Electrochemical study of the quasi-binary thallium(I) telluride–indium(III) telluride solid system

Electrochimica Acta 52 (2007) 8048–8054

Electrochemical study of the quasi-binary thallium(I) telluride–indium(III) telluride solid system E. Zaleska, Z. Sztuba, I. Mucha, W. Gaweł ∗ Department of Analytical Chemistry, Faculty of Pharmacy, Wrocław Medical University, Szewska 38, 50-139 Wrocław, Poland Received 18 April 2007; received in revised form 8 June 2007; accepted 30 June 2007 Available online 7 July 2007

Abstract The Tl2 Te–In2 Te3 solid system was studied using emf measurement of concentration cells to verify the phase diagram for the system published earlier by other authors. Partial molar thermodynamic functions of thallium at 443 K were determined for the ternary phases of this system. The study produced substantial evidence corroborating the formation of a new compound not found earlier. © 2007 Elsevier Ltd. All rights reserved. Keywords: Phase diagram; Thallium indium telluride system; Concentration cell emf

1. Introduction

2. Published data on the Tl2 Te–In2 Te3 system

Our previous studies showed [1,2] that the electromotive force (emf) measurement of concentration cells is a highly effective method for arriving at accurate and reliable results of phase studies on binary metal and semi-metal systems. Another major advantage of the method is the possibility of determining the thermodynamic characteristics of a system. The emf method can be successfully used to establish not only the exact number and composition, but also the range of existence and even the nature of the solid phases formed by the components of a system. Some of the numerous thallium(I) telluride quasibinary systems, i.e. those of the type Tl2 Te–Mx Tey , have been examined by our research group using the emf method; these are Tl2 Te–(Ag2 Te [3], Bi2 Te3 [4], Sb2 Te3 [5], SnTe [6], Cu2 Te [7], HgTe [8], and CdTe [9]). The results obtained, together with those of the relevant phase studies which involved thermal analysis, necessitate the verification of the phase diagrams for the systems described earlier. The aim of the present study was to investigate the Tl2 Te–In2 Te3 system (the next one in the Tl2 Te–Mx Tey “family”), using the emf method.

The title system was formerly examined by Babanly and Kuliyev [10]. In their study they employed differential thermal analysis (DTA), X-ray diffraction, and microhardness measurements and also constructed a phase diagram for this system, which is depicted in Fig. 1. From the diagram it follows that the components of the system formed one chemical compound at 50 mol% In2 Te3 , i.e. the TlInTe2 , which melts congruently at 1051 K and is rather a phase of variable composition (γ) with an existence range of 5 mol% and 3 mol% at 921 K and below 700 K, respectively. The terminal solid solutions α and β, on the Tl2 Te and In2 Te3 matrix, respectively, were formed 8 mol% wide each. It is essential to note that the above data were reported in 1976. The decades that followed have witnessed rapid technological advances in many fields, and this includes the experimental techniques used by Babanly and Kuliyev [10]. It seems therefore desirable to re-examine the system with a different method, the emf measurement of concentration cells. 3. Principles of emf measurement in studies on metal systems

∗

Corresponding author. Tel.: +48 71 7840305; fax: +48 71 7840307. E-mail address: [email protected] (W. Gaweł).

0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.06.078

In phase and thermodynamic studies on binary and multicomponent metal systems, use is made [11] of the following concentration cells:

E. Zaleska et al. / Electrochimica Acta 52 (2007) 8048–8054

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where n = valency of the cation Mn+ , F = Faraday’s constant, and E = emf. When the measurements are performed over a broad temperature range, the temperature coefficient of the emf (dE/dT) can be determined with sufficient accuracy, which enables the partial molar entropy of the metal M (SM ) to be calculated: SM = nF

dE . dT

The partial molar enthalpy of the metal M (HM ) can be calculated from the relation:   T dE −E . HM = GM − TSM = nF dT Fig. 1. Phase diagram for the system Tl2 Te–In2 Te3 (according to [10]).

