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Low temperature specific heat of rocksalt thorium compounds
V. Maurice, J. Boutard, D. Abbe
To cite this version:
V. Maurice, J. Boutard, D. Abbe. Low temperature specific heat of rocksalt thorium compounds.
Journal de Physique Colloques, 1979, 40 (C4), pp.C4-140-C4-141. �10.1051/jphyscol:1979445�. �jpa-
00218841�
JOURNAL DE PHYSIQUE Colloque C4, supplément au n" 4, Tome 40, avril 1979, page C4-140
Low temperature specific heat of rocksalt thorium compounds
V . M a u r i c e , J. L . B o u t a r d C) a n d D . A b b e (n) SESI, CEN, B.P. 6, 92260 Fontenay aux Roses, France
(+) SEAMA, CEN, B.P. 6, 92260 Fontenay aux Roses, France — (presently: D Tech, CEN Saclay, B.P. 2, 91190 Gif sur Yvette, France)
(f t) OM, ONERA, 92320 Chatillon, France
Résumé. — Des mesures de chaleur spécifique de ThCo,75, ThC0,6N0,4 et ThP ont été effectuées entre 2 et 15 K.
On en a déduit la valeur de la température de Debye 0D et de la densité d'états au niveau de Fermi NF de ces trois composés. NF croît avec la concentration en lacunes dans ThCi-, et est minimale pour ThC0,6N0,4 par rapport à ThC et ThN.
Abstract. — The specific heat of ThC0 75, ThC0.6N0.4 and ThP samples has been measured in the range 2-15 K.
From the results, the Debye temperature 0D and the density of states at the Fermi level N¥ of the three compounds have been calculated. NF increases with vacancy content in ThC,^x and is minimal for ThC06N04
compared to ThC and ThN.
1. Introduction. — T h o r i u m r e a c t s w i t h c a r b o n a n d nitrogen t o f o r m face c e n t r e d cubic T h C and T h N ; T h C is stable o v e r a large range of c o m p o s i - tion. M o r e o v e r , T h C a n d T h N a r e mutually soluble a n d c a n f o r m T h CxN , _x c o m p o u n d s . T h e electronic s t r u c t u r e of t h e s e c o m p o u n d s a n d o t h e r s a s T h P is still n o t well k n o w n [10]. B y making specific h e a t m e a s u r e m e n t s at low t e m p e r a t u r e , t h e d e n s i t y of s t a t e s at t h e F e r m i level h a s b e e n e v a l u a t e d for T h C0 7 5, T h C0 6N0 4 a n d T h P ; f u r t h e r , its stoichiome- t r y a n d V E C ( V a l e n c e E l e c t r o n C o n c e n t r a t i o n ) d e - p e n d e n c e h a s b e e n i n t e r p r e t e d .
2. Experimental. — T h e specific h e a t of T h C0 7 5, T h C0 6N0 4 and T h P s a m p l e s h a s b e e n m e a s u r e d in t h e r a n g e 2-15 K , using an adiabatic calorimeter [1].
T h e samples w e r e p r e p a r e d b y t h e classical p r o c e s s of reaction-sintering. T h e i r lattice p a r a m e t e r s (called a) a r e given in table I.
3 . Results. — A t l o w t e m p e r a t u r e , t h e specific h e a t C c a n generally b e interpreted in t e r m s of C = yT 4- a T3, w h e r e y is p r o p o r t i o n a l t o t h e elec-
t r o n i c d e n s i t y of states at t h e F e r m i level NF, a n d a is proportional t o 1/9^, dD being t h e D e b y e t e m p e - r a t u r e linked w i t h t h e a c o u s t i c m o d e s of t h e lattice v i b r a t i o n s . F i g u r e 1 s h o w s t h e o b s e r v e d variation of C/T versus T2 in t h e r a n g e 2-10 K for T h C0 7 5 ; b y a linear fit of t h e e x p e r i m e n t a l d a t a b e l o w 7 K , w e
Fig. 1. — Low temperature specific heat of ThC075 : C/T versus T2 (•: experimental data ; — : linear fit ; - - : J. Danan's experi- mental curve for ThC [2]).
