Stoichiometry of the chevrel phase SnMo6S8

0022-36!i7/82/030177-osso3.ooM) Pergamon Press Ltd.

I. Phys. Chem Solids Vol. 43, No. 3, pp. 177-181, 1982 Printed in Great Britain.

STOICHIOMETRY OF THE CHEVREL PHASE SnMo,& H. A. WAGNER IV. Physikalisches Institut der Universitat, Sonderforschungsbereich 126 Giittingen-Clausthal, Gottingen, West Germany

H. C.

fkEYHARDT

Institut ftir Metallphysik and Kristall-Labor der Physikalischen Institute, SFB 126Gottingen-Clausthal, Gottingen, West-Germany (Receiued 2 December 1980; accepted in revised form 18 September 1981)

Abstract-The phase field of the Chevrel phase Sn,Mo,Ss was investigated by metallographic methods. A shift of the phase field away from the “ideal” compound SnMosSs towards the molybdenum-rich side of the phase diagramwas observed.The degreeof off-stoichiometrycould not yet be determined,however, the c/a ratio of the hexagonallattice parameters can be used to characterize the samples. Measurements of the superconducting transition temperature indicate two superconducting “phases”, which differ in the c/a ratio of the lattice parameters. While phase A showed a linear variation of T: with c/a in the region from 11.0to 13.0K phase B was Independentof the-c/a ratio with a Tf = 10.8+ 0.2 K. 1. INTRODUCTION

Ternary molybdenum chalcogenides M,Mo&, (M = metal, X = S, Se, Te) became known because of their outstanding superconducting properties. The upper critical field, Hc2, at 4.2 K reaches values beyond 60 T and the critical temperature, T,, can be as high as 15K. Furthermore, this class of materials gained interest because of lattice instabilities observed at low temperatures and magnetic ordering effects in Rare-Earth compounds REMo& (see Fischer [ 11)below 1 K. All existing models for this class of compounds use the concept of a molecular crystal built from M atoms and MO& clusters. The experiments, however, clearly indicate a deviation from the stoichiometric formula MMo&. Small metal atoms (e.g. Cu) can occupy up to 4 atomic sites per MO& unit and can form several lowtemperature phases[ l-51, whereas the bigger atoms-like Pb, Sn and RE, to which we restrict our investigationsare found in only one lattice site. For Sn and Pb compounds, the Chevrel phase exists in a narrow homogeneity region with regard to the concentration of metal and X atoms. As a consequence, the stoichiometry of the synthesized compound very sensitively depends on the preparation conditions. Uncontrolled off-stoichiometry causes a scatter of the experimental results and can prevent the comparison between experiment and theory[l]. Presently it is not possible to prepare compound M,Mo,S, with a certain desired x : y : z ratio. Because of the high vapor pressure of sulfur and of some of the metal sulfides the ternary compounds can only be produced in closed systems which prevent the simultaneous control of sulfur and metal activities. As a result the defects of the different sublattices depend strongly on the particular way of sample preparation. However, to control the activities one needs information on the thermodynamic data and the phase diagram. For the investigations the system Sn,Mo,S, was chosen, because the hfossbauer isotope ‘19Snprovides a microscopic probe, which allows to detect the local PCS Vol. 43, No. 3-A

environment of the tin atoms and, therefore, defects in the lattice (e.g. Sn precipitates, Sn oxides, possibly interstitials and vacancies). In this contribution we report on sample preparation, T, measurements, and X-ray investigations; first Mossbauer-effect measurements are published elsewhere[6]. 1. SAMPLE PREPARATION The samples were prepared from Sn and MO metal powder (Koch-Light, particle size 4-8 pm, 99,95%) and from elemental S (Riedel-De-Hiien, 995%). The powders were thoroughly mixed, compacted into pellets, and sealed in quartz ampoules together with a tightly fitting quartz rod in order to reduce the free reaction volume. The ampoules were heated for 170h at Tp= 1000, 1130 and 125O”C,respectively, and were quenched in water. We renounced further heat treatments, which would lead to additional losses of Sn and S. The solubihty of Sn in MO, MO& and MO& was investigated by using powder samples, which were reacted at T, = 1130°Cin the same way. For Pb and Sn compounds deviations from the stoichiometric formula MMosXs were often reported[ 1,7]. This could partly be due to a high impurity content of the powder, in particular due to oxygen and H20, which are expected to be main impurities. To identify the oxygen, the Mossbauer line of SnOz was used, which is the most stable oxide in the Sn-Mo-S system. The oxygen content of the Sn powder amounted to 5 0.1 at %. To estimate the 0 content of MO, 4 at% of Sn were mixed with MO and heat treated as described above. From the SnOz formed in the reaction with MOORwe estimated an 0 content of 1.2 wt% for the MO powder. 3. CONCENTRATION-TEMPERATURFhDlAGRAh4 (XiTJd@m)

OF

Sn-MwSSYSl’EW

The ternary phase diagram (Fig. 1) was constructed in the temperature region between 1000 and 1300°C from the binary Mo-Sn[8], Mo-S [9], and Sn-S]lOl phase 177

H. A. WAGNERand

178

Fig.