where M and M are any metals. Reliable data can be obtained from the results of emf measurements of reversible cells only. The thermodynamic reversibility of the cell demands that: (1) the metal M should be less noble than the metal M so that no reaction is possible between metal M and Mn+ ions; (2) no process other than M  Mn+ + ne− should occur on the surfaces of the electrodes; (3) the cations Mn+ should be stable throughout the measurements; (4) the electrolyte should embody solely ionic conductivity; (5) the emf measurement should be performed in such a way that no current flow occurs in the cell; and (6) no thermoelectric effect should influence the emf of the cell. Accordingly, all the materials used in the construction of the cells must be robust enough to resist the action of the metals and the electrolyte at the applied temperatures of the measurement. The electrolyte and the inert gas within the cell body should be purified from moisture and oxidizing or reducing substances. The choice of the electrolyte depends on the nature of the metals and the range of the temperature to be employed, but any ionic liquid (such as molten salt) or any polar solvent (e.g. glycerin) that meets the above requirements can be used to serve as an electrolyte. To avoid thermoelectric effects, all the connecting wires should be made of the same metal (tungsten or molybdenum) and the whole electrolytic cell should be placed in a steel block within an electric furnace. The sole experimental criterion for the thermodynamic reversibility of the cell is a constant value of its emf at a given temperature regardless of the duration of the measurement and the preceding heating or cooling. Using the results of emf measurement of the cell, the partial molar Gibbs free energy of the metal M (GM ) may be calculated directly according to the known relation (as, for example, in [12]): GM = −nFE

The emf measurements of concentration cells are useful not only for the verification of phase diagrams for metal systems constructed by other methods, but also for the determination of the ordering state in the solid solutions formed in these systems, especially in the intermediate phases of variable composition. The dE/dT values (or the SM values of the less noble metal which can be calculated from them) are closely related to the ordering state. The potentials of the one-phase alloy electrodes decrease with increasing content of the less noble component in the alloy, whereas the potentials of the two-phase alloy electrodes are constant. When within the temperature range applied to the emf measurements a phase transition occurs in the alloy electrode or in the reference electrode, the temperature coefficients of the emf below and above the transition point will take on different values. Thermodynamic studies involving emf measurements of concentration cells to investigate ternary metal systems of the type M–M –M (where M is a metal other than M and M ) were initiated in the early 1960s. The quasi-binary Tl2 Te–In2 Te3 system represents a polythermal cross-section of the ternary Tl–In–Te system. 4. Experimental 4.1. Materials The components of the examined system, i.e. thallium telluride and indium telluride, were prepared from high-purity elements: 99.9% thallium, 99.9% indium, and 99.99% tellurium (all from Aldrich Chem. Co.). The metal tellurides were synthesized by simple fusion of stoichiometric quantities of the elements, weighed with an accuracy of ±0.0001 g in quartz tubes under purified argon atmosphere (5 N pure, BOC Gazy, Poznan) and then stirred for 15 min at a temperature ca. 100 K higher than the melting point of the respective metal telluride. 4.2. Apparatus and measurement The Tl2 Te + In2 Te3 solid alloys were investigated via the following reversible concentration cells:

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The scheme of the electrolytic cell is presented in Fig. 2. The principles underlying the functioning of the cell are identical to those of the early apparatus described elsewhere [13], but the design has been modified since then. The container for the electrolyte was a quartz crucible placed within the quartz body of the apparatus. The body was hermetically closed with a glass (Thermisil) head. Both the thallium reference electrode and the alloy electrodes were prepared by melting 1–2 g of the metal (alloy) in small test tubes (5 mm in diameter, 50 mm long) made of glass or quartz according to the melting point of the alloy and then letting them solidify. The tubes had small holes 15 mm above the bottom to enable contact between the electrode and the electrolyte. The electrolyte was a solution of a small amount of TlCl (99.9% pure, BDH Chemicals Ltd.) and NaCl (p.a., POCh) in glycerin (p.a., POCh). The glycerin was heated prior to use at 435 K for several hours to remove traces of moisture. A 5-g piece of the thallium metal was put into the container to prevent the Tl+ ions from being oxidized in the electrolyte. The apparatus was filled with pure argon. The electrodes, connected to the voltmeter (F-541 Meratronik) with tungsten wires sealed in quartz tubes, were immersed in the electrolyte. The voltmeter was connected to a computer for the processing and display of the experimental data.

