30 J.G. SERENI
The specific heat of Ce2Zn17 shows a sharp peak with a logarithmic divergence at T N = 1.6 K, as a sign of low dimensionality, while the curves are also fitted by a power law with critical exponents 7 = ~' _~ 0.1, corresponding to a three-dimensional (3D) Ising magnet (Sato et al. 1987, 1988a). These authors' argument that the anisotropy of the magnetic interactions leads to a lower dimensionality is supported by the significant short-range order effect above TN and the ratio of internal magnetic energy
~ll( T > TN)/qI(T < TN ~ 3) (0.71 for the 2D Ising model for spin ½), while the entropy AS(TN)/RIn2 = 0 . 8 2 is close to 0.84, the value for the 3D prediction (see, e.g., de Jongh and Miedema 1974). The entropy value may be an overestimate because of the lack of lower-temperature measurements ( T < 1.2 K), where the C / T ratio be- comes important and has to be compared with the reported linear term of 80 mJ K - 2/Ce atom extracted from T > 5 TN.
Finally, the CeB 6 specific heat has been intensively studied because of its double transition and the chance of it being one of the few cases having a guarded Fs-CF ground state. In the absence of an external magnetic field, CeB 6 shows a hump at 3.4 K and an antiferromagnetic transition at T N = 2.4 K. The high-temperature anomaly was recognized as resulting from a quadrupolar transition (Effantin et al.
1985), which shifts towards higher temperatures and gains intensity under an applied magnetic field. On the contrary, the antiferromagnetic transition at 2.4 K is strongly depressed under a magnetic field (Fujita et al. 1980, Peysson et al. 1986). The value of CM at TN (20 J K - 1/Ce atom) largely exceeds the mean-field prediction. The entropy associated with this transition is R ln2, while the R ln4 value is reached at the paramagnetic phase (Fujita et al. 1980). The temperature dependence of the anti- ferromagnetic B T 3 (with B = 950 m J K - a / C e a t o m ) was observed between 0.5 and 1K, while a linear term ( T L V = 3 0 0 m J K - Z / C e a t o m ) becomes dominant at T < 0.5 K as the antiferromagnetic excitation vanishes because of an anisotropy gap (Peysson et al. 1986).
LOW-TEMPERATURE BEHAVIOUR OF CERIUM COMPOUNDS 31 number of compounds included in the H F family because of their large 7LT coefficient (of the order of 100 mJ K - 2 / C e atom), which actually have to be considered as spin glasses due to the varying electronic environment around the Ce ion. Such an effect is produced by nonmagnetic-atom disorder (NMAD), which leads to a large CM/T peak near 1 K characteristic of the spin glass behaviour (Gschneidner et al. 1990). Some of these "false heavy-fermion compounds" will be discussed in sect. 8 within the pseudoternary group, where the Ce partner is partially substituted. Such atomic disorder is not possible in binary compounds and coincidentally there are only two well-recognized H F compounds (CeCu6 and CeA13). We shall introduce another candidate to this group: Ce3In, whose low-temperature specific heat has the charac- teristics of H F - c o m p o u n d behaviour.
One of the best examples of an H F compound is CeCu6, its C~/T ratio grows continuously by decreasing the temperature reaching a value of ~LT(0)=
1.67 J K - 2 / C e atom at T < 700 m K (Amato et al. 1987). Although the CM/T ratio shows no maximum, the experimental results are fitted with a CM/T = 7LT(0) -- A'T equation. The presence of such a term A'Tindicates that some maximum should occur below the experimental temperature range (i.e., T < 50mK). At temperatures T > 10 K, the 7HT term is large: 250 mJ K - 2 / C e atom (Penney et al. 1987), but the total entropy gain of R in 2 is reached at 15 K (including the 7m- contribution). This suggests that there is still a strong decoupling of the electrons involved in the interactions when the excited CF levels already contribute to the specific heat. The 7LT(0) term decreases quadratically with applied field (up to 4.5 T), with a ratio of 0.028 J K - 2 T - 2/Ce atom. F o r H > 5 T a maximum in CM/T appears as an effect of the strong polarization of a narrow band with Zeeman decoupling between the spin- up and spin-down bands (Amato et al. 1987). The 7LT(0) term also decreases under
I000
700
~ 4 0 0
100B~ 13
0
I I 1 I Os21r2 Ru 2
l V Rh3
N
2C3 Co 2
Ni 2 Rh2
~R, a
Sn 3 Pd 3 Ni s Ni !