T a b l e I.
y ( m J / m o l e . K2) N p ( s t / e V . at. T h ) 0„<K)
a (A)
1 04K -1 3C [ 8 ] 1 04K -1 5N [ 8 ] TT1 (s.K)-1 3C [8]
TT, (s.K)-I 5N [8]
T h C0 7 5
3.38 1.43 238
5.32 7.8
67
T h C [2]
2.12 0.9
262 5.34 7.6
74
ThC0.6N0.4
1.9 0.81
262 5.27 6 8.5
104 1 0 4 7
T h N [3]
3.12 1.32 284 5.16 8.8
785
T h P 2.86 1.21 214 5.84 4.4-31P 16.2-31P
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979445
LOW TEMPERATURE SPECIFIC HEAT OF ROCKSALT THORIUM COMPOUNDS C4-141 have calculated the values of y and a!, and deduced
8, and N F . The experimental data obtained for ThC,,N,, and ThP have been treated in the same way (I) ; all the results are given in table I, where they are compared to published literature values for similar compounds of other compositions. Besides, on figure 2 we can see that ThC,,N,, is a supercon- ductor. The bump observed in the experimental curve is not as sharp as a theoretical one ;-this might be due to a slight inhomogeneity of the sample.
Resistivity measurements (Fig. 2) show that at T, = 4.2 K , only a portion of the sample becomes superconducting ; the transition for the whole sam- ple occurs at T, = 3.8 K, temperature at which it becomes diamagnetic, as we checked by magnetic susceptibility measurements. Moodenbaugh et al.
have found ThP to be a superconductor [43 ; the transition temperature Tc = 0.2 K is too low to be observed in our svecific heat measurements.
Fig. 2. -Low temperature specific heat and resistivity of ThCo sNo ,. (a) : C / T versus T 2 ; (b) : C / T versus T .
4. Discussion.
-
There are five points that we want to discuss in the present section :4.1 First, we observed an increase of the density of states at the Fermi level with increasing vacancy
(') In the case of superconducting ThCo 6N0 4, on a C / T versus T diagram (Fig. 2b), the areas (A) and ( B ) on each side of the
y
+
a T 2 curve were found equal as predicted theoretically : at the transition, the entropy remains a continuous function of T.content in The,-,, in agreement with Satow's [5] and Aronson's [6] magnetic susceptibility measure- ments. The fact that the 13C Knight shift K and the spin-lattice relaxation rate TI do not vary from ThC to ThC,,, (see Table I) shows that the electronic structure of carbon is practically unmodified by non-stoichiometry ; the increase of N , is then attri- buted to thorium 6d states.
4.2 Furthermore, NF is minimal for ThCo.,N0:,, compared to ThC and ThN. The same conclusion had been obtained from previous I3C and 15N MNR experiments 17, 81 (minimum of K and maximum of
TT, for the composition ThCo,,NO.3. This minimum is not observed for the magnetic susceptibility which increases continuously with increasing VEC for tho- rium carbonitrides [63. From our results, using a rigid band model, ThC,,N,, would correspond to the VEC value for which the 2p-6d bonding band is filled
4.3 In addition to this, ThCo,No., being a super- conductor, the enhancement of y due to the electron-phonon interaction can be evaluated ; using the McMillan equation [9] with T, = 3.8 K , and ta- king 0.13 for the electron-electron coupling constant as it is commonly done for transition metals and alloys, we obtained the bare density of states at the Fermi level ; its value is
instead of 0.81 as it is shown in table I.
4.4 ThP and ThN, which are isoelectronic, have very close y values and therefore very similar electronic properties.
4.5 The measured densities of states at the Fermi level for stoichiometric
ThC,
ThN and ThP are in qualitative agreement with those calculated by Imo- to et al. [lo] by the tight-binding method.Acknowledgments.
-
We wish to thank Dr. R.Caudron for his help in the experiments performed at the ONERA, and Dr. C.-H. de Novion for stimu- lating discussions. Dr. F. A. Wedgwood is acknowledged for the loan of the ThP sample.
References
[I] ABBE, D., Thesis (1976), University of Paris-Jussieu. [7] BOUTARD, J. L., DE NOVION, C.-H., Plutonium 1975 and [2] DANAN, J., J. Nucl. Mater. 57 (1975) 280. other actinides (North Holland publishing Company) [3] DE NOVION, C.-H., COSTA, P., J. Physique 33 (1972) 257. 1976, p. 505.
[4] MOODENBAUGH, A. R., JOHNSTON, D. C., VISWANATHAN, R., [8] BOUTARD, J. L., thesis (1976) University Paris-Sud Orsay,
Bull. Am. Soc. 22 (1977) 52. Report CEA-R-4797 (1976).
[5] SATOW, T., J. Nucl. Mater. 21 (1967) 343. [9] MACMILLAN, W. L., Phys. Rev. 167 (1968) 331.
[61 ARONSON, S., AUSKERN, A. B., J. Chem. Phys. 48 (1968) [lo] IMOTO, S., ADACHI, H., HORI, T., J. Nucl. Sci. Technol. 12
1760. (1975) 71 1.