1. Schematic phase diagram temperatures between

of the Sn-Mo-S 1000 and 1300°C.

system

H. C. FREYHARDT

at

diagrams by using thermodynamic considerations[ 1l] (see part 4) and the assumption that the Chevrel phase is the only stable ternary phase. This assumption was checked by preparing compounds in the three-phase regions. The solubility of a third element in the binary phases is considered to be small as was observed in the Pb-Mo-S system[71. To check these assumptions, “?Sn in Mo,Mo& and MO& was investigated by Mossbauereffect measurements. At 1130°Cthe solubility of Sn was smaller than 1 at% in all cases. A ternary compound MMo&, mentioned by Chevrel for V, Cr, Fe and Co[12], does not exist in the Sn-Mo-S system. The phase region of the Chevrel phase (Fig. 2) was investigated by light microscopy and by X-ray diffractometry. The light-microscopic detection limits were 1 wt% for MO and the melt, but were much higher for MO& and MO&. By X-ray diffractometry only 510 wt% of the additional MO, MO&, MO& and Sn can be detected if the particles are not too small. For the graphic presentation of the single-phase field

(Fig. 2) we chose a perfect S sublattice (Sn,Mo,&). If one chases, however, a perfect MO sublattice, one gets a picture which is quite similar to what is known for Pb,Mo&[7]. Considering the boundaries of the phase held shown in Fig. 2, one should keep in mind that the experimental errors (O?, Hz0 contaminations, uncertainties in the detection of MO& and MO&) and the absence of a complete local equilibrium (see Section 4) lead to an error in x and y of Z-O.1and 50.2, respectively. In spite of these experimental errors one clearly can see that the Sn,Mo,& phase field is shifted away from SnMo&s in direction of the molybdenum-rich side of the phase diagram. The boundaries agree reasonably well with the values reported by Bychkov et al. [ 131who investigated a series of SnxMo6.,Ss compounds (0.27 2 x 5 1.6). By a modified tube-in-tube-in-tube method after Moh [9], the phase field could be investigated up to 1650°C. At 1520*3O”Ca peritectic or a peritectoid point was found, which is comparable to the value of 1530°C given by Hauck[7] for Pb,Mo& If the temperature is slowly increased, starting at 4OO”C,the sulfur melt was observed to disappear at 570 + 2O”C,which agrees well with the 580°C reported by Bychkov et a1.[13]. This temperature obviously represents the lower boundary for the phase field. It is quite similar to the values for CuMo& (594°C) and FeMo& (535’C) determined by differential thermoanalyses (DTA). A decomposition of Sn,Mo,S, below 570°C could not be detected by DTA[14]. Hence the Chevrel phase is metastable at room temperature and exhibits the same behavior as MO& with a lower boundary at 61o”C, an upper boundary at 1700°C and transformations at 1560 and 161O”C[9]. 4. THERMODYNAMK CONSIDERATIONS In a three-component system the ternary compound Sn,Mo,S, has 4 degrees of freedom, e.g. temperature 7’. total pressure p, Sn and SZ activity (as,, a&. In the literature, however, as well as in our experiments, the temperature, the total volume V and the concentrations can and cs are selected instead. The activities as” and as2 can then deduced from T, V, and This method is equivalent to controlling the activities if an equilibrium is reached during the reaction. In a non-equilibrium situation as, and as2 change as a function of the reaction path, i.e. the final state becomes dependent on the starting values. The used method of fixing T, V, cSn and cs, however, gives a reproducibility for the critical temperature T, of the specimens of typically 0.2 K[ 15, 161. This corresponds to Ax and Ay fluctuations of 0.05 and 0.1, respectively, in Sn,Mo& (compare Fig. Sa, b). A first step to fix the activities externally was made by Schollhorn et al.[17, 181 who investigated the redox behavior of Cu,Mo&, Li,Mo.& Na,Mo& by electrochemical methods. From the phase diagram, however, it is obvious that not only a control of the Sn activity is necessary but that the Sz or the MO activity must be fixed as well, because the properties of Sn,Mo,& seem to be strongly influenced by the defects of the MoaSs clusters. The Sz activity of the Chevrel phase can be expected to be the same as for the coexisting phases MO, CS”

1

.

60

.

