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Low temperature decomposition of PbMo6S8 and Cu2Mo6S8
Mat. Res. B u l l . , Vol. 17, p p . 943-947, 1982. Printed in t h e USA. 9025-5408/82/070943-05503.00/0 C o p y r i g h t (e) 1982 Pergamon P r e s s Ltd.
LOW TEMPERATURE DECOMPOSITION OF PbMo6S 8 AND Cu2Mo6S8
J. Hauck and M. Heiderich Institut fur Festk~rperforschung Kernforschungsanlage JHlich D-5170 J0lich, FRG
(Received May 17, 1982; Communicated b y A. Rabenau)
ABSTRACT:
The reaction of Mo, S and respectively Pb or Cu was studied under hydrothermal conditions. At temperatures below %500 °C Pb and Cu react readily to give the binary sulfides, while the reaction of Mo is very sluggish. For longer reaction times or higher temperatures the equilibrium three-phase assemblage~ Cu,MoS2,Mo and Pb,MoS2,Mo or Pb,Mo2S3,Mo (above %610 ~C) are obtained The ternary phases PbMo6S ~ and Cu2Mo6S 8 form above 713 and 590 vC, respectively.
Introduction The superconducting ternary molybdenum sulfides PbMo6S 8 and Cu2Mo6S 8 are usually prepared by reaction of the elements in sealed silica tubes or sealed m o l y b d e n u m capsules (1-4). The reaction is very slow and must be performed above %1OO0 °C to get complete reaction within reasonable time. Materials prepared at high temperatures exhibit rather large deviations from stoichiometry, giving rise to different physical properties of the samples (1-4). The composition of the phases can be expressed by the formula PbxMO6S8-y and CUxMO6Ss-y, if the S and Pb or Cu contents are related to ~he Mo6 clusters of both structures. Usually the range of homogeneity of ternary compounds is smaller at lower temperatures, and therefore low temperature preparation should yield more stoichiometric compounds. The hydrothermal reaction technique seemed to be suitable for low temperature preparation, since binary sulfides such as PbS, CuS and MoS 2 can be obtained by this technique, - even as small crystals -, at temperatures as low as ~3OO °C (5,6). The present experiments however, showed, that the ternary compounds can not be synthe943
944
J . HAUCK, et al.
sized at low t e m p e r a t u r e s bility.
because
of their
Vol. 17, No. 7
thermodynamic
insta-
Experimental The h y d r o t h e r m a l e x p e r i m e n t s w e r e p e r f o r m e d in a B r i d g m a n autoclave at t e m p e r a t u r e s b e t w e e n 150 and 800 °C w i t h a m a x i m u m p r e s s u r e of 800 bar (5,6). M o s t e x p e r i m e n t s w e r e p e r f o r m e d in sealed silica tubes. Runs in Au tubes w e r e less s u i t a b l e especially at t e m p e r a t u r e s above 500 °C b e c a u s e of the f o r m a t i o n of PbAu and CuAu alloys. The e q u i l i b r a t i o n of the copper c o m p o u n d in Cu tubes i n c r e a s e d the copper c o n t e n t of the sample d r a s t i cally b e c a u s e of an r a p i d r e a c t i o n of the sulfur w i t h the copper tube at the b e g i n n i n g of the e x p e r i m e n t . The finely p o w d e r e d s t a r t i n g m i x t u r e s and a c e r t a i n a m o u n t of water were sealed in the s i l i c a (or Au or Cu) tubes and slowly h e a t e d to the d e s i r e d t e m p e r a t u r e of e q u i l i b r a t i o n . The p r e s s u r e inside the tubes was c a l c u l a t e d f r o m steam tables and had to be c o m p e n sated by an a d e q u a t e argon p r e s s u r e in the autoclave, if it exceeded 60 bar. A f t e r