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Submitted on 1 Jan 1978
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HEAT CAPACITY OF RARE EARTH
SESQUISULFIDES
J. Ho, S. Taher, G. King, J. Gruber, B. Beaudry, K. Gschneidner, Jr
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-840
HEAT CAPACITY OF RARE EARTH SESQUISULFIDES
J . C . Ho, S.M.A. Taher, G.B. King, J . B . Gruber , B . J . Beaudry and K.A. Gschneidner,Jr.
Wiohita State University, Wiahita,Kansas,U.S.A. North Dakota State University, Fargo, North Dakota,U.S.A. Ames Laboratory-DOE, Iowa State University, Ames, Iowa,U.S.A.
Résumé.- On a effectué des mesures de chaleur spécifique entre 2 et 20K environ, sur une série de sesquisulfures de terres rares, R Sa, presque stoechiomêtrique, avec R = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, ou Lu. Aucun ordre magnétique n'a été remarqué. Des anomalies supplémentaires de chaleur spécifique en plus de celles dues au réseau et aux électrons de conduction sont présentes dans tous les cas sauf dans les composés de La et de Lu. Pour la plupart, de telles contributions sont du type Schottky et dues à des effets de champ cristallin.
Abstract.- Heat capacity measurements between about 2 and 20 K have been made on a series of nearly stoichiometric rare earth sesquisulfides RSj^5 with R = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm or Lu. No magnetic ordering was observed. Extra heat capacity contributions beyond those due to the lattice and conduction electrons occur in all cases except in the La and Lu compounds. Such contributions are mostly of the Schottky type associated with crystal field effects.
INTRODUCTION.- In this paper we are concerned with a study of low temperature heat capacity of the re-fractory semiconducting R2S3(RSi>5) phases. The compounds, where R = La through Dy, crystallize with the bcc Th3Pi, defect-type structure and are general-ly called the y-phase/1/. The compounds H0S1 5 through TmSi 5 have the "6" or the monoclinic 6-Dy2 S3 type structure, YbSi 5 and LuSj^g have "e" or the rhombohedral a-Al203 type structure. The resulting materials are often semiconducting or semimetallic in nature, and have received increasing attention in recent years with particular interest centered on their transport, optical, and magnetic proper-ties HI. To .help elucidate, among others, their crystal fields and magnetic behaviours, low tempe-rature heat capacity data should be most useful, but only very limited studies have so far been re-ported. This work was therefore initiated as a pre-liminary survey over the almost entire set of rare earth elements.
EXPERIMENTAL.- Listed below are eleven RS samples with their chemically analysed composition (± 0.01 in x for compositions given to three significant figures of ± 0.005 in x for compositions given to four significant figures) :
R La Ce Pr Nd Gd Tb Dy Ho Er x 1.499 1.457 1.471 1.45 1.47 1.496 1.49 1.48 1.491
_ __
_ _ .
x 1.470 1.494
While those with R = Nd, Gd, and Dy are 1-2 g sin-gle crystals grown by flame fusion in an argon plas-ma or by Bridgplas-man techniques/3/, others were
prepa-red by direct combination of the elements between 600 and 900°C and grown as 10-20 g crystals or in-gots from the melt (1700-2200°C). X-ray powder pat-terns confirm the crystal structures reported by Elahaut et al./l/ and as noted above. The Th$P^ structure thus obtained appears stable against any phase transformations down to liquid helium tempe-ratures .
Heat capacity measurements by adiabatic ca-lorimetry were made between about 2 and 20 K. Pul-sed heating and germanium thermometry were uPul-sed. Overall uncertainties of the heat capacity values associated mainly with addenda corrections were es-timated to be within I or 2 %, depending on the si-ze and the magnitude of heat capacity of a given sample.
RESULTS AND DISCUSSION.- Figure 1 shows the linear dependence of C/T versus T2 for LaSi.^gs and LuSi^gi, typical for normal- solids at low enough temperatures. The different Debye temperatures of 320 K and 265 K, respectively, are not consistent with the mass difference between La and Lu, a Debye temperature of 293 K for LuSi^gi, would be expected relative to that for LaSi#499 if the lattice stif-fness were only dependent on the lanthanide mass. The lower observed Debye temperature for LuSi^gij is probably due to the fact that the compounds have
3w
low 20 K i n t h e h e a t capacity d a t a and t h i s has beenFig. 1 : C/T versus T~ f o r LaS1.1,99 and L u S l e b 9 ~
.
