Chemistry

V. I. Ivlieva, N. Kh. Abrikosov

Investigation of Phase Equilibrium in Systems Formed by Antimony Chalcogenides

(Presented by Academician I. V. Tananaev, June 19, 1964)

Chalcogenides of elements of group V of D. I. Mendeleev’s periodic system, and especially solid solutions based on them, have long attracted the attention of researchers. A large number of works have been devoted to a detailed study of bismuth chalcogenides, as well as of complex systems based on them.

Earlier investigations of bismuth chalcogenides made it possible to gain a deeper understanding of the nature of conductivity in these substances, to establish the relation between their crystal-chemical nature and the physical properties of the phases formed, and also to find a whole series of new materials for use in photo- and thermoelements.

Phase equilibria in systems formed by antimony chalcogenides have not been investigated up to the present time. Only the properties and crystal structure of individual compounds have been studied.

In the present work the phase equilibria and certain physical properties in the systems Sb₂S₃—Sb₂Se₃ and Sb₂Se₃—Sb₂Te₃ have been investigated. The compound Sb₂S₃ melts with an open maximum at 555° (¹). This compound crystallizes in an orthorhombic lattice, structural type \(D_5^8\), with constants \(a = 11.22\) Å, \(b = 11.30\) Å, and \(c = 3.84\) Å (²). The compound Sb₂Se₃ melts with an open maximum. The melting temperature of Sb₂Se₃, according to data of various authors, differs and lies in the range 575—617° (¹). This compound is isomorphous with Sb₂S₃; lattice constants: \(a = 11.62\) Å, \(b = 11.77\) Å, and \(c = 3.962\) Å (³). The compound Sb₂Te₃, like the two preceding compounds, melts with an open maximum at 622° (¹). According to the data of work (⁴), this compound at room temperature deviates from stoichiometry and corresponds to a composition of 59.17 mole % Te.

In contrast to the two preceding compounds, Sb₂Te₃ crystallizes in the tetradymite structure, structural type C33 (⁵). Lattice constants: \(a = 4.26\) Å and \(c = 30.42\) Å.

The starting materials used were: antimony SU-000 with contents of Te, As, Pb, Al, Cu (about \(1 \cdot 10^{-4}\%\)); sulfur, twice sublimed in vacuum, with contents of Mg < 0.001, As < 0.01, Fe and Al < \(1 \cdot 10^{-4}\%\); reactive selenium with contents of Pb, Fe, Cu < 0.01, Te < 0.1%; tellurium, remelted from powder and twice sublimed in vacuum, with contents of Mg, Pb, Cu, Bi, Ca, Fe, Al—traces. The content of impurities in the starting materials was determined by qualitative spectral analysis. The alloys were prepared from charges corresponding to the composition of the binary compounds. The Sb₂Te₃ charge was prepared taking into account the deviation of the compound from stoichiometry, according to work (⁴). Charges and alloys were prepared in evacuated sealed quartz ampoules. After melting of the charge, the ampoules with alloys of the Sb₂Se₃—Sb₂Te₃ system were removed from the furnace and rapidly cooled in air. Alloys of the Sb₂S₃—Sb₂Se₃ system, after melting, were cooled together with the furnace. To bring them to the equilibrium state, the alloys were annealed for a long time over the course of several months in sealed ampoules filled with argon, at a temperature of 500° for alloys of the Sb₂S₃—Sb₂Se₃ system and 520—560° for the Sb₂Se₃—Sb₂Te₃ system. Greater tem-

To the article by V. I. Ivlieva and N. Kh. Abrikosov

Fig. 3. Microstructure of alloys, ×200.
a — alloy 90 mol.% Sb₂S₃, 10 mol.% Sb₂Se₃, annealed; b — alloy 75 mol.% Sb₂S₃, 25 mol.% Sb₂Se₃, annealed; c — alloy 58.5 mol.% Sb₂Te₃, 41.5 mol.% Sb₂Se₃, as-cast; d — alloy 58.5 mol.% Sb₂Te₃, 41.5 mol.% Sb₂Se₃, annealed; e — alloy 16 mol.% Sb₂Te₃, 84 mol.% Sb₂Se₃, as-cast; f — alloy 20.2 mol.% Sb₂Te₃, 79.8 mol.% Sb₂Se₃, as-cast.

