Crystallography of mixed Chevrel phases

Mat. Res. Bull. Vol. 14, pp. 1525-1528, 1979. Printed in the USA. 0025-5408/79/121525-04502.00/0 Copyright (c} 1979 Pergamon P r e s s Ltd.

Crystallography of Mixed Chevrel Phases*

F. Y. [>adin and J. W. Downey Argonne National Laboratory Argonne, IL 60439

(Received October 3, 1979; Refereed}

ABSTRACT Chevrel phases containing both cations located at the point of inversion symmetry and cations displaced from the center of the unit cell are investigated crystallographically. We have established the existence of single phase compounds containing both large (Sn) and small (Zn or Fe) cations.

Introduction The ternary intermetallic compounds of the Chevrel phase structure have received much attention in recent years because of the many interesting physical properties that the compounds exhibit. These properties include high field superconductivity, long range magnetic order, coexistence of superconductivity and antiferromagnetism, soft lattice modes, transitions from metallic to semiconducting behavior, and fast ion transport. Yvon (i) has crystallographically characterized Chevrel phases of formula MxMo6X 8 (M = cation, X = chalcogen) according to the degree of displacement of the cation from the center of inversion symmetry (the center of the rhombohedral unit cell). That is, as the rhombohedral angle a changes from a value ~ 90 ° (e.g. Pb, Sn or rare earth cations) to a value > 90Q (e.g. In, Fe, Co, Ni, Cu or Zn cations) the rhombohedral cell becomes contracted along the principal direction forcing the cation to sit in potential minima removed from the center of inversion symmetry. It has been found (2) in CUxMO6S 8 that there is a ring of six positions about (0,0,0) referred to as Cu-I positions and a second ring of six positions referred to as Cu-ll positions. The latter are related to the special positions at (1/2,0,0). MSssbauer effect studies (3) of mean squared displacements of Sn I19 in SnMo6S 8 first pointed to the highly anharmonic and anisotropic nature of the atomic vibrations in the centrosymmetric cation variant of the Chevrel phases. Also, SchSllhorn et al. (4) have shown that the room temperature mobility of cations in the displaced cation variant of the Chevrel phases is high. As part of a program to use Sn I19 and Fe 57 MSssbauer effect to study localized phonon mode anharmonicity and atomic mobility, we have investigated the crystallography of mixed Chevrel phases based on either SnMo6S 8 or FeMo6S 8.

Work supported by the U.S. Department of Energy. 1525

1526

F . Y . FRADIN, et al.

Vol. 14, No. 12

Because many of the interesting physical properties of the Chevrel phase compounds are intimately associated with the M cation stoichlometry and lattice location, we have undertaken to establish the feasibility of forming single phase compounds with both large eations (Sn) that prefer to sit at the center of inversion symmetry and small cations (Zn or Fe) that prefer to be displaced from the center of the cell. These mixed cation Chevrel phases serve as prototypical compounds for the study of the defects of importance in fast ion transport and in magnetic clusters in the Chevrel phases. We have also investigated the crystal chemistry of the Fe and mixed cation Fe, Zn Chevrel phases. Experimental Procedure Samples weighing three grams were prepared in an argon glove box from -250 mesh 5N7 + tin filings, 5N+ zinc filings, 3N5+H 2 annealed molybdenum powder and 6 ~ vacuum melted sulfur powder. A powdered iron-sulfide master alloy made from 4~7 + iron foil was utilized in all iron containing samples. The loose powders and filings were mixed and placed in a quartz capsule. The evacuated capsules were slowly heated from room temperatures to 650°C and held overnight. The capsules were removed from the furnaces and the powders were mixed by shaking and then heated from 650 to 1000°C and held at the latter temperature for 48 hours. The samples were then ground, mixed, compacted in a 1/2" die and sealed in evacuated quartz capsules. The capsules were then heat treated at I050°C for 48 hours. The final procedure included grinding the compacts into powder, mixing, sealing the loose powder in evacuated quartz capsules, and annealing the powders at IO00=C for 24 hours. The samples were examined by x-ray diffraction, using CuKa and CrKa radiation. The Debye-Scherrer powder patterns indicated a hexagonal (rhombohedral) R3 structure. The lattice parameters were calculated from patterns taken with CrKa. The superconducting transitions were measured by a standard inductance bridge at a frequency of ~! kHz. Temperatures were measured with a calibrated Ge resistor. Superconducting onsets are listed in Table I. Discussion of Results Our principal crystallographic results are presented in Table I. It is to be noted that samples containing Fe are not superconducting down to 1.5 K. ~nls result is interesting in the light of the fact (5) that for Sno.5Eu0.5~6S8 the superconducting transition temperature T c is only depressed by 0.3 K compared to SnMo6S 8. It is known that Eu is in the divalent state with a localized moment of S = 7/2.(5) Clearly 4f wave functions of the rare earths are more highly localized than the 3d wave functions of Fe, but the crystallographic isolation of Eu at the (0,0,0) position from the molybdenum (shielded by the sulfur) must be more complete than that of Fe at the displaced positions. In addition to the new (Sn,Fe) Mo6S8, (Sn,Zn) Mo6S8, and FeZnMo6S 8 phases listed in Table I, we have found that a nominal composition of Snl.0Fel.~[o6S 8 is multiphase. Also a nominal composition of Sn0.sFeo.5Mo6S 8 is multiphase. We have likewise found that nominal compositions of FexMo6S 8 with 1.8 < X < 4 are multiphase and strongly paramagnetlc or ferromagnetic at ro--om~emperature.

