be seen, most of the compounds fall into the intermediate Ce-Ce spacing regime, while none of them shows heavy fermion behavior. This means that a favorable Ce-Ce spacing for heavy fermion behavior is a necessary condition but not a sufficient one. In summary, we report the low-temperat~e heat capacity and magnetic susceptibility data of a series of cerium intermetallic compounds. No heavy fermion behavior is found. CEF plays an important role in the heat capacities of CeGaz and CeCd,,. To our knowledge, the antiferromagnetic phase transitions in CeCd, and CeCds are reported for the first time. The origin of the interesting behavior of CePtGas needs to be determined.
Acknowledgments The authors would like to thank B. J. Beaudry, N. M. Beymer and K. Funke for their assistance in preparing the samples. This work was supported by USDOE, Office of Basic Energy Sciences, Division of Materials Sciences, under Contract No. W-7405-ENG-82.
References 1 H. H. Hill, in W. N. Miner (Ed.), Plutonium 1970, The Metallurgical Society of the AIME, New York, 1970, p. 2. 2 G. P. Meisner, A. L. Giorgi, A. C. Lawson, G. R. Stewart, J. 0. Willis, M. S. Wire and J. L. Smith, Phys. Reu. Left., 53 (1984) 1829. 3 K. Ikeda, K. A. Gschneidner, Jr., B. J. Beaudry and U. Atzmony, Phys. Rev. B, 25 (1982) 4604. 4 R. J. Stierman, K. A. Gschneidner, Jr, T.-W. E. Tsang, F. A. Schmidt, P. Klavins, R. N. Shelton, J. Queen and S. Legvold, J. Magn. Magn. Muter., 36 (1983) 249. 5 J. Tang and K. A. Gschneidner, Jr., J. Magn. Magn. Mater., 75 (1988) 355. 6 T. H. Tsai, J. A. Gerber, J. W. Weymouth and D. J. Sellmyer, J. Appl. Phys., 49 (1978) 1507. 7 W. H. Dijkman, Thesis, University of Amsterdam, 1982. 8 P. Burlet, M. A. Fremy, D. Gignoux, G. Lapertot, S. Quezel, L. P. Regnault, J. RossatMignod and E. Roudaut, J. Magn. Magn. Mater., 63 & 64 (1987) 34. 9 M. Takahashi, H. Tanaka, T. Satoh, M. Kohgi, Y. Ishikawa, T. Miura and H. Takei, J. Phys. Sot. Jpn., 57 (1988) 1377. 10 M. Olivier, T. Siegrist and S. P. McAlister, J. Magn. Magn. Mater., 66 (1987) 281. 11 A. Schroder, R. van den Berg and H. v. Lohneysen, Solid State Commun., 65 (1988) 99. 12 K. H. J. Buschow, Rep. Prog. Phys., 42 f19’79) 1641.