Single crystals of chevrel-type compounds: Growth, stoichiometry and electrical resistivity

Mat. Res. Bull. Vol. 13, pp. 743-750, 1978. P e r g a m o n P r e s s , Inc. Printed in the United States.

SINGLE CRYSTALS OF CHEVREL-TYPE COMPOUNDS: GROWTH, STOICHIOMETRY AND ELECTRICAL RESISTIVITY R. Flflkiger*, R. Baillif and E. Walker D~partement de Physique de la Matihre Condensc~e U m v e r s l t e de Geneve - 24, Quai E r n e s t Anserrnet 1211 Geneve 4, Switzerland •

•

•

%

(Received June 12, 1978; Communicated by B. T. Matthias)

ABSTRACT A s e r i e s of rhombohedral Mo chalcogenides (Chevrel compounds) based on S o r Se have been melted under argon p r e s s u r e s up to 100 aim. We have found that single c r y s t a l s of an appreciable size (1 to 3 cm3) can be grown by the Bridgman-Stockbarger technigue. Density m e a s u r e m e n t s on these c r y s t a l s , t o g e t h e r with x - r a y and m i c r o s c o p i c observations lead to the conclusion that the f o r m u l a MxMo6X 8 d e s c r i b e s c o r r e c t l y the g r e a t m a j o r i t y of these compounds. The sulfides with M = Pb, Sn as possible exceptions s e e m to be stabilized by a s m a l l e x c e s s of Mo relative to the s t o i c h i o m e t r i c ratio Mo:S=6:8. No evidence f o r the p r e s e n c e of S defects was found. The e l e c t r i c a l r e s i s t i v i t y of PbMo6.2S8 has been found to v a r y l i n e a r l y as a function of t e m p e r a t u r e in the range T c < T ~ 50 K, the lowest m e a s u r e d value at the n o r m a l state being o = 90 ~f~ cm. Introduction A considerable n u m b e r of investigations on rhombohedral t e r n a r y molybdenum chalcogenides of the f o r m u l a MxMo6X 8 belonging to the space group R3 has been published since the d i s c o v e r y of t h e s e compounds. (I, 2) In this f o r m u l a , M stays for a l a r g e n u m b e r of elements, as Pb, Sn, Cu, Ag, Zn, R . E . , . . . . , x is the content of the M e l e m e n t and X r e p r e s e n t s S, Se, Te o r combinations of these e l e m e n t s with I and Br. (3)

*Present a d d r e s s : M a s s a c h u s e t t s Institute of Technology, F r a n c i s Bitter National Magnet L a b o r a t o r y , 170 Albany Street (NW-14) Cambridge, MA 02139. 743

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The main i n t e r e s t of the superconducting Chevrel compounds r e s i d e s in t h e i r e x c e s s i v e l y l a r g e c r i t i c a l magnetic fields, which can attain values up to 600kOe. (4, 5) in view of the possible use of these compounds in superconducting m a g n e t s , it is not only important to know t h e i r superconducting p r o p e r t i e s , but also t h e i r mechanical behavior. These p r o p e r t i e s can be studied m o s t conveniently on single c r y s t a l s . We will d e s c r i b e in this paper a c r y s t a l growing technique which allows the f o r m a t i o n of appreciably l a r g e single c r y s t a l s of rhombohedral Mo chalcogenides.