Si2_ x
I Cu 6 IAi;~ I 3In --2~C011 t
5 4 3
D(41
Fig. 12. CejX k compounds of region IV, labelled as jX k and c~-Ce.
32 J.G. S E R E N I
o
< 0
"~
2
.~,
0
0
0
X ~ ,
"G"
0
0
G)
¢q
¢xl ¢xl ¢~
Z
~ ~ i ~.. ~ ~
A A A
A A
z
6 ~ u ~ u
" ~ Z Z Z
6 6 6 6 6
L O W - T E M P E R A T U R E B E H A V I O U R O F C E R I U M C O M P O U N D S 33
0 0 0 0 0 0 0 0 0 0 0 0 O 0
d d d d d d d d d d d d d d
~ , m m, ~
o ~
A ~ ~ A
~ A
~ < ~ < ~ ~
~
0 ' ~
~ 0
o ~ ' "
~ =
~ o ~
0
e~ . 0... ,.c=
o '~ '~
<<8
~__~ .-.-..~
II 0 a r - ' h
8 ~ ~.~
e ~
o
._=
-I
r ' - h ¢ - q
~ , .,.-,
~= . 2 . 2 u u 8 , - o , - o
,-.! ,-!
o o
< Z Z ~
a s ~ . ~
. 8 \
~ 0 o x l.~ c~ o o 0 o o . ` u o ~
o o ~ ~ t ' q o ~ ~
r -,,,,~ ~ c,:i r~ ~ o o "-'~ ~ o'~
• ~ ~ ~ ' < 0 ,~ ~ " ~ : ~ ' =
~ . , ,.~ ~ - . ~ '---:-. D
~ .'-,, ~ ~ ,
, . ~ ~ ~ . ~ ~ ~ r~ C,~ x O I-~
'~' ,.,~ o o ~ .,..~ . ~ ~ ~ ,-.~
34 J.G. SERENI
applied pressure (becoming 7LT(0) = 0.8 J K - 2 / C e a t o m under a pressure of 8.8 kbar) together with the 7nr contribution (Phillips et al. 1987).
The CeA1 a compound shows a yET(0) value of 1.2 J K - Z / C e atom, but the CM/T ratio increases up to 2 J K - 2/Ce atom at 0.5 K, which signals the transition to the coherent coupling of the K o n d o resonances in a K o n d o lattice (Bredl et al. 1984).
Under an applied field the CM/T maximum value is reduced to 1.7 J K - 2 / C e atom for H = 4 T and lightly shifted to lower temperatures (see also Bredl et al. 1984). The effect of pressure on 7LT(0) is similar to that in CeCu6, reducing its value to 0.55 J K - 2 / C e atom under 8.2 kbar, but it is significant that the CM/T maximum disappears under only 0.4 kbar of pressure.
The low-temperature specific heat of Ce3In is presented in a CM/T representation in fig. 13, after p h o n o n subtraction (Sereni et al. 1989a, b). The extrapolated 7LV(0) value is about 0.5 J K - Z / C e atom , with the maximum value of CM/T = 0.68 J K 2/Ce atom at 2 K. The signature of the H F - c o m p o u n d behaviour is given by the ratio (zT/CM)(rck/#eff) 2 = 2.6, which is constant for T < 5 K and close to the value pre- dicted for a spin-½ K o n d o system (from magnetization measurements, #eff was found to be about 1 g~ for this compound) (Sereni et al. 1990b). At higher temperatures there is another contribution to C M, which reaches its maximum at 18 K with a value of
C M (18 K ) = 5.1 J K - 1/Ce atom. The total entropy gain at 30 K is 0.8 R In 4, indicating that two doublets are involved in the low-temperature properties of this compound. In fact the cubic Cu3Au-type structure gives the possibility of a quartet CF ground state.
Desgranges and Rasul (1985) had studied the case of a K o n d o system with CF splitting of the order of the characteristic temperature TK; the experimental results from Ce3In are compared with the theory in fig. 14. The fact that CM shows a higher and sharper maximum at 18 K than any one of the values predicted by the theory may be due to the fact that CeaIn actually has a F 8 quartet CF ground state that undergoes a quadrupolar transition at 18 K (as in the case of CeB6) and therefore it becomes a real doublet ground-state system only at T < 18 K (Sereni et al. 1989b).
O.B
v
0.4
u 0,2
f
~ .-; I I Ce31n I I I0 lO 20 30
¢'4 C C~
T(KI
Fig. 13. Magnetic specific heat (CM = Cp - f i T 3) of C%In in a C M / T versus T representation.