To enable the electrode alloys to be homogenized, the emf measurements did not commence until 7 days after the cell had been assembled and heated at 400 or 450 K (according to the temperature range applied). Throughout the experiment, the temperature of the cell was alternately raised and then decreased within the appropriate temperature range. The accuracy of measurements with the Ni/Ni,Cr thermocouple was ±1 K. The emf readings were taken every hour with an accuracy of ±0.1 mV, and then the temperature was increased or decreased by ca. 10 K. Each series of measurements took 2–3 weeks. The electrode reactions are: (−) reference electrode (anode) Tl → Tl+ + e− (+) alloy electrode (cathode) Tl+ + e− → Tl, and then the deposited thallium metal reacts with the alloy electrode material: In2 Te3 + 6Tl → 2In + 3Tl2 Te. Generally, in the case of any alloy composition, the cell reaction is: [nTl2 Te + mIn2 Te3 ] + 6zTl → 2zIn + [(n + 3z)Tl2 Te + (m − z)In2 Te3 ] where 0 < m ≥ z. This reaction results in changing the electrode’s alloy composition, i.e. it changes the proportion of the two coexisting phases within a given two-phase region. The emf value does not change, however, as long as the crystal structures of the phases remain the same. Within another two-phase region, where the specific structure of one of the phases is different, the emf assumes another value. Within the limits of a solid solution (one-phase alloy), the change in composition manifests in a continuous change in crystal lattice parameters and, consequently, the emf decreases continuously with increasing content of the less noble component. In principle, this cell reaction is nothing but the simple reduction of indium(III) with thallium metal. From the practical point of view it is worthless, while it is important in phase examinations of multicomponent metal systems. It is essential to note that the exact measurement of the emf of a concentration cell demands that no current flow occurs in the cell. Consequently, an electrode reaction cannot in fact occur, so the emf represents solely a tendency of the reaction. Nevertheless, both the emf and the dE/dT values enable an exact determination of the thermodynamic parameters of the reaction. In the case of the present study, the symbols GTl , STl , and HTl are changes in the Gibbs free energy, entropy, and enthalpy of thallium, respectively, in the electrode reaction. 5. Results and discussion

Fig. 2. The scheme of the electrochemical cell used in the examinations of binary metal telluride systems.

Thirty-six different thallium(I) telluride–indium(III) telluride alloys of compositions covering the whole concentration range (from 2.00 to 97.00 mol% In2 Te3 ) were investigated using the concentration cell emf method within the temperature range of 393–493 K. In all cases the emfs showed a linear temperature

E. Zaleska et al. / Electrochimica Acta 52 (2007) 8048–8054

dependence (Fig. 3a and b) that can be expressed in the form of the equation E = a + bT, where E = emf; T = temperature in K; and a and b are constants, b being the temperature coefficient of E (b = dE/dT). However, the E versus T plots for the cells containing alloy electrodes in the range of 45.00–52.00 mol% In2 Te3 show a “break” at 443 K (Fig. 3b), which is why in this concentration region the emf measurements were conducted separately within two temperature ranges, 393–443 and 443–493 K. The experimental and calculated results are summarized in Tables 1 and 2. The tables include the compositions of the alloys examined, the relevant phase regions, the equations E = a + bT, and the emf values at 443 K (E443 ). The temperature 443 K was chosen as it falls in the middle of the measurement temperature range (393–493 K). The fact that this value is identical to the break-point mentioned above, is sheer coincidence. The values for the constants a and b of the equations and for E443 (including relevant errors) were calculated by the least squares method using the experimental data obtained from the particular series of measurements. A major advantage inherent in the concentration cell emf method is the possibility of determining the thermodynamic characteristics of the alloys examined. Knowing the E443 and dE/dT values made it possible to calculate the following partial molar thermodynamic functions of thallium in the solid Tl2 Te + In2 Te3 alloys at 443 K: free enthalpies (GTl ), entropies (STl ), and enthalpies (HTl ). Relevant values are shown in Tables 3 and 4. From Tables 1 and 2 it can be inferred that the emf (E443 ) and temperature coefficient (b) values for the telluride alloys are approximately constant within the concentration regions 29.0–42.0 mol% In2 Te3 , 45.0–48.0 mol% In2 Te3 , and

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Fig. 3. Examples of the temperature dependence of emf of the cells with Tl2 Te + In2 Te3 alloy electrodes: (a) 5.0 mol% In2 Te3 ; (b) 52.0 mol% In2 Te3 .