C r y s t a l Growing The c u r r e n t l y used p r e p a r a t i o n technique f o r obtaining single-phase Mo chalcogenides c o n s i s t s of sintering m i x t u r e s of the c o r r e s p o n d i n g e l e m e n t s M, Mo and X in evacuated quartz tubes at t e m p e r a t u r e s between 400 and 12000 C. After this t r e a t m e n t , the g r a i n size does not exceed 0.2 rnm, even a f t e r prolonged annealings at 1200 ° C. Chevrel (6) r e f e r s to different t r a n s p o r t reactions which allow to obtain single c r y s t a l s of molybdenum chalcogenides up to a volume of 3 ram3. It is c e r t a i n l y possible to f u r t h e r i n c r e a s e the grain size by these methods, but they have the disadvantage that they a r e specific to the studied compound: the t e m p e r a t u r e gradients and the t r a n s p o r t agents have to be d e t e r m i n e d f r o m case to case, while the reaction time can r e a c h s e v e r a l weeks. Hauck (7) has r e c e n t l y r e p o r t e d the formation of rhombohedral single c r y s t a l s (up to 3 m m 3) by m e l t i n g in sealed Mo tubes. A s e r i o u s disadvantage of this method a r i s e s , however, in the occasional b u r s t i n g of the Mo tube above the melting point of the compound. As shown by our own e x p e r i m e n t s , this is not only due to the reaction between the tube and the m e l t , but also to the r e a c t i v i t y of the vapors of S, Se, Pb, Sn, . . . . . . . at these t e m p e r a t u r e s (1550-1750 ° C). We have found e a r l i e r (8, 9) that this problem can be avoided by melting the Chevrel compounds in a high frequency furnace under a m o d e r a t e l y high argon p r e s s u r e (20-100 arm. ). This p r e s s u r e c a u s e s , in principle, a slight i n c r e a s e of the p a r t i a l p r e s s u r e of the volatile components of the compounds, but the d r a s t i c d e c r e a s e of the diffusion speed of the evaporated a t o m s in the argon a t m o s p h e r e l a r g e l y c o u n t e r - b a l a n c e s this effect, thus r e s u l t i n g in a lower evaporation r a t e . By applying the Bridgrnan-Stockbarger technique with fixed crucible and furnace, we have f o r m e d a s e r i e s of single c r y s t a l s with sizes varying f r o m 1 m m 3 (LaMo6S8, GdMo6S8, GdMo6Se 8) to 1 cm 3 (CuxMo6S8, PbMo6:2S8, Mo6Se 8, PbMo6Se 8, SnMo6Se 8, AgMo6Se8, ZnMo6Se8). A CUl. 8Mo6S 8 c r y s t a l g r e a t e r than 3 cm 3 was obtained. The device used in the p r e s e n t work is a f u r t h e r developed v e r s i o n of that d e s c r i b e d in Refs. 8 and 9. It c o n s i s t s e s s e n t i a l l y of a cylindrical molybdenum box of 50 rnm d i a m e t e r and 1 m m wall thickness, in which up to 6 samples can be melted simultaneously. A Ta s u s c e p t o r of 0. 1 m m thickness is situated around the Mo cylinder. The t e m p e r a t u r e is m e a s u r e d at the c e n t e r by a W-3~ Re vs. W-25~o Re thermocouple. T h e r e is a considerable gradient of t e m p e r a t u r e between the top and the bottom of the box, the bottom being the colder part. This gradient is controlled by v a r y i n g the n u m b e r s of r e f l e c t o r s at the top and at the bottom of the box.

