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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 1 2022 ISSN 0005-2531 (Print)
UDC 544.31:546.81'86'24
THERMODYNAMIC PROPERTIES OF Sb2Te3-BASED SOLID SOLUTIONS IN THE
SnTe-Sb2Te3 SYSTEM
A.E.Seyidzade, A.A.Aghayeva, E.N.Orujlu, S.Z.Imamaliyeva
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
aynseyidzade@yahoo.com
Received 20.10.2021 Accepted 26.11.2021
The SnTe-Sb2Te3 system was investigated by using EMF measurements of reversible concentration cells relative to the SnTe electrode in the 300-400 K temperature range. The formation of solid solutions (up to 20 mol%) based on Sb2Te3 was confirmed. Based on EMF measurements, the equations of the temperature dependences of the EMF for solid solutions based on Sb2Te3 with compositions of 5, 10, and 20 mol% SnTe were obtained and these results were furtherly used to calculate the partial thermo-dynamic functions of SnTe in alloys. The standard thermodynamic functions of the formation of solid solutions of the above compositions were calculated by graphical integration of the Gibbs-Duhem equation along the SnTe-Sb2Te3 section.
Keywords: Sn-Sb-Te system, phase diagram, solid solutions, EMF method, thermodynamic properties.
doi.org/10.32737/0005-2531-2022-1-83-88 Introduction
The ternary tetradymite-like compounds formed in the A^Te-B^Tes (AIV-Ge, Sn, Pb; BV-Sb, Bi) systems have been intensively studied for many years as promising thermoelec-trics with low thermal conductivity [1-4] and as materials with a phase transition [5, 6]. Due to complex multilayer structures, they have lower values of the lattice thermal conductivity compared to the starting binary compounds [1, 2]. In addition, it has been recently established that these compounds are three-dimensional topo-logical insulators and are very promising for use in various areas of high technologies, from spintronics and quantum computing to medicine and security systems [7-10].
The development and design of new mul-ticomponent materials are based on reliable complexes of mutually consistent data on phase equilibria and thermodynamic functions of the corresponding systems [11, 12]. One of the most effective ways to obtain similar complexes is the use of the electromotive force (EMF) method. Various modifications of the EMF method with liquid [13-18] and solid [13, 14, 1921] electrolytes are successfully used in such complex studies of chalcogenide systems.
Phase equilibria in the Sn-Sb-Te ternary system have been studied in a number of works and most of these results are summarized in
[22]. In [23-25], the phase diagrams of the SnTe-Sb2Te3 quasi-binary section, and in [25], a complete diagram of the Sn-Sb-Te system is presented. According to [23-25], a single ternary compound - SnSb2Te4 with incongruent melting at 879K [23] or 876 K [25] is formed in the SnTe-Sb2Te3 quasi-binary system. The ther-modynamic properties of the SnSb2Te4 compound are studied by the EMF method in [26].
In [27], we presented a new refined phase diagram of the SnTe-Sb2Te3 system and there is a new compound with the SnSb4Te7 composition, in addition to known SnSb2Te4. Both compounds are formed by peritectic reactions at 868 K and 866 K, respectively. The system also revealed wide ranges of solid solutions based on the initial compounds SnTe (~10 mol%) and Sb2Te3 (-20 mol%).
This work presents the results of studying the thermodynamic properties of solid solutions based on Sb2Te3 in the SnTe-Sb2Te3 system using EMF method.
Experimental part
Alloys for EMF measurements were obtained by co-melting of high purity elementary components from Alfa Aesar in evacuated quartz ampoules. After melting, the alloys were quenched in icy water at 1000 K and then subjected to stepwise annealing at 650 K (1000 h.) and 400 K (100 h.).
In this way, alloys with compositions of 5, 10, 20 mol% SnTe with addition of 5 at% tellurium excess were prepared. Electrochemical cells
(-) SnTe(solid) | liquid electrolyte, Sn2+(SnTe-Sb2Te3- Te) (+) (1)
were composed and their EMF was measured in the 300-400 K temperature interval.
The electrodes of the (1) type cells were prepared by pressing powdered SnTe (left electrode) and annealed alloys of the studied system (right electrodes) in the cylinder form of tablets with a diameter of ~6 mm and a thickness of 2 mm.