Table 1 Results of electrochemical studies on the Tl2 Te–In2 Te3 solid alloys within the temperature range 393–493 K Composition (mol% In2 Te3 )

Phase region

Equation E = a + bT

E443 (mV)

Terminal solid solution α (one-phase area)

(147.70 ± 0.97) + (0.1868 ± 0.0047)T (192.91 ± 0.42) + (0.1036 ± 0.0021)T (188.33 ± 0.71) + (0.1356 ± 0.0039)T (115.62 ± 1.57) + (0.3144 ± 0.0081)T (207.74 ± 0.53) + (0.0973 ± 0.0031)T (141.95 ± 1.66) + (0.2862 ± 0.0099)T (73.51 ± 2.10) + (0.4437 ± 0.0116)T (60.41 ± 1.66) + (0.5372 ± 0.0119)T (81.87 ± 1.59) + (0.5370 ± 0.0134)T

230.5 238.8 248.4 255.9 250.8 268.7 270.1 298.4 319.8

± ± ± ± ± ± ± ± ±

1.0 0.4 0.7 1.6 0.5 1.7 2.1 1.6 1.6

α + compound 4Tl2 Te·3In2 Te3

(494.56 ± 1.73) − (0.2390 ± 0.0366)T (510.42 ± 1.28) − (0.2688 ± 0.0369)T (499.69 ± 49.7) − (0.2373 ± 0.0347)T (492.54 ± 0.95) − (0.2223 ± 0.0276)T

388.4 391.3 394.6 394.1

± ± ± ±

1.6 1.3 1.1 0.9

Arithmetic mean for the phase region:

(499.30 ± 13.41) − (0.2419 ± 0.0340)T

392.1 ± 1.2

55.0 65.0 75.0 90.0 90.0

γ +β

(321.41 ± 2.29) + (0.3634 ± 0.0499)T (291.59 ± 1.76) + (0.4332 ± 0.0402)T (331.09 ± 1.55) + (0.3384 ± 0.0362)T (302.81 ± 1.92) + (0.4059 ± 0.0433)T (314.35 ± 1.73) + (0.3819 ± 0.0389)T

482.4 483.5 481.0 482.7 483.5

Arithmetic mean for the phase region:

(312.25 ± 1.85) + (0.3846 ± 0.0417)T

482.6 ± 1.6

95.5 97.0 97.0

Terminal solid solution β (one-phase area)

(309.53 ± 2.46) + (0.4170 ± 0.0458)T (217.48 ± 4.29) + (0.6374 ± 0.0647)T (190.98 ± 5.08) + (0.7087 ± 0.0706)T

494.3 ± 2.3 499.9 ± 4.1 504.9 ± 4.9

2.0 5.0 8.0 10.0 12.0 15.0 18.0 20.0 25.0 29.0 30.0 35.0 42.0

± ± ± ± ±

2.0 1.6 1.4 1.7 1.5

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Table 2 Results of electrochemical studies on the Tl2 Te–In2 Te3 solid alloys within the concentration range 45–52 mol% In2 Te3 Composition (mol% In2 Te3 )

Phase region

Equation E = a + bT

E443 (mV)

Compound 4Tl2 Te·3In2 Te3 + ␥B

(433.74 ± 0.70) − (0.0580 ± 0.0311)T (444.06 ± 0.50) − (0.0937 ± 0.0165)T (433.39 ± 0.27) − (0.0628 ± 0.0073)T (432.91 ± 0.31) − (0.0612 ± 0.0100)T

408.1 402.5 405.6 405.8

Arithmetic mean for the phase region:

(436.03 ± 0.45) − (0.0689 ± 0.0162)T

405.5 ± 0.5

49.0 50.0 51.0 52.0

(423.80 ± 0.37) + (0.0698 ± 0.0121)T (455.57 ± 0.31) + (0.0506 ± 0.0067)T (460.93 ± 0.22) + (0.0392 ± 0.0040)T (466.73 ± 0.37) + (0.0329 ± 0.0067)T

454.7 478.0 478.3 481.3

(397.38 ± 0.16) − (0.0022 ± 0.0026)T (407.32 ± 0.15) − (0.0074 ± 0.0025)T (405.11 ± 0.13) − (0.0066 ± 0.0020)T

396.4 ± 0.2 404.0 ± 0.2 402.2 ± 0.1

Temperature range: 393–443 K 45.0 46.0 47.0 48.0

γ B (one-phase area)

Temperature range: 443–493 K 45.0 47.0 48.0

Compound 4Tl2 Te·3In2 Te3 + γ A

Arithmetic mean for the phase region: 49.0 50.0 51.0 52.0

γA (onephase area)

55.0–90.0 mol% In2 Te3 . It is essential to note, on the other hand, that in these regions E443 takes on different values (392.1, 405.5, and 482.6 mV, respectively) which, in turn, is an indication that the phase compositions differ from one region to another. Although the difference between the first two figures is relatively small (no less than 11.7 mV, taking into account the maximal errors 392.1 ± 1.2 and 405.5 ± 0.5 mV, respectively), it is sufficiently large to be considered as evidence supporting the existence of two different phase regions. This conclusion was additionally confirmed by the difference in the temperature coefficients dE/dT: within the range 29.0–42.0 mol% (Table 1) the difference is over three times that within 45.0–48.0 mol% In2 Te3 (Table 2) at lower temperatures (393 K–443 K) and over 44 times the one at higher temperatures (443 K–493 K).

