Phase transitions in transition-metal chalcogenides

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Solid State Communications, Vol. 4, pp.419 -421, 1966. Pergamon Press Ltd. Printed In Great Britain.

PHASE TRANSITIONS IN TRANSITION-METAL CHALCOGENIDES C. Haas Philips Research Laboratories N. V. Philips’ Gloeilampenfabrieken, Eindoven, Netherlands (Received 6 June 1966 by G. W. Rathenau)

In many transition metal chalcogenides the anions are hexagonally close packed, the cations occupying octahedral sites. In compounds of the composition M1_~X(M=inetal, X = chalcogene ) the octahedral sites are partly vacant. If the vacant sites are distributed randomly over all octahedral sites, the crystal has the full symmetry of the NiAs-structure (B8). If the vacant sites are completely or partially ordered the crystal symmetry is lower. Ordered phases have been observed for transition metal chalcogenides with approximate compositions M,X9, M5X6, M3X4, M2X3, M5X8. ~‘ The relation between phases of this type is investigated in this paper.

A CHANGE from one crystal structure to ancther, either by a change of composition or by a change of temperature, can occur as a first-order phase transition, as a continuous a second-order change without phase phase transition, transition. or

A first-order phase transition is possible between any two structures. For a second-order phase transition8 there A continouos are very change stringent without symmetry conditions. phase transition is possible only between crys-

Table 1 NiAs and related structures

space group (100), (010), (001) (100), (010), (001)

D 1d 3

(100), (011), (011)

Cib

E

(110), (120), (002) (220), (011), (211)

Ce,,

Ui

(rOl), (011), (111)

C~,

x CrS

(110), (120), (100), (010), (001)

C~ C2~ 419

a

—

observed in the compounds NIAs-structures: (B8) Cd(OH) 2-structure: (C6) 4, and many cther compCr3S1 ounds Cr 4, Cr 4 2S3-trig. 555 Cr,8e 1, Fe 8 7~, Cr7Te9 ~, Fe7S8 Cr7Se5 3S3-rhomb.’ 4 Jahn-Teller distortion: Cr

420

PHASE TRANSITIONS IN TRANSITION-METAL CHALCOGENIDES

tals with the same space group and the same unit cell.

Vol. 4, No. 9

and the unit cell of structure a. These compounds can therefore go over into each other either without any phase transition (by a continuous change of composition), or by a firstorder phase transition.

A first-order phase transition always leads to a two-phase region, separating the existence regions of the two phases. Such a two-phase region does not occur for a second-order phase tran-

Cr2S3-trigonal (a) and Cr3S,-rhombohedral (w) have the same ordered arrangement of atoms

sition or for a continuous change. Table 2

Possible phase transitions between NiAs and related structures 0: 1: 2: U:

continuous change without phase transition first-order phase transition second-order phase transition, type I second-order phase transition, type II

a.

a

w

x

CrS

a

0.1 1.2 1.2

1.2 0.1 1.2

1.2 1.2 0.1

1.2 1.2 1.2

1.2 1 1

1 1 1

1.11 1.11 1

1.2 1 1

E

a

1.2 1.2

1.2 1

1.2 1

0.1 1

1 0.1

1 1

1 LU

1 1

Ui

1

1

1

1

1

0.1

1.11

1

x

1.11 1.2

1.11 1

1 1

1 1

1.11 1

1.11 1

0.1 1

1 0.1

Crs

Some of the structures observed in transition metal chalcogenides are given in Table 1. Translations are always expressed in terms of the translations of the NiAs- structure. For completeness the structure of CrS, which is obtamed from the NIAs-structure by a Jahn- Teller distortion, is also considered. The possible transitions between these structures are given in Tahie 2. As mentioned above, for a second order phase transition to be possible, certain symmetry conditions, given by Landau should be fuifilled. The application of these conditions is often quite cumbersome ~ ~ requiring the use of the theory of representations of space groups. The results of these considerations for transitions between NiAs and some related structures are given in Table 2. ~,

and CrIn the Cr-S system the compounds Cr5S9 2S3-trigonal both have the space group *

E

in planes perpendicu]ar to the (001)-direction, and differ onl’y in the stacking of these layers. ~ It is found that a transition from a to w is not possible as a normal second- order phase transition of type * In such a case an intermedtate structure x can be formed by a secondorder phase transition of type U, and x can transform by another second-order phase transition of type U into structure ~ A similar situation obtains for the transitions ~ a, a w. The intermediate structure x has the same type of layers as aand w, but the stacking of these layers along the (001)direction has no transitional periodicity. Consequently this structure is not described by a threedimensional, but by a two-dimensional space group [point group C31, and primitive translations (110), (120):1. Structures of this type have been observed in some3,metal but notyet alloys ~“ ~and in magnetic materials’ in compounds of the type considered here. A ~•

-.

-.

-.

For the definition of type I and II second-order phase transitions see Ref. 10.

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PHASE TRANSITIONS IN TRANSITION-METAL CHALCOGENIDES

transition from structure 8 to ~ has been reported in Cr~S6at 590°C. ~‘ It would be interesting to investigate whether the intermediate struc-

421

ture ~ really occurs in this case (if, at least, the transition is not a first-order one.)

References 1. BERTAUT E. F., Acta Crvst.

~,

557 (1953).

2. BRUNIE S. and CHEVRETON M., Com. Rend. Acad. Sd. Parts

~,

5847 (1964).

3. CHEVRETON M. and BRUNIE S., Bull. Soc. franc. Miner. Cryst. 87, 277 (1964). 4. 3ELL1NEK F., Acta Cryst. 10, 620 (1957). 5. CHEVRETON M., MURAT M., EYRAUND C. and BERTAUT E. F., J. Physique 3j, 443 (1963). 6. CHEVRETON M., BERTAUT E. F. and JELLINEK F., Acta Cryst. 11, 431 (1963). 7. OKAZAKIA., J. Phys. Soc. Japan~, 1162 (1961). 8. LANDAU L. D. and LIFSHITZ E. M., Statical Physics, Chap. 14, Pergamon, London (1962). 9. HAAS C. , J. Phys. Chem. Solids 26, 1225 (1965). 10. HAAS C.,Phy~. Rev. 140, A863 (1965). 11. GUTTMAN L., Solid State Physjç~, (F. Seitz and D. Turnbull Eds.) Academic, New York (1956). 12. SATOH. and ROTH R. S., Phys. Rev.

~4., 1833

(1961).

13. ELLIOTT R. J., Phys. Rev. 124, 346 (1961). 14. van BRUGGEN C. F. and JELLINEK F., Proc. Int. Conf. Transition Metal Conipounds (Paris), (1965). In vielen bergangs Metall Chalkogenen stud die Antonen hexagonal enge gereihtm während die Kattonen oktaedrische Stellen besetzen. In Zusanimensetzungen der M X Art (M = Metall, X = Cha]kogene) sind die oktaedrischen Stellen teliweise leer. Diese unbesetzten Stellen sind entweder ungoerdnet in welchem Falle das Kristall die symmetrische NIAs Structur hat (B8), sind die Stellen teilweise oder ganz goerdnet 1st die Kristalisymmetrie niedrigerer Ordnung. Geordnete Phasen wurden für tibergangs-Metall- Chalkogene der Zusammensetzung M7Xe, M5X8, M3X4, M2X3, M5X8, “boebachtet. Das Verhältnts der Phasen wird in dem Artikel untersucht. -

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