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CHEVREL-TYPE COMPOUNDS

745

The o t h e r f a c t o r s affecting d i r e c t l y the grain size were found to be the cooling speed (between 5 and 30 ° C / m i n ) and the shape of the crucible. The best r e s u l t s were obtained in c y l i n d r i c a l crucibles having a round o r a conical bottom. The crucible m a t e r i a l is of p a r t i c u l a r importance,we g e n e r a l l y used A1203, which was substituted by BeO f o r the higher melting compounds. Both c e r a m i c s showed no reaction with the Mo chalcogenides, except if the M component was a r a r e earth. In this c a s e , boron n i t r i d e o r t h o r i u m oxide had to be used. Generally speaking, our (RE)Mo6x 8 samples were of poor quality, and the m a x i m u m size of the obtained c r y s t a l did not exceed 1 m m 3. This could be due to a slight reaction even with BNor ThO2, o r to the f o r m a t i o n of the compounds R . E . Mo6X8, which we think to be p e r i t e c t i c , in analogy to PbMo6.2S 8 (next section). The melting t e m p e r a t u r e s of the i n t e r m e d i a t e phases Mo2S 3 and MoS 2 were found to be substantially higher (> 1800 ° C) than those of the t e r n a r y sulfides c r y s t a l lizing in the Chevrel phase. F o r the l a t t e r , the m e l t i n g t e m p e r a t u r e d e c r e a s e s with that of the M e l e m e n t , the lowest one being m e a s u r e d f o r M=Pb (around 1550 ° C). The m e l t i n g t e m p e r a t u r e s of the selenides a r e m a r k e d l y lower than those of the c o r responding sulfides, but show the s a m e tendency if M v a r i e s . F r o m the c r y s t a l l o g r a p h i c and m i c r o s c o p i c observation of our compounds we deduce that CuxMo6S8, NixMo6S8, Mo6Se8, CuxMo6Se8, PbMo6Se8,SnMo6Se8, AgMo6Se8, and ZnMo6Se 8 f o r m congruently, while s e v e r a l sulfides as PbM°6.2S8, SnMo 5 +yS8 , and AgMo6S 8 f o r m p e r i t e c t i c a l l y . All the compounds containing r a r e e a r t h s a r e ttmught to f o r m p e r i t e c t i c a l l y , but the definitive a n s w e r r e m a i n s open. Stoichiometry An important question which a r i s e s on p r e p a r i n g single c r y s t a l s of compounds c r y s t a l l i z i n g in the Chevrel phase is that of s t o i c h i o m e t r y . It is known that the content x in the f o r m u l a s MxMo6S 5 and MxMo6Se 8 m a y v a r y considerably, depending on the M cation, while it is u s u a l l y a s s u m e d that the ratio between Mo and S or Mo and Se c o r r e s p o n d s to the s t o i c h i o m e t r i c one, which is 6:8. An a n a l y s i s of all our melted compounds shows that this assumption is in g e n e r a l justified with some possible exceptions which a r e d i s c u s s e d below. It has to be noted that t h e r e is some difference between the melting behavior of sulfides and selenides. The situation is b e s t c h a r a c t e r i z e d by comparing the b i n a r y compounds Mo2S 3 (Mo3S 4 is not stable) and Mo3Se4: the f i r s t compound shows p r a c t i c a l l y no melting l o s s e s , while 1% Se o r m o r e a r e lost on the second. (10) The same tendency of i n c r e a s e d evaporation of Se is encountered on t e r n a r y compounds, thus suggesting a s t r o n g e r bonding between Mo and S than between Mo and Se. Neverthel e s s , it can be said f r o m the p r e s e n t data that all selenides f o r m v e r y close to the s t o i c h i o m e t r i c ratio 6:8 between Mo and Se, independently f r o m the n a t u r e of the M cation. If t h e r e is a deviation f r o m this r a t i o , it is probably due to Se defects, but this amount is c e r t a i n l y lower than 1% of the Se a t o m s , as it follows f r o m density m e a s u r e m e n t s (Table I). The situation is m o r e complex for the t e r n a r y molybdenum sulfides, where the m e l t i n g l o s s e s of the M e l e m e n t a r e higher. In g e n e r a l , they r e f l e c t the e l e m e n t a r y vapor p r e s s u r e of the l a t t e r at the m e l t i n g point of the r h o m b e h e d r a l phase. The

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stoichiometric ratio 6:8 between Mo and S could thus only be confirmed for M = Cu (9, 11), Ni (14), Co (14), Ag (14), La (12), and Gd (12), while the exact composition of the sulfides with M = Pb, Sn, Zn, for which melting losses a r e excessive, is diffi cult to determine. We found that appreciably large Pb-Mo-S and Sn-Mo-S single crystals could only be obtained if the initial Mo content exceeded the stoichiometric ratio 6:8. For Fo, the largest crystals were obtained for nominal compositions ranging between Pbl. 2Mo6.9S8 and Pbl. 2Mo7. 2S8 . Melted alloys of these compositions show two distinctly different zones, one containing the eutectic Mo2S3 + PbMo6.2S8, the other being PbMo6.2S8, with grain sizes up to 1 cm 3. The process of growing crystals of a phase which forms peritectically is thought to be the "traveling solvent zone p r o c e s s " : during the slow cooling after melting, the interface (rhombohedral phase + liquid Pb-Mo-S) vs. rhombohedral phase is moved from the w a r m e r to the cooler region of the crucible, the solid crystals being deposited behind this interface. From the analysis of the evaporated material (essentially Fo) and from the chemical analysis (13) of several single crystal pieces, it follows that the reaI composition of this compound corresponds to the formula PbMo6+yS 8. Indeed, from the average density, D = 6.38 :~ 0.08 g / c m 3, obtained on 8 small single crystal pieces, the molybdenum excess can be calculated to y = 0.3 (Table I ). The result

TABLE !