6.5

70

75

Y

Fig. 2. Phase field of Sn,Mo,Ss at Iooo”C determined by light microscopy. Full and open squares indicate pure A- and Bsamples, respectively. For the half-open squares both phases are observed simultaneous (see texti The perfect compound SnMo&, indicated by a hatched square, lies in a three phase field.

CS.

Stoichiometryof the ChevrelPhase SnMo&

00

179

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Fig. 3. S2activityversustemperature.The boundarybetweenthe phase fields of the binary phases are calculated from thermodynamicdata[l3]. The shaded area indicatesfhe values of SZ activities which can be expected for the ternary compound

MO&, MoSz and Sn (Fig. 1). Therefore, we calculated SZ activities as a function of temperature for the binary sulfides, using thermodynamic data[ 191and the assumption that ail solid phases possess an activity equal to 1. For example: 2M+ S27+2MS K,

=exp_~$+ A4 532

1 aa2

(1)

K, is equilibrium constant at constant pressure. The curve calculated from eqn (1) determines the phase boundary between M and MS in the activitytemperature diagram (Fig. 3). Figure 3 shows the sequence of Sn and MO sulfides with increasing sulfur activity[ 111. The shaded area indicates ihe SZ activity expected for the Chevrel phase. The Sn activity of SnxMoySz, however, which is the second parameter in the phase field is not yet known.

5. BUPERCONWCTlNG BEILiVIOlk

Fig. 4. Inductivelymeasuredsuperconducting/normaI transition curve for a two phase sample.

to a variation of the Sn and S content of x = 0.05 and

y = 0.1, respectively (Fig. Sa,b). Pure A-phase samples were found at the molybdenumrich side and B-phase samples at the molybdenum-poor side of the phase field shown in Fig. 2. Because of the experimental uncertainties, however, no sharp phase boundary and no systematic variation in the relative abundance of phases A and B in the two-phase area could be detected. At room temperature the two phases could not be distinguished by differing structures. All samples exhibited the same rhombohedral Rg structure, however, with a variation of the lattice parameters. In Fig. 6 T, is plotted vs c/a (a, c: hexagonal lattice parameters at room temperature). To a good approximation, c/a is equivalent

DEPENDENCE ON LATIICE

The superconducting critical temperature T, was measured inductively as a function of the composition. Quite frequently transition curves with two jumps (Fig. 4) were observed, indicating two different superconducting modifications or “phases”, which we shall refer to in the following as phase A and B for the purpose of convenience. However, a double transition can be caused under certain circumstances also by strains, by compositional inhomogenities, or by additional impurities like C, N and 0. In our samples no indication for such effects are found (see discussion at the end of this chapter). On the other hand we used quotation marks to indicate that the reason for the observed double transition is not yet fully clarified. The T, values (onset) lay in the range ll.OK~T:513.0K and 10.4KsTf$ 11.3K for phase A and B, respectively. While Tt depends on the Sn and S content (Fig. 5a,b) TF does not. The uncertainty of Tt of about 0.2K corresponds

C IKI

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130

g=1oooT

a

~.113O"C

0

$.=125o'c

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B

\

11.5.

Y Fig. 5. (a) SuperconductingtransitiontemperatureT: as a function of x for Sn&&.&,. (b) SuperconductinghnSiliOn tcmpe.ratureT: as a functionof y for Sno.rJ%&.

H. A. WAGNER

180

nc S%“%S~Fradinet oTc SnMoS

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two

1000

1100

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a 1150 000~1250 phase

phases

presently checked whether a layer of impurity phases cover the Chevrel-phase powder particles thus leading to an erroneous determination of the volume of the second in the ac susceptibility superconducting phase measurement of T,. At present it is considered likely that one of the transitions is produced by a phase formed via a phase transformation below room temperature. For the clarification of this question further X-ray studies at low temperatures are required.

A T,*

1

123L

1238

1

1

12l.2

Fig. 6. Superconducting transition temperature function of the ratio c/a.

.