The value of Debye temperature OD i s calculated from the slope of t h e s t r a i g h t l i n e f i t t e d t o t h e d a t a .d i f f e r e n t c r y s t a l s t r u c t u r e s . Whereas LuS1,1,94 be- haves a s an i n s u l a t o r , t h e non-zero e l e c t r o n i c h e a t capacity of LaSl.499 i s unexpected and n o t under- stood. A t p r e s e n t time we can not r u l e out a devia- t i o n from t h e s t o i c h i o m e t r i c composition o r compo- s i t i o n a l inhomogeneity.
Fig. 2 : Heat c a p a c i t i e s of various r a r e e a r t h ses- q u i s u l f i d e s between about 2 and 20 K. For c l a r i t y most s e t s of d a t a a r e represented by smooth curves.
confirmed by magnetic s u s c e p t i b i l i t y r e s u l t s . With- out any u n p a i r e d 4f e l e c t r o n , LaS1 . 4 9 9 and LuS1, ,,94
should have only t h e l a t t i c e and e l e c t r o n i c h e a t ca- pacity. Indeed, the observed temperature dependence of h e a t c a p a c i t y i s monotonic, and can be used a s base l i n e s (i.e., t h e l a t t i c e a n d s e l e c t r o n i c h e a t capacity) f o r o t h e r compounds w i t h various degrees of anomaly. Even though some of t h e samples a r e known t o become magnetically ordered a t higher tem- p e r a t u r e s / 4 / and have t h e r e f o r e magnetic contribu- t i o n s t o t h e t o t a l h e a t capacity, t h e observed fea- t u r e s of temperature dependence w i t h broad peaks and i n f l e c t i o n p o i n t s suggest t h a t t h e e x t r a h e a t capa- c i t i e s i n t h e temperature range being cofisidered he- r e a r e mostly of t h e Schottky type caused by c r y s t a l f i e l d e f f e c t s . Such e f f e c t s occur when t h e c r y s t a l f i e l d removes t h e degeneracy of t h e ground J - m u l t i - p l e t , s p l i t t i n g i t i n t o a ground s t a t e and one o r more e x c i t e d s t a t e s , a l l of which may o r may n o t be degenerate. The magnitude of t h i s Schottky contribu- t i o n t o h e a t c a p a c i t y t h e r e f o r e depends on t h e crys- t a l s t r u c t u r e i n terms of t h e symmetry about the ra- r e e a r t h ions and t h e s t r e n g t h of t h e c r y s t a l f i e l d . While no c e n t e r of symmetry e x i s t s i n t h e Th3P4 s t r u c t u r e / 5 / , o p t i c a l s t u d i e s have shown t h a t one can approximate t h e p o i n t group symmetry of t h e r a r e e a r t h i o n s with t h e symmetry group Oh. C r y s t a l f i e l d parameters can a l s o be obtained from o p t i c a l s p e c t r a .
For NdS1.45 and DySl.49 good agreement between t h e
h e a t c a p a c i t y d a t a ' a n d t h e c a l c u l a t e d values based on such an approach has been previously reported/4/. O p t i c a l s t u d i e s on o t h e r compounds would be u s e f u l f o r f u r t h e r comparisons. Heat capacity measurements extended t o lower temperatures on some of them, e.
g., Ce, Er, and Tm, thus t o generate more complete
Schottky anomalies would a l s o be d e s i r a b l e i n t h i s r e s p e c t .
Figure 2 summarizes t h e experimental d a t a .
References
/I/ Flahaut,J., Guittard,M., Patrie,M., Pardo,M.P., Golabi,S.M and Domange,L., Acta Cryst.
19
(1965) 14 and2
(1966) 431 121 Taher,S.M.A. and Gruber,J.B., Phys. Rev. B16
(1977) 1624and references therein
131 Henderson,J.R., Muramoto,M., Loh,E., and Gruber,J.B., J. Chem. Phys.
2
(1967) 3347141 Taher,S.M.A., Gruber,J.B., H0,J.C. and Yeh,D.C., to be published in the Proceedings of the 13th Rare Earth Research Conference,Oct. 16-20, 1977
151 Flahaut,J. and Laruelle,P., Progress in the Science and Technology of the Rare Earths (PergamonPress, New York)