e q u i l i b r a t i o n the a u t o c l a v e was p u l l e d out of the f u r n a c e and cooled by a s t r e a m of c o m p r e s s e d air, the argon b e i n g v e n t e d off after cooling. Results M i x t u r e s of the e l e m e n t s or b i n a r y sulfides to give the n o m i n a l c o m p o s i t i o n P b M o 6 S 8 and Cu2Mo6S 8 served as s t a r t i n g m a t e r i a l s . Pb m e t a l r e a c t e d r e a d i l y w i t h sulfur to PbS, w h i l e the r e a c t i o n of Mo an~ S p r o c e e d e d v e r y s l o w l y e s p e c i a l l y at t e m p e r a t u r e s below 500 C (reaction I). For longer r e a c t i o n times e.g. 80 h at 500 °C or 10 h at 700 °C the Mo had r e a c t e d w i t h the w h o l e a m o u n t of sulfur and PbS was d e c o m p o s e d to Pb (reaction 24 . A b o v e ~ 6 1 0 °C (7,8) M o 2 S 3 (reaction 3) and above 713 ~ 8 C P b M o 6 S 8 w e r e f o r m e d (reaction 4). (I) Pb + 6 Mo + 4 S 2
~
7 5 PbS + ~ MoS 2 + ~ M o
(2)
)
Pb
+ 4 MoS 2 + 2 Mo
(3)
P
Pb
2 + ~ M o 2 S 3 + ~ Mo
(4)
~ PbMo6S 8
The copper m e t a l of the C u 2 M o 6 S 8 - m i x t u r e r e a c t e d first to CuS, the copper s u l f i d e w i t h the h i g h e s t S content, w h i c h is stable b e l o w 507 °C (8) (reaction 5). For longer h e a t i n g times and h i g h e r t e m p e r a t u r e s CuS was r e d u c e d to Cu s u l f i d e s w i t h lower S c o n t e n t such as CuL8S, Cu2S and f i n a l l y to Cu w h i l e the sulfur r e a c t e d w i t h Mo to give MoS 2 (reaction 6 and 7). A b o v e 590 8 °C the t e r n a r y phase was formed (reaction 8). (5)
2 CU
+
6 Mo
+
4 S2
P 2 CuS + 3 MoS 2 + 3 Mo
(6)
¢Cu2S
7 5 + ~ MoS 2 + ~ Mo
(7)
~2 Cu
+ 4 MoS 2 + 2 Mo
(8)
; Cu2Mo6S 8
Vol. 17, No. 7
PbMo6S8 AND Cu2Mo6S8
945
T h e t e r n a r y phases, p r e p a r e d at 720 a n d 600 °C, t h a t is s l i g h t l y a b o v e the t e m p e r a t u r e of d e c o m p o s i t i o n s h o u l d e x h i b i t a w e l l def i n e d s t o i c h i o m e t r y , if the k i n e t i c p r o b l e m s of the s l o w r e a c tions c o u l d be o v e r c o m e . The e x a c t s t o i c h i o m e t r y of t h e s e s a m p l e s is u n k n o w n . I m p u r i t i e s of M o S 2 i n d i c a t e a s u l f u r d e f i c i e n c y of the s a m p l e s (2,3). The T c = 9.9 K v a l u e of the Cu c o m p o u n d is typ i c a l for an a p p r o x i m a t e Cu c o n t e n t x = 1.8 (10). The T c = 9.2 K v a l u e of the Pb c o m p o u n d can be r e l a t e d to the a p p r o x i m a t e s u l f u r c o n t e n t 8-y = 7.6 (3). Discussion Fig. I shows the G i b b s free e n e r g y for the d i f f e r e n t r e a c t i o n s 1-8 w i t h a linear i n t e r p o l a t i o n w i t h i n the t e m p e r a t u r e r a n g e 700 - 11OO K (9). B e c a u s e of the small c h a n g e of ~ G for r e a c t i o n 3 and s e e m i n g l y a l s o for 4 and 8 r e l a t i v e to 2 and 7, the thermod y n a m i c d a t a are r e l a t e d to the r e a c t i o n s 2 and 7. G a s e o u s S 2 w a s t a k e n as r e f e r e n c e for all r e a c t i o n s . The p r e s s u r e d e p e n d e n c e
AGn - AG2 J kJlgfw 150 -
1/,0,I
4030-
ren~tion I
10
"-----....._
0
-10
~reaction
"---~---.... ---------....
reaction 2 _
"-----...,....,.
7
-
A62 =-1590÷0727 T
reaction 3
-20
r'-'~.~ reaction 8
590° 610° 700
6
800
713°C
900 FIG.
~'
-
,~,~ ~
1050
1100 TIK
I
Gibbs free e n e r g y at 700 - 11OO K of r e a c t i o n r e a c t i o n 2 and 7.