Glycerol with KCl solution was used as the electrolyte. Chemically pure glycerol, KCl, and SnCl2 were used for its preparation. Taking into account the inadmissibility of the presence of moisture and oxygen in the electrolyte, the glycerol was thoroughly dehydrated and degassed by pumping at a temperature of ~400 K, and anhydrous KCl and SnCl2 were added.
The techniques of assembling an electrochemical cell and measuring EMF are described in detail in [13, 14, 16]. EMF measurements were carried out in a cell with an inert atmosphere using a Keithley 2100 6 1/2 digital multimeter.
The first equilibrium EMF values were obtained after keeping the cell at ~400 K for 40-60 h. The subsequent values were obtained every 3-4 h. after a certain temperature was es-
tablished. The values that did not differ from each other by more than 0.2 mV, when measured repeatedly at a given temperature regardless of the direction of temperature change were taken as equilibrium EMF values.
Results and discussion
The results of the measurements of the EMF of (1) type cells were reproducible. The formation of up to 20~ mol%. Sb2Te3 - based solid solutions was confirmed. Figure 1 presents a powder diffraction pattern of an alloy with a composition of 5 mol% Sb2Te3.
Taking into account the linearity of the EMF dependences on the composition (Figure 2), they were processed using the Excel computer program by the least-squares method and presented in the form of linear equations of the type
E = a + bT ± t[(S2 /n) + S2 • (T - T)2 J12 (2) recommended in [13, 14]. Here n - is the number of pairs of values E (mV) and T (K); SE and Sb -are the variances of individual measurements of EMF and coefficient b, respectively; T - average absolute temperature, t - Student's t-test. At a confidence level of 95% and the number of experimental points n>20, Student's test is t<2.
Table 1 shows the steps of processing the experimental data of the T and E pairs for an alloy with a composition of 5 mol% SnTe. Table 2 presents the obtained equations of type (2) for all the studied alloys.
♦ ♦ - Sb2Te3 : v- Te
\ 7
• ♦ •
♦ 1 V V A . vtU- ♦ iL ♦ ♦ ♦
10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 Diffraction Angle [°26]
Fig. 1. Powder diffraction pattern of an alloy with a composition of 5 mol% Sb2Te3.
THERMODYNAMIC PROPERTIES OF Sb2Te3-BASED SOLID.
85
Fig. 2. Temperature dependences of the EMF of (1) type cells for alloys of the SnTe-Sb2Te3 system. Phase areas: 1 - P(Sn0.05Sb1.9Te2.9)+Te; 2 -P(Snc.iSbi.9Te2.8)+Te; 3 - p^Sb^Te^+Te.
Table 1. Results of computer
processing of EMF measurements for a sample with a composition of 5 mol% Sb2Te3
Ti, K E1, mV T; - T E1( T - T ) (T; - T)2 E E; - E (E; -E)2
299.7 204.25 -51.71 -10561.09 2673.58 203.57 0.68 0.47
301.8 203.24 -49.61 -10082.06 2460.82 203.75 -0.51 0.26
305.6 204.53 -45.81 -9368.84 2098.25 204.09 0.44 0.20
307.1 204.96 -44.31 -9081.09 1963.08 204.22 0.74 0.55
311.7 204.12 -39.71 -8104.92 1576.62 204.62 -0.50 0.25
315.6 205.31 -35.81 -7351.47 1282.12 204.97 0.34 0.12
319.4 204.75 -32.01 -6553.37 1024.43 205.30 -0.55 0.31
323.3 206.23 -28.11 -5796.44 789.98 205.65 0.58 0.34
329.5 205.65 -21.91 -4505.11 479.90 206.19 -0.54 0.29
334.7 206.86 -16.71 -3455.94 279.11 206.65 0.21 0.04
337.3 206.02 -14.11 -2906.26 199.00 206.88 -0.86 0.74
341.8 207.22 -9.61 -1990.69 92.29 207.28 -0.06 0.00
346.9 206.45 -4.51 -930.40 20.31 207.73 -1.28 1.63
349.1 207.32 -2.31 -478.22 5.32 207.92 -0.60 0.36
350 208.55 -1.41 -293.36 1.98 208.00 0.55 0.30