Lattice Constants, Calculated and Measured Densities, Vickers Microhardness and T c for Several Chevrel Compounds. The Data of Hauck (7) for FoMo6S 7 Have Been Added for Comparison

Compound

a

c

Dcalc

Dmeas

(~)

(~)

(g/cm 3)

(g/cm 3)

CUl. 3Mo6S8

9.479

10. 340

5.67

5.64

261

10.3/5.6

Cu2Mo6S 8

9.605

10. 225

5.86

5.82

276

10.8

Cu3Mo6S 8

9.720

10.213

6.08

6.05

AgMo6S8 6.38:k 0.08

Hv (kg/mm 2)

Tc (K)

5.5 301

7.5

300

12.0

PbMo6.2S8

9. 193

11. 473

6.30

FoMo6S 7 (7)

9. 174

11.431

5.99

Mo6Se 8

9.567

11. 176

6.80

6.81

286

6.3

CUl. 4Mo6Se8

9.865

10.800

7.04

7.04

261

5.7

Cu2.6Mo6Se 8

10. 001

10.746

276

<1.2

Pbl. 2Mo6Se 8

9.533

11.936

331

12.0

6.75

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C H E V R E L - T Y COMPOUNDS PE

747

of the chemical a n a l y s i s was Pbl. 02Mo6.5S8. Taking into account that some t r a c e s of undissolved Mo w e r e always p r e s e n t in our s a m p l e s (,-,2 %), the value of y has to be slightly c o r r e c t e d to y --~ 0 . 2 leading to the f o r m u l a PbMo6.2S8 . This r e s u l t a g r e e s quite well with the composition PbMo6.4S8 r e p o r t e d by Chevrel (14). It has to be noted that the composition P b 92Mo6S7.5, determined by Marezio et al. (15) from x - r a y s t r u c t u r a l r e f i n e m e n t s and the formula PbMo6S 7 recently r e p o r t e d by Hauck (7) would r a t h e r suggest the p r e s e n c e of sulfur vacancies. However, these compounds would have densities of 5.98 and 5.99 g / c m 3, r e s p e c t i v e l y , which a r e significantly l o w e r than 6.38 g/era3, our m e a s u r e d value. This strong argument, together with o u r r e s u l t s f r o m x - r a y a n a l y s i s , chemical analysis and m i c r o s c o p i c o b s e r v a t i o n on single c r y s t a l s , excludes the p o s s i b i l i t y of the p r e s e n c e of sulfur v a c a n c i e s within the limits of e r r o r (1 ~o). In addition it is interesting to note that the formula given by Marezio et al. (15) can be r e w r i t t e n to Pb. 98Mo6.4S8, which e s s e n tially c o r r e s p o n d s to that given by Chevrel (14) and is c l o s e to our proposed formula PbMo6.2S8. A single c r y s t a l x - r a y refinement of the s t r u c t u r e of our molybdenum lead sulfide by Yvon et al. (16) confirms the a b s e n c e of sulfur vacancies. I

I

I

I

I

0.~

0.~

PbNo6.2SB+

18% Mo2S 3

\ Pb I'4o6.2 S 8 ( single crystol)

0.~

0.20

0.,7 //"////"

0.2

/

M; 0.10

,,

0.1

OI

20

0

~

I

50

I

100

I

150

40

I

200

60

I

250

TFK] T~K]

i 8 80

I

300

FIGURE 1. The e l e c t r i c a l r e s i s t i v i t y of a PbMo6.2S8 single c r y s t a l , a Pb-Mo-S sample containing 18~o of Mo2S 3 and a Mo2S 3 p o l y c r y s t a l . The linearity of 0 (T) below 50 K is only o b s e r v e d for the pure r h o m b o h e d r a l phase. The p r e s e n c e of even s m a l l amounts of Mo2S 3 leads to a negative c u r v a t u r e of o(T). The lattice t r a n s f o r m a t i o n of the Mo2S3 phase at 195 ° K is shown.