I-8 r e l a t i v e
to
946
J . HAUCK, et al.
Vol. 17, No. 7
of AG was neglected. Stoichiometric compositions of the ternary compounds were considered for the reactions because of the unknown exact compositions. The enthalpy of reaction 3 had to be decreased by 0.3 % to get agreement with the experimental temperature of decomposition of Mo2S 3 at 610 °C (7,8). It can be seen from these data that reaction I, 5 and 6 are non-equilibrium reactions because of the less-negativ AG. The Gibbs free energy of PbMo6S 8 and Cu2Mo6S 8 at the temperature of decomposition are equal to the values of the decomposition products, that is ~G(PbMo6S8 )986 = -875 kJ/gfw and ~G(Cu2Mo6S8) 863 =-963 kJ/gfw. For CUxMO6S8-y low temperature phases are reported below 60 269 K (10). These phases might be considered metastable similar as Mo2S36 which exhibits metastable phase transitions below 37 a n d -80 C (11). The instability of CUxMO6Ss-y in the hydrothermal experiments agrees with the decomposition temperature 594 ± 5 °C which was determined in a dry sulfide system (8,12). Mineralogists had investigated the ternary phase diagrams Cu-Mo-S and Fe-Mo-S on search for ternary compounds with the approximate composition CuMo2S 5 and FeMo5SII, which are known as the minerals castaingite and femolite and which might be stabilized by small amounts of Pb and Bi in solid solutions (12). Cu2Mo6S 8 or Fe2Mo6S 8 and PbMo6S 8 are not known as minerals (12) probably because of their instability at low temperatures and because of the 10w valency of the metal atoms. Natural MoS2 frequently exhibits inclusions of copper, iron and lead sulfide (12) but no metallic Cu, Pb and Mo as would be exspected as decomposition products (reaction 2, 3 and 7). The decomposition of the ternary iron compound was reported at 535 ± 15 °C (8). X-ray diffraction measurements on m a n y ternary m o l y b d e n u m sulfides with different metal atoms yielded evidence for lattice instabilities of all these compounds, as was suggested from a systematic delocation of the Pb, Cu, etc. metal atoms (13). The instability of superconducting materials is quite common and can be related to the soft modes of these compounds (14).
Acknowledgements The authors are grateful to Dr. H. Pentinghaus, Karlsruhe and Dr. G. Scheidler, JHlich for the hydrothermal equipment, to B. Bischof, JHlich for experimental support and to H. Schr~der, JUlich for the T measurements. c References I. R. Chevrel, 31 (1980)
C. Rossel and M. Sergent, J. Less-Common Met.
2. K.Y. Cheung and B.C.H.
Steele, Mat. Res. Bull.
3. J. Hauck, Mat. Res. Bull.
12,1015
4. H.A. Wagner and H.C. Freyhardt, 177 (1982)
72,
I--5,1717 (1980)
(1977)
J. Phys. Chem. Solids 4_~3,
Vol. 17, No. 7
PbMo6S 8 AND Cu2Mo6S 8
5. K.-Th. Wilke, Kristallz~chtung, senschaften Berlin, 1973
947
VEB Deutscher Verlag der Wis-
6. C.J.M, Rooymans r in Preparative Methods in Solid State Chemistry (P. Hagenmuller, ed.) Academic Press, New York 1972, p. 71 7. H. Rau, J. Phys. Chem. Solids 41,765
(1980)
8. G.H. Moh in Topics in Current Chemistry Vol. 76 p. 108 Springer-Verlag, Berlin
(1978)
9. I. Barin and O. Knacke and O. Kubaschewski, Thermochemical Properties of Inorganic Substances, Springer-Verlag, Berlin 1973, 1977 10. R. Fl~kiger, R. Baillif, J. Muller and K. Yvon, J. Less-Common Met. 72,193 (1980) 11. R. de Jonge, T.J.A. Popma, G°A. Wiegers and F. Jellinek, J. of Solid State Chemistry 2,188 (1970) 12. B. Grover and G.H. Moh, 529 (1969)
N. Jahrbuch f. Mineral, Monatshefte
13. K. Yvon, Solid State Commun. 25,327 14. H. Rietschel,
Fortschr. Miner. 5_~7 18
(1978) (1979)
LOW TEMPERATURE DECOMPOSITION OF PbMo6S 8 AND Cu2Mo6S8
J. Hauck and M. Heiderich Institut fur Festk~rperforschung Kernforschungsanlage JHlich D-5170 J0lich, FRG
(Received May 17, 1982; Communicated b y A. Rabenau)
ABSTRACT:
The reaction of Mo, S and respectively Pb or Cu was studied under hydrothermal conditions. At temperatures below %500 °C Pb and Cu react readily to give the binary sulfides, while the reaction of Mo is very sluggish. For longer reaction times or higher temperatures the equilibrium three-phase assemblage~ Cu,MoS2,Mo and Pb,MoS2,Mo or Pb,Mo2S3,Mo (above %610 ~C) are obtained The ternary phases PbMo6S ~ and Cu2Mo6S 8 form above 713 and 590 vC, respectively.