The continuous curve is the normalized entropy gain.
L O W - T E M P E R A T U R E B E H A V I O U R O F C E R I U M C O M P O U N D S 35
0.4
,'t.
(J 0.2
l 3 lO 30 T ( K )
[ 1 5 I
/// ~\ Ce31n A/T k = 0 / , / ] ~
I / -/,/S ~ . / '\ \ \
K,;Y
\ /,, x.x
I;;-'/" "-.3- \
i'" ~ I i i I "'~-
10-2 ~-1 10 10
T/Tk
0.6
t Y
0.4 -~.
0.2
Fig. 14. Comparison of the C%In specific heat (solid line) with the model of Desgranges and Rasul (1985) (dashed lines),
7.2. Intermediate-valence compounds
All the following compounds have a magnetic susceptibility that does not follow the Curie law, but shows a broad maximum (at Tin). The values of Tm are found between 100 K up to above the range of the measurements ( T > 103 K) (see table 5). In all these compounds the Ce atoms have equivalent sites in the lattice, with a coordination number of 12. There are no s elements among the Ce partners. Coincidentally with the fact that large T m values indicate larger energy scales than those of the H F com- pounds, the intermediate-valence compounds (IV) show much smaller 7 terms, between 10 and 100 mJ m o l - 1 K - 2 . The meaning of TLT and 7HT will not be the same as for the compounds that order magnetically and they will be considered in each particular case.
The most important family of IV compounds is that formed with transition metals with the Laves structure. The volume reduction of these compounds (with respect to their related lanthanide compounds) is significant, reaching values up to 8%, see table 5. This leads to Ce-Ce spacings comparable to and smaller than those of the compounds of region I, see also table 5. The fact that the IV compounds are only cubic when D < 3.5 A (more specifically fcc as 0~-Ce), suggests that the local symmetry of the Ce ion plays an important role in the f-conduction-electron hybridization (Sereni 1985). The low-temperature specific heat of these compounds is shown in figs. 15 and 16. CeRu2 shows the highest temperature for the superconducting transition among the Ce compounds at T~c = 6.18 K (Joseph et al. 1972b, Sereni et al. 1989a). The ? term extrapolated from T > T~c is 7aT ~- 65 mJ m o l - 1 K - 2. Because such a value of Vnx is not consistent with the AC/TsoV = 1.43 prediction of the BCS theory (where AC is the specific heat jump at Tsc nor with the thermodynamical condition that AS(T~¢) for the superconducting phase and for the normal electronic contribution has to be equal, i.e.
AS(Ts¢) = VTs¢, the experimental results were interpreted in two different ways. Joseph et al. (1972b) evaluated a 7LT term of 40.8 mJ mol -a K -2, which approaches both
36 J.G. S E R E N I
160
£ ~20
,-- 80
40
.4",
.v ". C e R u 2
o- ,
:- . .~ x
: , , * x
w
I o ; " "
.~- • """ x x
~" ":°'""" x x CeOs2 o
x o o o
× X
0
," X o
X X o o
• o x MgCu 2
~o.." ~ , ~ , , × , , x o oO
- f ' ~ . . . oo o 9 MgZn 2 Fig. 15. Total specific heat of
superconductive Ce Laves phases.
C e O s 2 is s h o w n for b o t h allotropic
I I 1 I I forms, after Torikachvili et al.
0 20 40 80 120 (1984). CeRu 2 was taken from
T2( K 2) Sereni et al. (1989).
conditions: kC/T~cyt T = 1.33 and AS(T~o) = yLTT~c (with AC = 336 mJ mo1-1 K - l ) . Sereni et al. (1989a) claim CeRu 2 to be an unconventional superconductor, with a measured value of YLT = 6 m J m o l - 1 K - z at T < 0.1T~c, AC = 522 mJ tool -1 K -1 and the ratio AC/T~c7H r = 1.3, close to the value predicted by Hirschfeld et al. (1986) for the "axial" superconductors. In the theory, a linear temperature dependence for T < T~¢ is also predicted. Note that the meaning of the 7LT term is quite different in the two interpretations. In addition, in fig. 15 we show the specific heat o f C e O s 2 in a Cp/T versus T 2 representation of its two allotropic forms, the cubic MgCu2-type structure and the superconducting hexagonal MgZn2-type structure. The ~HT term is similar for both phases: 24 and 22 mJ m o l - * K - 2 , respectively, and the jump AC(T~¢) exhibited by the MgZn 2 phase is approximately one third of the value expected from the BCS theory (Torikachvili and Maple 1984).