Vol. 14, No. 12

MIXED

CHEVREL

PHASES

1527

The results shown in Table I indicate that as Fe is dissolved in S~io6S 8 the rhombohedral angle increases towards 90 ° and the volume ~f the unit cell increases by ~0.5%. This may be compared to Fel.0Mo6~ , where the volume of the unit cell is ~2.6% smaller than SnMo6S 8 and the rP~mbohedral angle is ~94.6. As Zn is dissolved in S ~ 6 S 8 , T c is slightly suppressed with very little change in lattice parameter as compared to the Fe d~ped compounds. There are probably two effects depressing T c in the Fe doped Sr~io6S8. First, spin disorder scattering due to paramagnetic Fe is quite effective. Second, Fe is most likely dissolving in the trivalent state yielding a less favorable electron concentration or position of the Fermi level in t~e d-band density of states. ZnMo6S 8 has about the same lattice parameters as FeMo6S 8 and we have been abl to further increase the rhombohedral angle (and cell size of FeMo6S 8) by dissolving Zn in FeMo6S 8. Presumably the cation solmbility is increased because Zn is divalent whereas Fe is probably trivalent im the Chevrel phase. TABLE I Lattice Parameters and Superconducting Transition Temperatures of Chevrel Phases (R3)

Compound

Volume (A) 3

Hexagonal o

ao(A )

Rhombohedral o

Co (A)

ao

(i)

a(degrees)

T

c (K)

Snl.2Mo6S8(6)

829.8

9.173(1)

11.388(1)

6.516(1)

89.48

14.10

Snl.0Mo6S8(6)

828.6

9.174(1)

11.368(1)

6.512(1)

89.55

12.36

Snl.0Zn0.2}~6S 8

829.4

9.176(2)

11.374(2)

6.515(2)

89.54

12.29

Snl.0Zn0.4Mo6S 8

829.2

9.177(2)

11.369(2)

6.514(2)

89.56

11.69

Snl.0Fe0.1Mo6S 8

830.8

9.186(2)

11.369(2)

6.518(2)

89.60

<1.5

Snl.0Feo.4Me6S 8

832.4

9.223(1)

11.300(1)

6.523(1)

89.99

<1.5

Fel.0Mo6S 8

806.9

9.522(1)

10.276(1)

6.477(1)

94.62

<1.5

Fe~1.8~6S8

811.2

9.541(1)

10.290(1)

6.489(1)

94.64

<1.5

Fel.0Znl.0Mo6S 8

817.8

9.586(1)

10.276(1)

6.509(1)

94.85

<1.5

ZnMo6S7(7)

811.3

9.545

10.282

6.489

94.683

Znl.2Mo6S7.2(8)

815.2

9.553

10.314

6.50

94.6

FeI.2~6S7.2(7)

813.8

9.564

10.273

6.497

94.783

Fel.32Mo6S8(9)

814.9

9.569

10.278

6.497(1)

94.78(2)

3.6

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F . Y . FRADIN, et al.

Vol. 14, No. 12

Preliminary MSssbauer effect measurements (i0) on Snl.0Fe0.4Mo6S 8 indicate that there are two different nearest neighbor environments for the Sn indicating that ~60% of the Sn atoms have no Fe neighbors and that ~40% of the Sn atoms have Fe neighbors probably in channel-type positions. Conclusions Single phase Chevrel phase compounds containing both cell centered (Sn) and displaced (Fe or Zn) cations have been prepared and characterized. An addition of 0.I mole fraction Fe to SnMo6S 8 depresses T c below 1.5 K. An addition of up to 0.4 mole fraction Zn to SnMo6S 8 results in a small depression of T c. The F e x ~ 6 S 8 and the FeZnMo6S 8 Chevrel phases have been crystallograph ically characterized at room temperature. The rhombohedral angle in FeMo6S 8 can be increased by additions of Zn.

References i. 2. 3. 4. 5. 6. .

8. 9. i0.

K. Yvon, Solid State Comm. 25, 327 (1978). K. Yvon, A. Paoli, R. FIUkiger, and R. Chevrel, Acta Cryst. B33, 3066 (1977) C . W . Kimball, L. Weber, G. Van Landuyt, F. Y. Fradin, B. D. Dunlap, and G. K. Shenoy, Phys. Rev. Lett. 36, 412 (1976). R. Sch~llhorn, M. KHmpers, and J. O. Besenhard, Mater. Res. Bull. I_~2, 781 (1977). F . Y . Fradin, G. K. Shenoy, B. D. Dunlap, A. T. Aldred, and C. W. Kimball, Phys. Rev. Lett. 38, 719 (1977). F. Y. Fradin, J. W. Downey, and T. E. Klippert, Mater. Res. Bull. Ii, 993 (1976). R. Chevrel, M. Sergent, J. Prigent, J. Solid State Chem. ~, 515 (1971). A. C. Lawson and R. N. Shelton, ~ t e r . Res. Bull. 12, 375 (1977). P. J. Guillevic, O. Bars, and D. Gradjean, Acta Cryst. B32, 1338 (1976). B. Stafford, C. D. Barnet, C. W. Kimball, and F. Y. Fradin, in Proceedings of the 1979 Conf. on Superconductivity in d- and f-band Metals, La Jolla, California, June 21-23, 1979 (to be published).