Alloys lying in the region of the solid solution based on Sb₂Te₃ were subjected to annealing.

The alloys of the Sb₂Se₃—Sb₂Te₃ system reached equilibrium more rapidly when vibrational stirring of the melt was used, followed by holding for 8 h in the liquid–solid state and subsequent cooling together with the furnace in which the melts were prepared. Attainment of equilibrium in the systems under study was monitored by examination of the microstructure. In the Sb₂S₃—Sb₂Se₃ system the polished sections were etched with a 5% solution of FeCl₃ in HCl, while alloys rich in antimony selenide were etched with a 5% solution of NaOH in water. In the Sb₂Se₃—Sb₂Te₃ system, the polished sections of cast alloys were etched with a solution of aqua regia in alcohol (1 : 1/2), and those of annealed alloys—in the region of solid solution—with a 5% solution of FeCl₃ in HCl.

Fig. 1. Phase diagram of the Sb₂S₃—Sb₂Se₃ system

Differential thermal analysis was carried out by recording heating curves of equilibrium alloys on an N. S. Kurnakov pyrometer in Stepanov quartz vessels. The chemical compound SnTe served as the standard in the thermal analysis.

Because of the large supercooling of the alloys of the Sb₂S₃—Sb₂Se₃ system observed during crystallization, the liquidus and solidus lines for this system were constructed from heating curves of equilibrium alloys.

Fig. 2. Phase diagram of the Sb₂Se₃—Sb₂Te₃ system.
a — single-phase alloys, b — two-phase alloys

For alloys of the Sb₂Se₃—Sb₂Te₃ system, the liquidus was constructed from cooling curves, and the solidus from heating curves.

The phase diagram of the Sb₂S₃—Sb₂Se₃ system, constructed from thermal-analysis data and microstructural study, is shown in Fig. 1. A continuous series of solid solutions with a minimum is formed in this system. The minimum lies at a concentration of 27 mol.% Sb₂Se₃ and 545°. Annealed alloys over the entire concentration range had a single-phase polyhedral structure with uniform grain coloration (Fig. 3 a, b). X-ray analysis confirmed that all the alloys studied had the structure of antimony sulfide.

Measurements of the microhardness and thermal conductivity of alloys annealed at 500° showed a continuous variation of these properties as a function of alloy composition. The microhardness has a shallow maximum lying somewhat closer to the Sb₂S₃ compound and corresponding to a value of the order of 140 kg/mm². The thermal conductivity has a shallow minimum lying closer to the Sb₂Se₃ compound and corresponding to a value of about 2 cal/cm·sec °C.

The phase diagram of the Sb₂Se₃—Sb₂Te₃ system, of eutectic type with limited regions of solid solutions, is shown in Fig. 2. The eutectic is formed at a concentration of 18 mol.% Sb₂Te₃ and a temperature of 560°. At the eutectic temperature, about 5 mol.% Sb₂Te₃ dissolves in solid Sb₂Se₃. On the Sb₂Te₃ side there extends a broad region of solid-

solid solutions, lying within the concentration range 34–100 mol.% Sb₂Te₃.

The phase diagram of the Sb₂Se₃—Sb₂Te₃ system, constructed from thermal-analysis data, is well confirmed by the study of the microstructure. Figure 3b shows the microstructure of a cast nonequilibrium alloy containing 58.5 mol.% Sb₂Te₃. This alloy lies in the region of the solid solution based on Sb₂Te₃. A zonal solid solution, formed because of slow diffusion in solid alloys, is clearly visible. The darker central part of the crystals is enriched in Sb₂Te₃; these regions of the crystals crystallize first. As crystallization proceeds, the alloy becomes enriched in Sb₂Se₃; in the photograph these regions of the crystals appear light in color. After annealing, the alloy has a polyhedral, single-phase structure (Fig. 3c). Figures 3d, e show the microstructure of two alloys lying in the two-phase region. The first of these alloys, containing 16 mol.% Sb₂Te₃, lies in the region of primary crystallization of the solid solution based on Sb₂Se₃; the crystals of the solid solution are white. The second figure shows the microstructure of an alloy with 20.2 mol.% Sb₂Te₃, lying in the field of primary crystallization of the solid solution based on Sb₂Te₃. The crystals of the solid solution separate in the form of needle-like crystals. In both figures the eutectic is clearly visible.