In fig. 16, we also show the low-temperature specific heat of the Laves CeX2 compounds (here X are T M belonging to the Co and Ni columns), in a Cp/Tversus T 2 representation. The c o m m o n feature of these compounds is an anomaly at T ~- 6.2 K, which is usually attributed to the presence of C e 2 0 3 impurities in the sample. The hexagonal C e 2 0 3 compound, however, orders antiferromagnetically at T N = 8.5 K (Westrum and Justice 1968). The observed anomaly may thus be the result of the allotropic cubic C e 2 0 3 phase, which is known to form preferentially at the surface of the sample, and is referred to by Besnus et al. (1983b). We should remark that such an anomaly appears in Ce IV compounds with partners belonging to the Co and Ni columns, including CePd 3 (Besnus et al. 1983b) and CeRh (Sereni and Kappler 1989), but not in those with partners belonging to the Fe column (Ru and Os) or in a Ce itself. Considering that this is an extrinsic effect, we find that the y term extrapolated from T > 6.5 K (TnT) ranges between 38 and 23 m J m o 1 - 1 K -2, including CePd3, which anyway is not far from the C/T values for T--+ 0 (or YLT) which range between 40 and 24 mJ t o o l - 1 K - 2 . The linear terms of CeRh and CeRh 3 are somewhat lower than these values: 7nx = 12.7 mJ m o l - ~ K - 2 for CeRh (Sereni and Kappler 1989), and 7nT = 14 mJ tool -1 K -2 for CeRh3 quoted by Mihalisin et al. (1981).
L O W - T E M P E R A T U R E B E H A V I O U R O F C E R I U M C O M P O U N D S 37 120
80
4O
12o
80
40
I T
CeCo 2 •
.°. °
. . , ° " . . • j " . , - "
t " " • '
CeNi 2
T2(K 2)
q I
20 40
°
. ; • .
... . . . ? " "
. . . : " •" . • • . . ".',% •• •
..f.." .• •
, • . . •
..:."" ...:5).:..:'".'.'" "'" • •" . . .
;-..;'.5 ....
I I
60 80
• de lr 2
C e R h 2 . . . . • .
Ce Rh
p I I I
Fig. 16. Total specific heat of the intermediate valence Ce Laves phases and CeRh, in a Cp/T
versus T 2 representation. CeNi 2 and CeCo 2 are taken from Machado da Silva and Hill (1972).
We have to mention that CePd 3 forms congruently, within a concentration of 24 26.5% of Ce. The 7HT term depends on the composition and varies between 7 H T = 3 4 m J m o l - l K -2 for 24.5% of Ce, has a maximum value of 7HT = 38.6 mJ mo1-1 K -2 at 25% of Ce and then decreases to 7HT = 36 mJ mo1-1 K -2 for 25.7% of Ce (Besnus et al. 1983b). Concerning the reference compound, it should be taken as YPd 3 (with ~ = 3.5 mJ m o l - 1 K - 2 ) , because L a P d 3, which has an extremely low value (7 = 0.28 mJ mol-~ K - 2 ) , is not the appropriate reference since CePd3 and L a P d 3 do not mix throughout the whole'range of concentrations•
Another three IV compounds are formed with Ni, they are CeNi, CeNi2 and CeNi 5 . In the case of CeNi the specific heat was measured up to room temperature by Gignoux et al. (1983), from where a value of 7LT = 6 5 m J m o l ~ K -2 and
~&T = 80 mJ mol 1 K 2 were extracted• The 7HT value will be discussed in sect. 9, in reference to C F effects in IV compounds• CeNi 2 has a 7Hx coefficient of 30 mJ mol-~ K -2, with the mentioned anomaly at 6.2 K (Machado da Silva and Hill 1972). In the case of CeNi 5 the value OfVLT = 40 mJ tool -~ K -2 has to be compared with that of LaNi 5 (TLT = 34.3 mJ mo1-1 K -2) and PrNis(TL T = 37 mJ mol -~ K -2) (Nasu et al. 1971). Here we see that the 7 term has almost 80% of the "band"
contribution and only 20% ( ~ 5 mJ m o l - ~ K -2) of Ce IV contribution. If this is the correct interpretation, CeNi 5 shows the smallest 7 contribution owing to the hybri- dized 4f orbital, i.e., ~ 5 m J m o l -~ K -2. Finally, CeCo2 with a 7nT coefficient of 40 mJ m o l - 1 K - 2 (Machado da Silva and Hill 1972) has a superconducting phase at T~c = t.5 K (Luo et al. 1968). Concerning the Ce superconductors, one finds that all of them are fcc-type with D < 3.4 ~., including u- and ~'-Ce.