± ± ± ±

± ± ± ±

0.9 0.5 0.2 0.3

0.3 0.3 0.2 0.4

(403.27 ± 0.15) − (0.0054 ± 0.0024)T

400.9 ± 0.4

(370.92 ± 1.40) + (0.1943 ± 0.0536)T (374.93 ± 1.13) + (0.2190 ± 0.0426)T (292.47 ± 2.11) + (0.4334 ± 0.0569)T (305.38 ± 1.40) + (0.3985 ± 0.0403)T

457.0 472.0 480.0 481.9

± ± ± ±

0.8 1.0 1.9 1.4

The constancy of the emf values within the above phase regions provides evidence for the assertion that these are twophase regions. It must be noted, however, that there are some regions (2.0–25.0 mol% In2 Te3 , 49.0–52.0 mol% In2 Te3 , and 95.5–97.0 mol% In2 Te3 ) where the emf values were found to vary with each change in composition. This substantiates the occurrence of one-phase alloys. The above data are plotted in Fig. 4, which relates the emf (E443 ) to the In2 Te3 content. The plot makes it clear that there are different phase regions in the Tl2 Te–In2 Te3 system, thus enabling us to arrive at important conclusions. The results provide strong evidence for the formation of a new compound that has not been found earlier. The composition of the compound is in the range 42.0 < mol% In2 Te3 < 45.0, its

Table 3 Partial molar thermodynamic functions of thallium at 443 K in the Tl2 Te–In2 Te3 solid system within the temperature range 393–493 K Phase region

−GTl (kJ mol−1 )

2.0 5.0 8.0 10.0 12.0 15.0 18.0 20.0 25.0

Terminal solid solution α (one-phase area)

22.2 23.0 24.0 24.6 24.2 25.9 26.1 28.8 30.9

29.0–42.0

α + 4Tl2 Te·3In2 Te3

37.8 ± 0.1

55.0–90.0

γ +β

46.6 ± 0.2

37 ± 10

30.1 ± 4.4

Terminal solid solution β (one-phase area)

47.7 ± 0.2 48.2 ± 0.4 48.7 ± 0.5

40 ± 10 60 ± 10 70 ± 10

29.9 ± 4.5 21.0 ± 4.7 18.4 ± 4.7

Composition (mol% In2 Te3 )

95.5 97.0 97.0

± ± ± ± ± ± ± ± ±

0.1 0.0 0.1 0.2 0.1 0.2 0.2 0.2 0.2

STl (J mol−1 K−1 ) 18.0 9.9 13.1 30.3 9.4 27.6 42.8 51.8 51.8

± ± ± ± ± ± ± ± ±

0.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

−23 ± 9

−HTl (kJ mol−1 ) 14.3 18.6 18.2 11.2 20.0 13.7 7.1 5.8 7.9

± ± ± ± ± ± ± ± ±

0.1 0.0 0.1 0.1 0.1 0.2 0.2 0.2 0.1

48.2 ± 2.3

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Table 4 Partial molar thermodynamic functions of thallium at 443 K in the Tl2 Te–In2 Te3 solid system within the concentration range 45–52 mol% In2 Te3 Composition (mol% In2 Te3 )

Phase region

−GTl (kJ mol−1 )

STl (J mol−1 K−1 )

−HTl (kJ mol−1 )

Temperature range: 393–443 K 45.0–48.0

4Tl2 Te·3In2 Te3 + ␥B

39.1 ± 0.1

−6.7 ± 1

42.1 ± 0.0

γ B (one-phase area)

43.9 46.1 46.2 46.4

4Tl2 Te·3In2 Te3 + γ A

38.7 ± 0.0

γ A (one-phase area)