-

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Electrical Resistivity S y s t e m a t i c a l m e a s u r e m e n t s of the e l e c t r i c a l r e s i s t i v i t y on single c r y s t a l s and p o l y e r y s t a l l i n e samples of Chevrel compounds were effectuated in the t e m p e r a t u r e range T c g T ~ 320 K. The r e s u l t s on the r e s i d u a l r e s i s t i v i t y , Do, as defined just above the superconducting t r a n s i t i o n t e m p e r a t u r e shows a considerable variation f r o m one piece of the s a m e sample to another: values between 90 ~Zf~em and 300 Df~ cm were found. This is due to the excessive b r i t t l e n e s s of the s a m p l e s . A certain amount of m i c r o f r a c t u r e s f o r m e d during the cooling and cutting p r o c e s s e s , cannot be avoided. It was thus decided to r e p o r t the lowest value of Oo , which is of the s a m e o r d e r of magnitude f o r m o s t Chevrel compounds (17). At t e m p e r a t u r e s above 50 K, the elect r i c a l r e s i s t i v i t y of Chevrel compounds shows the well known deviation f r o m l i n e a r i t y which is common to all known superconductors (20). The behavior of •(T) for PbMo6.2S8 and o t h e r compounds (17) in the range T e g T ~ 50 K, however, shows an e s s e n t i a l l y l i n e a r dependence on T (Fig. 1, insert). We have found that the l i n e a r i t y of 0(T) can only be observed on single-phased s a m p l e s , the p r e s e n c e of the secondary phase Mo2S 3 contributes to a negative c u r v a t u r e . Fig. 1 i l l u s t r a t e s the influence of 18~o Mo2S 3 on 0(T). We think that this is the r e a s o n why the l i n e a r i t y of 0(T) below 50 K has not been reported before. (After the completion of our p r e s e n t work, we l e a r n e d that Woollam (18) has found the same behavior of 0(T) for lead molybdenum sulfide obtained by sintering. ) Although this l i n e a r i t y of 0(T) at low t e m p e r a t u r e s has been observed on other Chevrel compounds (17), it is not a g e n e r a l p r o p e r t y of all compounds c r y s t a l l i z i n g in this phase: in an e a r l i e r a r t i c l e , (8) we have shown that CuxMo6S 8 and CuxMo6Se 8 exhibit negative c u r v a t u r e s of 0(T) at T < 50 K. Conclusion A series of single crystals of Mo chalcogenides crystallizing in the rhombohedral Chevrel phase have been formed by the Bridgrnan-Stockbarger technique. Their size is in general sufficient for the measurement of most physical or mechanical properties. As an example, we cite the recent measurement of the anisotropy of Hc2 in PbMo6.2S8, PbMo6Se8 and Mo6Se8 by Decroux et al. (19) We have found that the great majority of these compounds can be described by the stoichiometric formula MxMo6X8. Onlythe sulfides with M = Pb, Sn, seem to be stabilized by a slight excess of Mo. From density measurements on our single crystals, the presence of sulfur defects can be excluded within the limits of error (i ~o). A variation of Tc between 6 and 15 K was observed on melted polyerystalline samples of Pb-Mo-S. In spite of the uncertainty in the final composition, the present data suggests that the variation of Tc is primarily due to a variation of the Pb content. However, this result has to be confirmed by additional measurements. We have found that most Chevrel compounds form congruently except the sulfides with M = Pb and Sn. Someparticularities in the high temperature formation conditions are also shown by the compounds (R. E. ) Mo6X8, since important amounts of R.E. sulfideor selenide are always present after melting in alloys of nominal compositions (R. E. ) Mo6X8 or (R. E. )I. 2M°6X8" In analogy to Pb Mo6.2S8, it is probable that single c r y s t a l s of l a r g e r sizes can be grown by the same m e c h a n i s m . F u r t h e r e x p e r i m e n t s a r e under way in in our l a b o r a t o r y . The r e a s o n s for the l i n e a r dependence of o(T) at low t e m p e r a t u r e a r e not yet understood. However, the fact that it is encountered on FoMo6.2S 8 and not on CuxMo6S 8 or CUxMo6Se 8 confirm the dif-