C/Cl

T: and TF as a

to the rhombohedral angle a (c/a = [9/4 sin2(a/2) - 31”‘) used by other authors[l,2]. Below c/a = 1.235 only phase B was observed with Tz = 10.8 t 0.2 K independent of the value of c/a. Samples with a Sn : MO : S ratio of 1 : 6: 8 lay in this region. Above c/a = 1.235 phase A occurred in addition and led to larger uncertainties of T? due to larger errors in the T,. determination. T: seems to depend linearly on c/a up to c/a = 1.239 where pure A-phase samples were observed. A c/a ratio higher than 1.235 can be ascribed to excess MO or to unintentional Sn and S losses, which lead to a deviation from the ideal formula SnMo&. All other data reported in the literature are included in Fig. 6. They show a reasonable agreement with our results, however, the double transition was not observed so far by other authors. This could be due to a c/a ratio outside of the region 1.235 I c/a 5 1.239 (e.g. Bolz et al.[20] and Fradin et al.[21]) or to the fact that sometimes only small amounts of the A-phase are present, which cannot be detected very easily. Because a well established correlation is found to exist between the lattice parameters and Tt, these transitions can be attributed to the rhombohedral Chevrel phase. The second transition at TF = 10.8 K cannot be explained by stress effects. Different stress (and impurity) levels generally lead to a broadening of the transition rather than to a well defined second T, value (stress-induced phase transformations are not considered here). The presence of bulk superconducting compounds-with a T, of 10.8 K-other than the ternary Sn-Mo-S Chevrel phases can be excluded. They should have led to additional lines in the X-ray spectra. It is

8. CONCLUSION For the Sn Chevrel phases deviations from the stoichiometric compound have a strong influence on the superconducting properties. From the measured changes of the lattice parameters and of T, it is obvious that the deviations from the formula SnMo6SR are connected with generation of defects in SnMo6S8 structure rather than with precipitations of additional phases. Because in different A- and B-phase specimens only one lattice site for Sn was observed by Miissbauer effect measurements[6], the off-stoichiometry could be due to Sn vacancies or defects in the MO& clusters (S vacancies or MO interstitials). Density measurement by Fliikiger et al.[22], however, on Pb,Mo,S, indicate that one should have excess MO rather than S vacancies for Chevrel phases with large atoms (Pb, Sn, RE), A control of these defects by fixing the Sz and Sn activities is presently not possible. This explains the scatter in the experimental data reported in the literature, which, however, can be classified by the c/a ratio. The question how the two “phases” A and B differ structurally is not yet resolved. As there is no indication of an additional lattice site occupied by Sn atoms, it is assumed that the two phases A and B differ in their MO& clusters, i.e. the clusters can have different defect concentrations or different structures for A and B phase, respectively. RFXERENCEs

I. Fischer O., Appl. Phys. 16, 1 (1977). 2. Yvon K., Current Topics in Materials Science 3, 53 (1979) 3. Fliikiger R., Junod A., Baillif R., Spitzli P., Treyvaud A.. Paoli A.. Devantav H. and Muller J.. Solid State Comm. 23. 699 (1977). . 4. Yvon K., Paoli A., Fliikiger R. and Chevrel R., Acta Cryst. 833. 3066 (1977). 5. Baillif R., Yvon K., Fliikiger R. and Muller J., 1. Low Temp. Phys. 37, I, 231 (1979). 6. Wagner H. A. and Freyhardt H. C., Physica 107B, 657 (1981). 7. Hauck J., Mat. Res. Bull. 12. 1015 (1977). 8. Shunk F. A., Constitution of Binary Alloys. McGraw-Hill, New York (1969). 9. Moh, G. H., Topics in Current Chemistry Vol. 76, p. 107. Springer-Verlag, Berlin-Heidelberg (1978). 10. Moh G. H., Private communication. 11. Vaughan D. J. and Craig J. R., Mineral Chemistry of Metal Sulfides, Chap. 7. Cambridge University Press, Cambridge (1978). 12. Chevrel R., Theses, University of Rennes (1974). 13. Bychkov Yu. F.. Gnuzin P. L., Yevstyukhina I. A. and Likhanin Yu. N., Phys. Met. Metall. 46, 4, 29 (1979). 14. Bente K., Private communication. 15. Sergent M., Chevrel R., Rossel C. and Fischer O., J. Less Common Metals 58, 178 (1978). 16. Culetto F. J., Diss. Techn. University Wien, Berichte der Kernforschungsanlage Jiilich. Nr. 1587 (1979).

Stoichiometry of the Chevrel Phase SnMo& 17. Sch~llhorn R., Kiimpers M. and Besenhard J. O., Mat. Ues. Bull. 12,781 (1977). 18. Schiillhom R., Kiimpers J., Lerf A., Umlauf E. and Schmidt W., Mat. Res. Bull. 14, 1039(1979). 19. Barin I. and Knacke O., Thermochemicnl Properties of Jnorganic substances. Springer-Verlag, Berlin (1973,1977).

181

20. Bolz J., Hauck J. and Pobell F., Z. Physik B25, 351 (1976). 21. Fradin F. Y., Downey .J. W. and Klippert T. E., Mat. Res. Bull. 1I, 993 (1976). 22. Fliikiger R., Baillif R. and Walker E., Mat. Res. Bull. 13,743 (1978). 23. Lawson A. C:. Mat. Res. Bull. 7,773 (1972).