353.2 207.95 1.79 372.92 3.22 208.28 -0.33 0.11
356.5 208.99 5.09 1064.46 25.94 208.57 0.42 0.17
359.5 209.54 8.09 1695.88 65.50 208.84 0.70 0.49
362 208.86 10.59 2212.52 112.22 209.06 -0.20 0.04
367.3 209.66 15.89 3332.20 252.60 209.52 0.14 0.02
371.4 210.56 19.99 4209.80 399.73 209.89 0.67 0.45
374.3 210.99 22.89 4830.26 524.10 210.14 0.85 0.72
379.7 209.58 28.29 5929.72 800.51 210.62 -1.04 1.08
382.9 210.69 31.49 6635.33 991.83 210.90 -0.21 0.04
386.1 211.23 34.69 7328.27 1203.63 211.18 0.05 0.00
389.4 210.44 37.99 7995.32 1443.49 211.47 -1.03 1.06
391.1 211.98 39.69 8414.19 1575.56 211.62 0.36 0.13
395.7 212.33 44.29 9404.80 1961.90 212.03 0.30 0.09
398.8 212.89 47.39 10089.57 2246.13 212.30 0.59 0.35
400.8 212.56 49.39 10499.05 2439.70 212.48 0.08 0.01
T=351.41 E=208.12
well-known technique [13, 14]. In equation (6); x - are molar fractions of SnTe; Af Z 0( Sb2Te3)
and AfZ0(SnTe) - are the standard thermodynamic functions of the formation of Sb2Te3 and SnTe.
In calculations by equation (6), we used literature data on the standard integral thermo-dynamic functions of SnTe and Sb2Te3 (Table 4). The sets of standard integral thermodynamic functions of these compounds used in the calculations are mutually consistent and completely reliable.
The standard heats of formation and entropy of both compounds, recommended in various reference books and review articles [28-33], practically coincide, and the values of their standard Gibbs free energies of formation, which we calculated from the enthalpy and entropy of formation, are very close to the data of works performed by the EMF method [30-33].
Table 2. Temperature dependences of EMF of (1) type cells for alloys of the SnTe-Sb2Te3 system in the 300-410 K temperature range_
Phase composition E = a + bT ±2SE(t)
P(Sno.o5Sb1.9Te2.9)+Te 177.16 + 0.0881 T ± 2 ¡(O.3^)+1.2-10 5(T-351.4)2^ 12
P(Sno.iSb1.9Te2.8)+Te 156.25 + 0.0561 T ± 2 [0 2^0)+ 9.6-10 6 (T -351.4)2^
P(Sno.2Sbi.9Te26)+Te 150.46 + 0.0320 T ± 2 (0 3^0)+1.2-10 5 (T -351.4)2^ 12
Table 3. Partial thermodynamic functions of SnTe in Sn-Sb-Te alloys at 298 K
Phase composition - AG SnTe - AH SnTe AS SnTe, J/(K-mol)
Kj/mol
P(Sno.o5Sbi.9Te29)+Te 39.26±0.08 34.19 ± 0.48 17.01 ± 1.35
P(Sno.iSbi.9Te28)+Te 33.38±0.07 30.15 ± 0.42 10.82 ± 1.19
P(Sno.2Sbi.9Te26)+Te 30.88±0.08 29.04 ± 0.48 6.18 ± 1.36
Table 4. Standard integral thermodynamic functions of ternary compounds of the SnTe-Sb2Te3 system
Phase -Af G 0(298 K ) -A, H 0(298 K ) Af S0(298 K ) S0(298 K )
Kj/mol J/(K-mol)
SnTe [28-3o] 60.8 ± 1.0 60.7 ± 0.8 - 101.3 ± 4.2
Sb2Te3 [28-3o] 58.6 ± 1.0 56.5 ± 0.8 - 246.4 ± 2.1
(SnTe)o.o5(Sb2Te3)o.95 60.7 ± 1.0 58.4 ± 0.9 7.7 ± 3.0 246.9 ± 5.2
(SnTe)o.i(Sb2Te3)o.9 62.3 ± 1.1 59.9 ± 1.0 8.0 ± 3.0 239.9 ± 5.3
(SnTe)o.2(Sb2Te3)o.8 65.2 ± 1.1 63.2 ± 1.1 6.7 ± 3.0 224.1 ± 5.4
From the data of Table 2 and using the following thermodynamic relations
AG, = -zFE,
AS, = zf( — I = zFb ,
l ST L
AHi = -zF
E - T
SE
JT, P
= -zFa
(3)
(4)
(5)
the partial molar thermodynamic functions of the SnTe in alloys at 298 K were calculated (Table 3). The calculation of the standard thermodynamic functions of the formation of P-solid solutions was carried out by graphical integration of the Gibbs-Duhem equation
Af Z0 (p - phase) = (1 - x) | Adx +
x
)J
+(1 - x)AfZ0 (Sb2Te3 ) + xAfZ0 (SnTe) (6)
along the section SnTe-Sb2Te3 according to the
0
THERMODYNAMIC PROPERTIES OF Sb2Te3-BASED SOLID
87
Errors were determined by the method of error accumulation.