Introduction The superconducting ternary molybdenum sulfides PbMo6S 8 and Cu2Mo6S 8 are usually prepared by reaction of the elements in sealed silica tubes or sealed m o l y b d e n u m capsules (1-4). The reaction is very slow and must be performed above %1OO0 °C to get complete reaction within reasonable time. Materials prepared at high temperatures exhibit rather large deviations from stoichiometry, giving rise to different physical properties of the samples (1-4). The composition of the phases can be expressed by the formula PbxMO6S8-y and CUxMO6Ss-y, if the S and Pb or Cu contents are related to ~he Mo6 clusters of both structures. Usually the range of homogeneity of ternary compounds is smaller at lower temperatures, and therefore low temperature preparation should yield more stoichiometric compounds. The hydrothermal reaction technique seemed to be suitable for low temperature preparation, since binary sulfides such as PbS, CuS and MoS 2 can be obtained by this technique, - even as small crystals -, at temperatures as low as ~3OO °C (5,6). The present experiments however, showed, that the ternary compounds can not be synthe943
944
J . HAUCK, et al.
sized at low t e m p e r a t u r e s bility.
because
of their
Vol. 17, No. 7
thermodynamic
insta-
Experimental The h y d r o t h e r m a l e x p e r i m e n t s w e r e p e r f o r m e d in a B r i d g m a n autoclave at t e m p e r a t u r e s b e t w e e n 150 and 800 °C w i t h a m a x i m u m p r e s s u r e of 800 bar (5,6). M o s t e x p e r i m e n t s w e r e p e r f o r m e d in sealed silica tubes. Runs in Au tubes w e r e less s u i t a b l e especially at t e m p e r a t u r e s above 500 °C b e c a u s e of the f o r m a t i o n of PbAu and CuAu alloys. The e q u i l i b r a t i o n of the copper c o m p o u n d in Cu tubes i n c r e a s e d the copper c o n t e n t of the sample d r a s t i cally b e c a u s e of an r a p i d r e a c t i o n of the sulfur w i t h the copper tube at the b e g i n n i n g of the e x p e r i m e n t . The finely p o w d e r e d s t a r t i n g m i x t u r e s and a c e r t a i n a m o u n t of water were sealed in the s i l i c a (or Au or Cu) tubes and slowly h e a t e d to the d e s i r e d t e m p e r a t u r e of e q u i l i b r a t i o n . The p r e s s u r e inside the tubes was c a l c u l a t e d f r o m steam tables and had to be c o m p e n sated by an a d e q u a t e argon p r e s s u r e in the autoclave, if it exceeded 60 bar. A f t e r e q u i l i b r a t i o n the a u t o c l a v e was p u l l e d out of the f u r n a c e and cooled by a s t r e a m of c o m p r e s s e d air, the argon b e i n g v e n t e d off after cooling. Results M i x t u r e s of the e l e m e n t s or b i n a r y sulfides to give the n o m i n a l c o m p o s i t i o n P b M o 6 S 8 and Cu2Mo6S 8 served as s t a r t i n g m a t e r i a l s . Pb m e t a l r e a c t e d r e a d i l y w i t h sulfur to PbS, w h i l e the r e a