On cooling below the eutectic temperature, the limiting concentration of both solid solutions decreases. The boundaries of the solid-solution regions were determined by studying the microstructure of alloys brought to an equilibrium state at a given temperature and quenched from that temperature. Four series of alloys were prepared, their compositions differing by 1 mol.%. These alloys were annealed at 500 and 300° and then quenched in ice water.

Study of the microstructure of an alloy of composition 2 mol.% Sb₂Te₃, quenched from 500 and 300°, showed that the alloy has a single-phase polyhedral structure and, consequently, lies in the region of the solid solution based on the compound Sb₂Se₃. In an alloy of composition 4 mol.% Sb₂Te₃, quenched from 500°, precipitates of a second phase are clearly visible: the solid solution based on the compound Sb₂Te₃. In an alloy of the same composition quenched from 300°, no significant increase in the amount of the second phase was observed. This study made it possible to place the boundary of the limiting solubility of the solid solution based on Sb₂Se₃ between the compositions 2 mol.% Sb₂Te₃ and 4 mol.% Sb₂Te₃. Study of the microstructure of an alloy of composition 34 mol.% Sb₂Te₃, quenched from 500°, showed that a small amount of the second phase—the solid solution based on the compound Sb₂Se₃—separates along the boundary of the solid solution based on Sb₂Te₃. In an alloy of the same composition quenched from 300°, the amount of the second phase increased considerably. An alloy of composition 35 mol.% Sb₂Te₃, quenched from 500°, has a single-phase structure; consequently, the boundary of the solid solution based on Sb₂Te₃ at 500° lies between the compositions 34 mol.% Sb₂Te₃ and 35 mol.% Sb₂Te₃.

In the microstructure of an alloy of composition 38 mol.% Sb₂Te₃, quenched from 300°, slight precipitates of the second phase are visible, whereas an alloy of composition 39 mol.% Sb₂Te₃, quenched from 300°, has a single-phase structure; consequently, the boundary of the solid solution based on Sb₂Te₃ at 300° lies between the compositions 38 mol.% Sb₂Te₃ and 39 mol.% Sb₂Te₃. The properties of alloys lying in the region of solid solutions based on Sb₂Te₃ were measured.

The microhardness has a shallow minimum at a composition close to 55 mol.% Sb₂Te₃. The microhardness of an alloy of this composition corresponds to a value on the order of 50 kg/mm². The lattice thermal conductivity decreases as the concentration of Sb₂Se₃ increases.

Similar changes in microhardness and thermal conductivity were observed by some authors (⁶) in a large group of compounds with the chalcopyrite structure, in the halide compounds of alkali metals.

The electrical conductivity decreases continuously as the content of \(\mathrm{Sb_2Se_3}\) increases.

The curves of the changes in properties in the investigated part of the system have no singular points indicating the presence of ordered structures in alloys annealed at \(560^\circ\).

Institute of Metallurgy
named after A. A. Baikov

Received
16 VI 1964

CITED LITERATURE

¹ M. Hansen, Constitution of Binary Alloys; N. Y.—Toronto—London, 1958.
² W. Hoffmann, Zs. Krystallogr., 86, 225 (1933).
³ N. W. Tideswell, F. H. Kruse, G. D. McCullough, Acta Crystallogr., 10, 99 (1957).
⁴ H. Kh. Abrikosov, L. V. Poretskaya, I. P. Ivanova, ZhNKh, 4, 2525 (1959).
⁵ W. R. Beke-brede, O. G. Guentert, J. Phys. Chem. Solids, 23, 1023 (1962).
⁶ V. P. Zhuze, T. A. Kontorova, ZhTF, 28, 1727 (1958).