Solid state demixing in Bi2Se3-Bi2Te3 and Bi2Se3-In2Se3 phase diagrams

Materials Research Bulletin, Vol. 31, No. 2, pp. 177-187, 1996 Copyright Q 1996 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/96 $15.00 + .OO

Pergamon

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SOLID STATE DEMIXING IN B&Se,-Bi,Te, AND Bi,SeJ-In,Se, PHASE DIAGRAMS

H. Ghoumari

Bouanani,

D. Eddike,

B. Liautard

and G. Brun

LPMS, ccO3, Universite Montpellier II, 34095 Montpellier cedex 05, France (Received July 11, 1995; Refereed)

ABSTRACT

The Bi,Te,-B&Se, and Bi,Se,-In,Se3 systems were studied by differential thermal analysis and X-ray powder diffraction. The phase diagrams of these systems were constructed; they are quasi-binary. In both cases, there are wide ranges of solid solutions based on the terminal compounds and an intermediate range of demixing. The solid solutions are of tetradymite type (rhombohedral RTm). Their limits are related to the substitution of the selenium-tellurium and bismuth-indium atoms in the layers. Electrical conductivity and thermoelectric power measurements confirm the boundaries of the solid solution regions deduced from the thermal and X-ray diffraction data. KEYWORDS: A. chalcogenides, A. electronic materials, C. differential scanning calorimetry, C. X-ray diffraction, D. phase equilibria

INTRODUCTION

Bi,Te, has the tetradymite-type rhombohedral structure, space group R5m; the hexagonal cell parameters are a = 4,395 A and c = 30.44 A. The lattice can be regarded as a hexagonal layered structure in which the unit cell is a sequence of three five-layer groups. The sequence of the layers is as follows Tel-Bi-Te*-Bi-Te’ so that the Bi atoms have three Te’ atoms as first neighbors (Bi-Te’ = 3.066 A) and three Te’ atoms as second neighbors (BiTe’ = 3.258 A) in the two adjacent layers. The Bi atoms of a same layer represent the only third neighbors (Bi-Bi = 4.383 A). In the case of Te’ atoms, the six first neighbors are the Bi atoms located in the upper and lower layers (Te2 being on the inversion center); the six second neighbors are the Te2 atoms of the same layer. A Te’ atom has three first neighbors 177

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as Bi atoms and three second Te’ (Tel-Tel = 3.647 A) in the adjacent layers; the Tel atoms of the same layer being the third. As can be noticed, each atom in Bi,Te, has three neighbors in the upper layer and three in the lower adjacent layer as in close cubic packing for the direction perpendicular to the (111) plane (1). The B&Se, structure is isomorphous (2) with smaller cell parameters (a = 4.13 pi, c = 28.7 A) and smaller bond lengths (Bi-Se’ = 2.99 A, Bi-Se* = 3.05 A, and Se’-Se* = 3.30 A). At room temperature, ln,Se, can crystallize in several different structures depending on synthesis conditions. We have pointed out that the substitution of the indium atoms by bismuth (or antimony) atoms, even in very low quantity (< 1%), results in the formation of the tetradymite-type rhombohedral structure (R5m) isomorphous with Bi,Te,. The cell parameters for the (In, 98Bi0&Se, composition are a = 3.972(2) A and c = 28.15(3) A, the In-Se octahedral bond length being about 2.95 A with a Se’-Se’ distance of355 a between adjacent layers (3). The equilibrium phase diagrams of the two binaries Bi,Te,-B&Se, and In,Se,-B&Se, have been redetermined by differential thermal analysis and X-ray crystallography. In the first case, the substitution only concerns the chalcogen and occurs in the Se’(Te’) and Se2(Te2) layers, whereas in the second system only the metallic atom layers Bi(In) are concerned.

EXPERIMENTAL Bi&-Bi,Te, Phase Diagram. The reference studies concerning the subject binaries were published between the years 1957 and 1965. They were performed using many techniques such as thermal analysis, X-ray structural determinations and physical properties (electrical and thermal conductivity, thermoelectric power or optical properties). Though the results at high temperature were consistent, the explanations about the room temperature phase diagram contain ambiguities and indicate the need for further investigation. At temperatures above 5OO”C, B&Se, andBi,Te, form a continuous solid solution (4,5). Microstructure studies and measurements of the physical properties established that when the temperature is lowered, the solid solutions become ordered and form the compound Bi,Te,Se. Diffraction studies (2,6,7) indicated a structure isomorphous with Bi,Te,S in which the Te* layers are occupied by selenium atoms only. The dependence of the thermoelectric power, electrical and thermal conductivity (8,9) on the composition of the alloys annealed at 500°C and 300°C also confirmed the presence of transformations in the solid solutions. In the same way, when Bi,Se, is added to Bi,Te,, the forbidden band width increases up to the point represented by the compound Bi,Te,Se (10). The questions addressed in the present study were: what kind of order exists at low temperature and for which defined composition or range of compositions, and what is the nature of the transition at elevated temperature. The X-ray diffraction diagram for the composition Bi,Te,Se (Figure 1) was obtained on a sample of that composition after three months of annealing at 300°C. It shows distinctly at high 13values the systematic splitting of the diffraction lines corresponding to the presence of two phases of different compositions. Refinement of the unit cell parameters