c t i o n of Mo an~ S p r o c e e d e d v e r y s l o w l y e s p e c i a l l y at t e m p e r a t u r e s below 500 C (reaction I). For longer r e a c t i o n times e.g. 80 h at 500 °C or 10 h at 700 °C the Mo had r e a c t e d w i t h the w h o l e a m o u n t of sulfur and PbS was d e c o m p o s e d to Pb (reaction 24 . A b o v e ~ 6 1 0 °C (7,8) M o 2 S 3 (reaction 3) and above 713 ~ 8 C P b M o 6 S 8 w e r e f o r m e d (reaction 4). (I) Pb + 6 Mo + 4 S 2
~
7 5 PbS + ~ MoS 2 + ~ M o
(2)
)
Pb
+ 4 MoS 2 + 2 Mo
(3)
P
Pb
2 + ~ M o 2 S 3 + ~ Mo
(4)
~ PbMo6S 8
The copper m e t a l of the C u 2 M o 6 S 8 - m i x t u r e r e a c t e d first to CuS, the copper s u l f i d e w i t h the h i g h e s t S content, w h i c h is stable b e l o w 507 °C (8) (reaction 5). For longer h e a t i n g times and h i g h e r t e m p e r a t u r e s CuS was r e d u c e d to Cu s u l f i d e s w i t h lower S c o n t e n t such as CuL8S, Cu2S and f i n a l l y to Cu w h i l e the sulfur r e a c t e d w i t h Mo to give MoS 2 (reaction 6 and 7). A b o v e 590 8 °C the t e r n a r y phase was formed (reaction 8). (5)
2 CU
+
6 Mo
+
4 S2
P 2 CuS + 3 MoS 2 + 3 Mo
(6)
¢Cu2S
7 5 + ~ MoS 2 + ~ Mo
(7)
~2 Cu
+ 4 MoS 2 + 2 Mo
(8)
; Cu2Mo6S 8
Vol. 17, No. 7
PbMo6S8 AND Cu2Mo6S8
945
T h e t e r n a r y phases, p r e p a r e d at 720 a n d 600 °C, t h a t is s l i g h t l y a b o v e the t e m p e r a t u r e of d e c o m p o s i t i o n s h o u l d e x h i b i t a w e l l def i n e d s t o i c h i o m e t r y , if the k i n e t i c p r o b l e m s of the s l o w r e a c tions c o u l d be o v e r c o m e . The e x a c t s t o i c h i o m e t r y of t h e s e s a m p l e s is u n k n o w n . I m p u r i t i e s of M o S 2 i n d i c a t e a s u l f u r d e f i c i e n c y of the s a m p l e s (2,3). The T c = 9.9 K v a l u e of the Cu c o m p o u n d is typ i c a l for an a p p r o x i m a t e Cu c o n t e n t x = 1.8 (10). The T c = 9.2 K v a l u e of the Pb c o m p o u n d can be r e l a t e d to the a p p r o x i m a t e s u l f u r c o n t e n t 8-y = 7.6 (3). Discussion Fig. I shows the G i b b s free e n e r g y for the d i f f e r e n t r e a c t i o n s 1-8 w i t h a linear i n t e r p o l a t i o n w i t h i n the t e m p e r a t u r e r a n g e 700 - 11OO K (9). B e c a u s e of the small c h a n g e of ~ G for r e a c t i o n 3 and s e e m i n g l y a l s o for 4 and 8 r e l a t i v e to 2 and 7, the thermod y n a m i c d a t a are r e l a t e d to the r e a c t i o n s 2 and 7. G a s e o u s S 2 w a s t a k e n as r e f e r e n c e for all r e a c t i o n s . The p r e s s u r e d e p e n d e n c e
AGn - AG2 J kJlgfw 150 -
1/,0,I
4030-
ren~tion I
10
"-----....._
0
-10
~reaction
"---~---.... ---------....
reaction 2 _
"-----...,....,.
7
-
A62 =-1590÷0727 T
reaction 3
-20
r'-'~.~ reaction 8
590° 610° 700
6
800
713°C
900 FIG.
~'
-
,~,~ ~
1050
1100 TIK
I
Gibbs free e n e r g y at 700 - 11OO K of r e a c t i o n r e a c t i o n 2 and 7.