44.1 45.5 46.3 46.5

49.0 50.0 51.0 52.0 Temperature range: 443–493 K 45.0–48.0 49.0 50.0 51.0 52.0

component molar ratio being Tl2 Te:In2 Te3 = 4:3 (at 42.9 mol% In2 Te3 ), to which the formula Tl8 In6 Te13 may be ascribed. Moreover, the negative STl values within the region from 29 to 48 mol% In2 Te3 (Tables 3 and 4) confirm the existence of any highly ordered phase, undoubtedly the 4Tl2 Te·3In2 Te3 compound. It was impossible, however, to determine the melting point of the compound in this study because of the relatively narrow range of the applied experimental temperature (393–493 K). In any event, the compound melts at a temperature higher than 493 K. The existence of the compound 4Tl2 Te·3In2 Te3 has been confirmed [14] by making use of two other independent techniques: thermal analysis and X-ray diffraction. Within the region 48.0 < mol% In2 Te3 < 55.0 a phase γ of variable composition is formed which manifests in the continuous changes of the E443 value within the concentration range 49.0 < mol% In2 Te3 < 52.0 over both temperature ranges (393–443 and 443–493 K) (Table 2). The E443 values become constant once the concentration of 55.0 mol% In2 Te3 has been reached. Moreover, the dE/dT values vary continuously within this region, thus indicating that the γ phase should be considered as an ordinary solid solution. Consequently, there is no evidence

± ± ± ±

± ± ± ±

0.0 0.0 0.0 0.0

0.1 0.1 0.2 0.1

6.7 4.9 3.8 3.2

± ± ± ±

1 1 1 1

−0.5 ± 0.1 19 21 41 39

± ± ± ±

10 10 10 10

40.9 44.0 44.5 45.0

± ± ± ±

0.0 0.0 0.0 0.0

38.9 ± 0.0 35.8 36.2 28.2 29.5

± ± ± ±

4.4 4.4 4.5 4.4

that the formula of the γ phase is TlInTe2 (50.0 mol% In2 Te3 as reported by Babanly and Kuliyev [10]). From the data in Table 2 it can be seen that in the case of concentration cells with alloy electrodes of an In2 Te3 content ranging from 45.0 to 52.0 mol%, the slopes of the straight lines (E = a + bT) within the range 393–443 K differ substantially from those observed within the range 443–494 K, the temperature coefficients of the emfs in the latter range being several times those in the former. This finding provides evidence for the occurrence of a polymorphic transition (A  B) in the γ phase. The temperature of the transition can be determined from the intersection of the respective lines E = a + bT for both temperature ranges (Fig. 3b). The mean value of the temperature is ca. 443 K. From Tables 3 and 4 it is apparent that at a given temperature the STl values differ from one-phase region to another. The differences are attributable to the different structures of the phases formed. The most important conclusion can be drawn, however, from the comparison of the STl values within the two temperature ranges (393–443 and 443–493 K) in the alloys from the one-phase region 48.0 < mol% In2 Te3 < 55.0. The differences in the STl values are indicative of a significant change in the crystal structure due to the polymorphic transformation of the γ phase. The alloys whose compositions range from 2.0 to 25.0 mol% In2 Te3 and from 95.5 to 97.0 mol% In2 Te3 (Table 1) show continuous changes in the emf (E443 ) and dE/dT values, which provides evidence for the occurrence of the terminal solid solutions α and β within the regions 0 < mol% In2 Te3 < 29.0 and 90.0 < mol% In2 Te3 < 100, respectively. 6. Conclusions

Fig. 4. The emf values at 443 K (E443 ) of the cells with alloy electrodes of compositions covering the whole concentration range of the system Tl2 Te–In2 Te3 .

The results obtained in this study enabled us to verify the phase diagram (Fig. 1) proposed by Babanly and Kuliyev [10]. The corrected phase diagram for the Tl2 Te–In2 Te3 system (shown in Fig. 5 of the present study) makes it clear that the application of the emf measurement method revealed new data on the system:

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Kuliyev approached 3 mol% (at the measurement temperatures applied in the present study); – the γ phase undergoes a polymorphic transition (A  B) at 443 K (not detected earlier). References

Fig. 5. Corrected phase diagram for the system Tl2 Te–In2 Te3 (this study).

– the terminal solid solution α covers the concentration range from 0 to 28 mol% In2 Te3 (3.5 times that reported by Babanly and Kuliyev [10]); – a new chemical compound 4Tl2 Te·3In2 Te3 (Tl8 In6 Te13 ) was formed in the system (not found by Babanly and Kuliyev [10]); – the range of existence of the γ phase covered approximately 7 mol%, while the one reported by Babanly and

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