38 J.G. SERENI
With the light p elements, Ce forms three IV compounds: CeB 4, Ce2C 3 and CeN.
Because of the difficulties in the sample preparation, to our knowledge, no specific heat results are available. Only a 7 = 8.3 mJ tool 1 K - 2 value for CeN was quoted by D a n a n et al. (1969).
Although the remaining Ce compounds that do not order magnetically: CeSi2_ x (0.05 < x < 0.14), CeSn3, CeBe13 and Ce24Co11, are not related in structure or composition, they show relatively large 7 values together with signs of spin fluctu- ations. The specific heats of CeSil.9o and CeSil.86 show the largest 7LT terms, 184.6 mJ tool -1 K -2 (Dhar et al. 1987b) and 203 mJ mo1-1 K - 2 (Sato et al. 1988b), respectively. In both cases the C / T versus T 2 plot shows the characteristic minimum described by the C / T = G + B T 2 + D T 2 in T equation. In a single-crystal sample, the cubic CeSn 3 was found to be an anisotropic spin fluctuator under an applied magnetic field (Tsang et al. 1984), with 7LT values of 75 mJ m o l - a K -2 at zero field, and 7LT = 63.4 and 5 9 m J m o l - ~ K -2 when a field of 1 0 T was applied in the directions [100] and [110], respectively. In a polycrystalline sample, the C / T versus T 2 dependence is fitted with the spin fluctuation equation for the specific heat with a characteristic temperature of T~ = 5.8 K (Ikeda and Gschneidner 1982b). The 7Lv term decreases under an applied field, from 70 mJ m o l - ~ K 2 for H = 0 to 53 mJ mol ~ K - 2 for H ~- 10 T, where the spin fluctuations are practically quenched.
Under fields stronger than 5 T a low entropic (10 3 R In 2) magnetic contribution appears at around 4.5 K, with similar characteristics in the T 3 dependence as observed in CeSix (x = 1.9 and 1.85) (Dhar et al. 1987b). Together with the relatively large value of 7L~ (>~ 50 mJ K - 2 / C e atom), another c o m m o n feature of the compounds that show spin fluctuation effects is their small volume contraction, A V / V <~ 1.5%, compared with other IV compounds where A V / V > 2.5%, see table 5.
The question why CeSn 3 does not order magnetically, while the isomorphic and isoelectronic neighbours CeIn 3, CePb 3 and CeT13 order antiferromagnetically, is still unanswered. N o t e w o r t h y is the fact that, for a Ce Ce spacing in a CeSn 3 compound with trivalent Ce (a = 4.74 A), the TN value extracted from eq. (8) is 4.6 K, in coincidence with the observed anomaly under a magnetic field.
The compound CeBet3, where the C e - C e spacing is the largest of this region (D = 5.19 A), is considered an archetype of the Ce IV compounds from its magnetic behaviour. Although the 7I~T value fits within the expected values, 58.6 mJ m o l - 1 K - 2 at around 5 K, there is a low entropic anomaly (Besnus et al. 1983a). The experimental results are fitted assuming that a Schottky contribution arises from magnetic clusters in the sample, nevertheless the possibility of spin fluctuations is not excluded by the authors.
Finally, Ce24COll shows a quite complicated hexagonal structure with 10 in- equivalent sites for Ce and extremely short C e - C e spacings (Larson and Cromer 1962). This compound does not order magnetically down to 0.5 K, but shows a hump in the specific heat centered at T m ~ 1.2 K. Below the hump C M tends to be linear with temperature, with a value of 7Lv ~- 4.8 J m o l - 1 K - 2 (note that one mole contains 24 Ce atoms), see inset in fig. 17. Between 3 and 9 K the specific heat can be fitted with the function C / T = G + B T 2 -t- D T Z l n T, see fig. 17, with G = 2.8 J m o t -1 K -z, B = 0.8 J m o l - 1 K -4 In K (Kappler and Sereni 1990). Because 10 different sites in the
LOW-TEMPERATURE BEHAVIOUR OF CERIUM COMPOUNDS 39
4 j -
N2 o 3
~2 i--
0 0
Ce24COll
i +
* ' * ~ " T(K)
0
20 40
T2(K 2)
0.5 1.0 1.5
~,1..:~ /
"'~'h-;.:... "-":..- ", "t I
60 30
Fig. 17. Total specific heat of
Ce24COll
in a Cp/Tversus T 2 representation in the range 0.5 9 K. The solid line is a fit to the relation Cp/T = G + B T 2 + DTZln T.