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10

15

20

179

25

:

FIG. 1. Bi,Te,Se X-ray diffraction pattern.

using the respective e values provided the results for the CLphase, a = 4.350(4) A and c = 30.14 (4) A, and for the a’ phase, a = 4.276(3) A and c = 29.82(2) A. Comparing these values with those in Figure 2 that represent the lattice parameters of the alloys in the Bi,Te3-B&Se3 system, it can be seen that the former correspond to the compositions 15 and 45 mole % of B&Se,. The differential scanning calorimetry (DSC) curve of the same sample shows a weak thermal event (2.4 KJ.Mol-‘) spread from 300” to 420” C, irreversible. The electrical conductivity curve of the Figure 3 shows a significant change between 3 10°C and 390°C. This confirms that there is, in that region a transformation from a disordered to an ordered state. The X-ray diffraction pattern of powdered alloys after heating to 400°C and rapidly quenching show peaks from only one phase. The refinement of the resulting unit cell parameters gives the values a = 4.302(4) A and c = 29.93(3) A, very close to the values given in the literature (1,2) and roughly the average of the a and a’ (Figure 2). In Figure 4, the B&Se,-Bi,Te, phase diagram is given in its present state. At temperatures above 500°C B&Se, and Bi,Te, can be dissolved into each other in any proportion and form a continuous solid solution. These results are very similar to those of Hugh (4) and Bankina (5). Near room temperature, the solid solution based on Bi,Te, extend from 0 to 15 mole % B&Se,. Up to 45 mole % of Bi,Te, can be dissolved in B&Se,. Both the solid solutions have the tetradymite-type rhombohedral lattice. The replacement of large tellurium atoms with smaller selenium atoms in the layers is limited and creates demixing in the solid state at low temperature.

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0A D

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Our results a=4.302 A G29.93 A Bland and Basinski (2) a=4.280 A c=29.86 A Nakajima (1) a=4.298.A c=29.77 A

l

29

28

-....., N.... ...... * *............,

\

I

I

20 Bi 2Te 3

t

a

304,

I Bi2Te2Se

40 4

I

so

60

IO

80

Mol %

a'

FIG. 2. Lattice parameters of alloys in the Bi,Te,- B&Se, system.

90 Bi2Se3

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BISMUTH INDIUM CHALCOGENIDES

181

Logo #

78

6,5

60 1,5

2,0

2,5

3,0

FIG. 3.

Electrical conductivity of the sample Bi,Te,Se. Bi,Se,-In,Se, Phase Diaeram. According to Likforman et al. (1 l), preparation of In,Se, by melting of the elements (from 600” to 800” C) with rapid cooling, without any special conditions, one always obtains the a phase, which is considered the stable phase at high temperature. This phase has two different polytipic structures: hexagonal, a = 4.02 A, c = 19.12 A; and rhombohedral, a = 4.02 A, c = 28.76 A. The substitution of indium by bismuth atoms, even in very low quantity (c 1%) stabilizes the rhombohedrai form, isomorphous with tetradymite RJm and therefore isomorphous with the Bi,Se, structure. The B&Se,-In,Se, phase diagram has previously been described by Beltskii and Demyanchuk (12). According to these authors, the two compounds form a continuous solid solution. Starting from those results, Lostak et al. (13) studied the physical properties of a series of single crystals Bi,.,In,Se,, which they prepared, in the interval (x = 0 to 0.66).

H. GHOUMARI BOUANANI et al.

I82

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‘K=C) 700

600

A

500

400

a,fi,

300

Bi2Tq

10

20

30

40

,,

,a',

60

50

70

80

90

BipSe3

MOL% t FIG. 4. Phase diagram of the Bi,Te,-B&Se,

system.

The B&Se, - I&Se, phase diagram and the lattice parameters of the alloys in the system obtained in the present study are given in Figures 5 and 6, respectively. Our results differ from those of earlier workers by the presence of a large demixing region. As in the previous example, the same space group is maintained whatever the composition but, as

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BizSej

BISMUTHINDIUMCIL4LCOGENIDES

1

2

3

4

5

6

7

183

a

9

In2Sej

FIG. 5. Phase diagram of the B&Se,-In,Se, system.

seen in Figure 7, there are two distinct ensemble of diffraction lines for the composition range between 46 and 90 mole % Bi,Se,. The lattice parameters of alloys in the limit of the demixing region for the 46 mole % In,Se, composition are a = 4.042(4) A and c = 28.5 12(2) A, and for the 90 mole % In,Se, composition are Q = 3.985(4) 8, and c = 28.166(2) A. Thermal events of low intensity are observed around 620°C for the compositions rich in In,Se,, which could correspond to a solid phase transformation.