I-8 r e l a t i v e
to
946
J . HAUCK, et al.
Vol. 17, No. 7
of AG was neglected. Stoichiometric compositions of the ternary compounds were considered for the reactions because of the unknown exact compositions. The enthalpy of reaction 3 had to be decreased by 0.3 % to get agreement with the experimental temperature of decomposition of Mo2S 3 at 610 °C (7,8). It can be seen from these data that reaction I, 5 and 6 are non-equilibrium reactions because of the less-negativ AG. The Gibbs free energy of PbMo6S 8 and Cu2Mo6S 8 at the temperature of decomposition are equal to the values of the decomposition products, that is ~G(PbMo6S8 )986 = -875 kJ/gfw and ~G(Cu2Mo6S8) 863 =-963 kJ/gfw. For CUxMO6S8-y low temperature phases are reported below 60 269 K (10). These phases might be considered metastable similar as Mo2S36 which exhibits metastable phase transitions below 37 a n d -80 C (11). The instability of CUxMO6Ss-y in the hydrothermal experiments agrees with the decomposition temperature 594 ± 5 °C which was determined in a dry sulfide system (8,12). Mineralogists had investigated the ternary phase diagrams Cu-Mo-S and Fe-Mo-S on search for ternary compounds with the approximate composition CuMo2S 5 and FeMo5SII, which are known as the minerals castaingite and femolite and which might be stabilized by small amounts of Pb and Bi in solid solutions (12). Cu2Mo6S 8 or Fe2Mo6S 8 and PbMo6S 8 are not known as minerals (12) probably because of their instability at low temperatures and because of the 10w valency of the metal atoms. Natural MoS2 frequently exhibits inclusions of copper, iron and lead sulfide (12) but no metallic Cu, Pb and Mo as would be exspected as decomposition products (reaction 2, 3 and 7). The decomposition of the ternary iron compound was reported at 535 ± 15 °C (8). X-ray diffraction measurements on m a n y ternary m o l y b d e n u m sulfides with different metal atoms yielded evidence for lattice instabilities of all these compounds, as was suggested from a systematic delocation of the Pb, Cu, etc. metal atoms (13). The instability of superconducting materials is quite common and can be related to the soft modes of these compounds (14).
Acknowledgements The authors are grateful to Dr. H. Pentinghaus, Karlsruhe and Dr. G. Scheidler, JHlich for the hydrothermal equipment, to B. Bischof, JHlich for experimental support and to H. Schr~der, JUlich for the T measurements. c References I. R. Chevrel, 31 (1980)
C. Rossel and M. Sergent, J. Less-Common Met.
2. K.Y. Cheung and B.C.H.
Steele, Mat. Res. Bull.
3. J. Hauck, Mat. Res. Bull.
12,1015
4. H.A. Wagner and H.C. Freyhardt, 177 (1982)
72,
I--5,1717 (1980)
(1977)
J. Phys. Chem. Solids 4_~3,
Vol. 17, No. 7
PbMo6S 8 AND Cu2Mo6S 8
5. K.-Th. Wilke, Kristallz~chtung, senschaften Berlin, 1973
947
VEB Deutscher Verlag der Wis-
6. C.J.M, Rooymans r in Preparative Methods in Solid State Chemistry (P. Hagenmuller, ed.) Academic Press, New York 1972, p. 71 7. H. Rau, J. Phys. Chem. Solids 41,765
(1980)
8. G.H. Moh in Topics in Current Chemistry Vol. 76 p. 108 Springer-Verlag, Berlin
(1978)
9. I. Barin and O. Knacke and O. Kubaschewski, Thermochemical Properties of Inorganic Substances, Springer-Verlag, Berlin 1973, 1977 10. R. Fl~kiger, R. Baillif, J. Muller and K. Yvon, J. Less-Common Met. 72,193 (1980) 11. R. de Jonge, T.J.A. Popma, G°A. Wiegers and F. Jellinek, J. of Solid State Chemistry 2,188 (1970) 12. B. Grover and G.H. Moh, 529 (1969)
N. Jahrbuch f. Mineral, Monatshefte
13. K. Yvon, Solid State Commun. 25,327 14. H. Rietschel,
Fortschr. Miner. 5_~7 18
(1978) (1979)