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749

f e r e n t m e c h a n i s m s between t h e s e two c l a s s e s of compounds, i . e . , the different d e g r e e of delocalization of the M ion with r e s p e c t to the origin of the unit cell (12), the diff e r e n t c r y s t a l s t r u c t u r e s at low t e m p e r a t u r e s and the different phonon spectra (21). The m i c r o h a r d n e s s data (Table I) show that the rhombohedral Mo chalcogenides a r e v e r y soft m a t e r i a l s compared to other s u p e r c o n d u c t o r s : for example the A15 compounds. However, the m e c h a n i c a l strength, in p a r t i c u l a r the yield s t r e s s is considerably lower than for the f o r m e r , thus r e n d e r i n g them e x t r e m e l y brittle. The selenides s e e m to be m o r e brittle than the sulfides, the m o s t brittle compound being the binary Mo6Se8, which is at the limit of mechanical stability. This suggests that even if the m e t a s t a b l e compound "'Mo6S8" could be synthesized, its use in superconducting magnets would be h i g h l y improbable, F o r this purpose, the study of the m e c h a n i c a l l y stabilizing role of the M e l e m e n t on Chevrel compounds has to be intensified. Acknowledgment The authors would like to thank P r o f e s s o r s J. Muller, q). F i s c h e r and K. Yvon for m a n y helpful discussions and t h e i r constant i n t e r e s t in this work. Reference s 1. R. Chevrel, M. Sergeant and J. Prigeant, J. Solid State Chem. 3, 515 (1971). 2.

B.T. Matthias, M. Marezio, E. Corenzwit, A. S. Cooper and H . E . Barz, Science 175, 1465 (1972).

3.

M. Sergeant, lb. F i s c h e r , M. Decroux, C. P e r r i n and R. Chevrel, J. Solid State Chem. ~ 87 (1977).

. ~). F i s c h e r , R. Odermatt, G. Bongi, H. Jones, R. Chevrel and M. Sergeant, Phys. Lett. 45A, 87 (1973). g). F i s c h e r , in"Proceedings LT14, Helsinki, 1975", (M. Krusius and M. Vuorio, ed.), Vol. 5, p. 172. 5.

S. Foner, E.J. McNiff, Jr. and E.J. Alexander, Phys. Lett. 49A, 269 (1974).

6.

R. Chevrel, Thesis No. Bl12, Universitg de Rennes, 1974.

7. J. Hauck, Mat. Res. Bull. 1_22, 1015 (1977). 8.

R. Flflkiger, H. Devantay, J . L . Jorda and J. Muller, IEEE T r a n s . Magn. 13, 818 (1977).

9.

R. Flflkiger, A. Junod, R. Baillif, P. Spitzli, A. Treyvaud, A. Paoli, H. Devantay and J. Muller, Solid State Commun. 23, 699 (1977).

10. The evaporated Se is deposited on the cold furnace walls under its red modification, which is highly toxic. 11.

K. Yvon, A. Paoli, R. F1//kiger and R. Chevrel, Acta Cryt. B33, 3066 (1977).

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12.

K. Yvon, Solid State Commun.

13.

The chemical analysis was p e r f o r m e d by Analyx S. A . , Geneve.

14.

R. F1//kiger, unpublished r e s u l t s .

15.

M. Marezio, P.D. D e r n i e r , J.P. Remeika, E. Corenzwit and B.T. Matthias, Mat. Res. Bull. 8, 657 (1973).

16.

K. Yvon and R. Fl~kiger, to be published.

17.

R. F1//kiger, R. Baillif and ~. F i s c h e r , to be published.

18. J.A. Woollam, private communication. 19.

M. Decroux, ~). F i s c h e r , R. Fl~kiger, B. Seeber, R. Delesclefs and M. Sergeant, Solid State Commun. 2__55,393 (1978). It has to be noted that the indicated compositions in this paper a r e the nominal compositions.

20.

Z. Fisk and G. W. Webb, Phys. Rev. Lett. 3_66, 1084 (1976).

21.

S.D. Bader, G.S. Knapp, S.K. Sinha, P. Schweiss and B. Renker, Phys. Rev. Lett. 3_77, 344 (1976).