The inset shows the lowest temperature data in a Cp/T versus T plot.
lattice may result in 10 different Ce environments, it is very speculative to attribute any particular CM versus Tdependence to certain Ce atoms. Qualitatively we can only say that some Ce atoms may behave as H F with a T m --- 1.2 K and others exhibit a spin fluctuation regime between 3 and 9 K with a tentative value of T s ~ 7 K. F r o m the z ( T ) behaviour at higher temperature (Canepa et al. 1989), one can observe some similarities with CeSi2_ x.
A detailed phenomenological analysis of the specific heat and the susceptibility of the systems that show spin fluctuation behaviour was made by Gschneidner and Ikeda (1983).
8, Related compounds
In this section we shall include the Ce compounds based on binary compounds that are transformed into pseudoternaries by including interstitial atoms into the lattice or by a partial substitution of the partner. In any case the Ce ion is not changed, neither in its lattice position nor in its concentration, but there is an intrinsic disorder arising from the nonperiodic distribution of the nonmagnetic atom. The nonmagnetic atom disorder (NMAD) concept was recently introduced by Gschneidner et al. (1990) in connection with pseudoternary compounds that, showing large 7tT and/or ~)I-IT values, were included within the heavy-fermion category.
8.1. I n t e r s t i t i a l s
The compounds with the antiperovskite LaPda B-type structure can be considered as a filled-up Cu3Au-type. The b o r o n atoms can be inserted in the T 6 octahedra of the binary c o m p o u n d L a P d a (or CePd 3 in our case), giving rise to a continuous solid solution of composition CePd3Bx, with 0 ~ x ~< 1 (Parth~ and Chabot 1984). Al- though the atomic radius of B (r = 0.98 A) is small, other light atoms with similar size, such as Be and Si (r -- 1.12 A and 1.32 A, respectively) can be also inserted in that
4 0 J . G . S E R E N I
interstitial position (Kappler et al. 1983). The respective concentrations are limited by their size.
As quoted before (cf. sect. 7), the starting or matrix compound CePd 3 is a well- known IV compound. The addition of the interstitial atoms produces an expansion in the cell volume, which is much larger in CePd3Bx than in LaPd3B x or GdPd3B~
compounds due to an induced change in the Ce valence (from mixed valence to trivalent) (Dhar et al. 1981b). The Ce ion becomes trivalent (i.e., magnetic) with more than two b o r o n nearest neighbours, giving rise to clusters of magnetic Ce (Baurepaire et al. 1983). The larger the interstitial, the stronger the volume effect and, with only 20% Si per unit cell, an onset of antiferromagnetic order is detected. With 30% Si concentration a magnetic transition, with the full entropy and a jump of ACM = 8 Jmo1-1 K -1 is observed (Nieva 1988), see fig. 18. N o long-range magnetic order is obtained by introducing Be as an interstitial for up to 45 % of Be per formula unit. At that concentration, where the entropy is about 0.85R In 2, the specific heat contribution of the magnetic Ce can be qualitatively described as that of a system with a random magnetic interaction. In fig. 19, we show the specific heat of the CePd 3 Beo.45 and the effect of an applied magnetic field, which shifts the temperature of the maximum of C M and reduces the value of the maximum (Sereni et al. 1986).
As mentioned before, because of its small size, B can be included as an interstitial up to one atom per formula unit. At low concentrations (x < 0.4) the onset of some kind of cluster or spin glass configuration is shown by the specific heat. At higher concentrations a qualitative change in the specific heat anomaly is observed in the reduction of the temperature and the value of the maximum of the specific heat, see fig. 20 (Sereni et al. 1986). In addition, under an applied magnetic field there is a qualitative difference between the compounds with low and high concentrations of B.
While the maximum of the magnetic contribution to the specific heat decreases with increasing magnetic field for x = 0.3 and 0.35, see Dhar et al. (1989) and Sereni et al.
& w
0 0
I I I
- C e P d
• • x Ce Pd3Si.2
• CePd3 Si.3
• . - - L a P d 3 S i . 2
# ~ • * " . 2 ~ . . . • x x X ~ X x X ~ e x °"-'F ,,, . " / •
I//i
"- ... .-.';-.,'1
8
T(K) Fig. 18. Specific h e a t o f C e P d 3 w i t h Si interstitiats.