DISCUSSION

AND CONCLUSION

From the viewpoint of energy, based on the bonding system in Bi,Te, and B&Se,, it follows that the most favorable position of the Te* atoms is that with six bonds with the nearest

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In2Se3

Bi2Se3 FIG. 6. Lattice parameters of alloys in the B&Se,-In,Se,

system

neighbors; the Te’ atoms have only three bonds. Replacement of the Te’ atoms with more strongly electronegative selenium should increase ionicity of the Bi-Te’ bond even more and reduce lattice energy. Therefore, when B&Se, is added to Bi,Te,, Se atoms replace first the atoms in the Te* layers; this is now well-known. For the same reason, we can suppose that the substitution of selenium atoms in B&Se, by tellurium atoms would be preferential

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FIG. 7.

X-ray diffraction pattern of 70 mole % B&Se, phase.

in the Se’ layers. What is new is the fact that both substitutions are never complete (for example, in the so-called compound Bi,Te,Se). In the equilibrium state, composition limits reached are as follows: Te’ a

Ze,

Bi Te’

(Se, 2Te0.8)’ Bi .Se, J

a’

Se2

FSe, zTeOA1

Figure 2 shows the lattice parameters of the alloys of the Bi,Te,-B&Se, system. Parameters a and c decrease from Bi,Te, to B&Se, composition. Outside the demixing region, the decrease of the cell parameters occurs smoothly as expected for a simple selenium-tellurium substitution in a layer. In the B&Se,-In,Se, system, there is only one symmetry site for the metal (Bi or In) and the substitution occurs preferentially either in the bismuth layers on the B&Se, side or in the indium layer on the In,Se, side. Figure 6 shows that the lattice parameters a and c decrease regularly up to a limit, by addition of indium atoms, while c/u increases. When

H. GHOUMARI BOUANANI et al.

186

‘LI.

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.. . . BIRKHOLZ L7) .......... ROSI et al. (1 fj > GOLDSMID (16) F+(wm-lk-1)

s CjlvK-‘)

. 2.0

180

140

100

Bi, Te,

Bi 2 Se,

FIG 8.

Thermoelectric

power in the Bi,Te,-B&Se, system.

bismuth atoms replace indium on the In,Se, side, the opposite change occurs. The increase or decrease of c/u indicates the change in the inter-layer bond distances. In the light of our results, we can now better understand the physical properties. Much can be explained by the formation of a new phase, Bi,Te,Se, in the first system and by recognizing demixing in the second system. The results of Bankina and Abrikosov (5) seem most important, the most frequently cited work concerning thermoelectric

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187

applications for these systems. The useful composition of the “n” side of the Peltier cells (about 5/6 Bi,Te,, l/6 B&Se,) has been determined by them. The thermoelectric power values S, measured in (5) are indicated in the Figure 8. It can be seen in what region the curves are in agreement with our results. On the same figure are gathered the thermal conductivity values measured by Birkholz (7), Rosi (9) and Goldsmid (14). In spite of slight differences, one still finds a good agreement between the first minimum and the limit of the Q solid solution. This is particularly clear with the Goldsmid and Birkholz results. The cz composition should be the best one to produce n type material for thermoelectric cooling with a high figure of merit, and we now know the reason for this. The results obtained by Lostak et al. (13) with single crystals of Bi,,In,Se, (0 < x I 0.66), show an unexpected increase of RH (the Hall constant parallel to the c axis) and of S (thermoelectric power) for x > 0.5, precisely the limit of the solid solution in the diagram. The compositions close to In,Se, (0.7 < x < 1) were not prepared and measured. The thermal conductivity (x) results of Belotskii (12) on alloys in the B&Se, - In,Se, system show a decreasing value for 0 < x < 0.5 and an increasing value for 0.85 < x < 1. Again, the physical properties depend critically on the structural behavior of the alloys according to the new phase diagram.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

S. Nakajima, J. Phys. Chem Solid 24, 475 (1963). J.A. Bland and S. Basinsky, Can. J. Phys. 39, 1040 (1961). S.A. Semiletov, Sov. Phys. Cristallogr. 6(2), 158 (1961). J.P. McHugh and W.A. Tiller, Trans. Metal. Sot. of AIME 215, 651 (1959). J.G. Bankina and N. Kh. Abrikosov, Rus. J. Inorg. Chem. 9, 509 (1964). V. Birkholtz, Z. Natur. For. 13A, 780 (1958). J.R. Drabble and C.H.L. Goodman, J. Phys. Chem. Solids 7, 142 (1958). F.D. Rosi, B. Abeles, and R.V. Jensen, Phys. Chem. Solids 10, 191 (1959). W. Fushels, J.N. Bierly, and F. Donahoe, Phys. Chem. Solids 8, 430 (1959). J.G. Audin, A. Sheard, J. Electr. Control 3, 236 (1957). D.P. Likforman, P.H. Fourcroy, M. Gittard, J. Flahaut, R. Poirier, and N. SZydlv, J. Solid State Chem. 33,91 (1980). 12. D.P. Belatskii and N.V. Demyanchuk, Izv. Akad. Nauk SSSR, Neorg. Mat. 5, 15 18 (1969). 13. P. Lostak, L. Benes, S. Civis, and H. Sussmann, J. Mater. Sci. 25, 277 (1990). 14. H.J. Goldsmid, J. Appl. Phys. 32, 2198 (1961).