Conclusion
The SnTe-Sb2Te3 system was investigated by EMF measurements of reversible concentration cells relative to the SnTe electrode in the 300-400 K temperature range. The formation of solid solutions (up to 20 mol%) based on Sb2Te3 was confirmed. Based on EMF measurements, equations of the temperature dependences of the EMF for solid solutions based on Sb2Te3 with compositions of 5, 10, and 20 mol% SnTe were obtained, from which the partial thermodynamic functions of SnTe in alloys were calculated. The standard thermodynamic functions of the formation of P-solid solutions of the above compositions were calculated by graphical integration of the Gibbs-Duhem equation over the SnTe-Sb2Te3 section.
Acknowledgments
The work has been carried out within the framework of the international joint research laboratory "Advanced Materials for Spintronics and Quantum Computing" (AMSQC) established between the Institute of Catalysis and Inorganic Chemistry of ANAS (Azerbaijan) and Donostia International Physics Center (Basque Country, Spain) and partially supported by the Science Development Foundation under the President of the Republic of Azerbaijan, grant № EIF-GAT-5-2020-3 (3 7)-12/02/4-M-02.
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SnTe-Sb2Te3 SISTEMINDO Sb2Te3 OSASINDA BORK MOHLULLARIN TERMODINAMIK XASSOLORI
A.E.Seyidzada, A.O.Agayeva, E.N.Oruclu, S.ZJmamaliyeva
SnTe elektroduna nazaran donar qatiliq dóvrolarinin EHQ-nin 300-400 K temperatur intervalinda ólgülmasila SnTe-Sb2Te3 sistemi óyranilmi§, Sb2Te3 asasinda 20 mol % bark mahlullar amala galmasi müayyan edilmiíjdir. EHQ ólgmalarindan Sb2Te3 asasinda 5, 10 va 20 mol% SnTe tarkibli bark mahlullar ügün EHQ-nin temperaturdan asililiq tanliklari alinmi§, onlardan SnTe-in xalitalarda parsial termodinamik funksiyalan müayyan edilmi§dir. Bark mahlullann standart amala galma termodinamik funksiyalari Gibbs-Duhem tanliyinin SnTe-Sb2Te3 kasiyi üzra qrafik inteqralanmasi ila hesablanmi§dir.
Agar sozlar: Sn-Sb-Te sistemi, faza diaqrami, bark mahlullar, EHQ metodu, termodinamik xassalari.
ТЕРМОДИНАМИЧЕСКИЕ СВОЙСТВА ТВЕРДЫХ РАСТВОРОВ НА ОСНОВЕ Sb2Te3 В СИСТЕМЕ
SnTe-Sb2Te3
А.Э.Сеидзаде, А.А.Агаева, Э.Н.Оруджлу, С.З.Имамалиева
Измерением ЭДС обратимых концентрационных цепей относительно SnTe электрода в интервале температур 300400 К исследована система SnTe-Sb2Te3. Подтверждено образование до 20 мол.% твердых растворов на основе Sb2Te3. Из данных измерений ЭДС получены уравнения температурных зависимостей ЭДС для твердых растворов на основе Sb2Te3 составов 5, 10 и 20 мол.% SnTe, из которых рассчитаны парциальные термодинамические функции SnTe в сплавах. Стандартные термодинамические функции образования твердых растворов указанных составов рассчитаны графическим интегрированием уравнения Гиббса-Дюгема по сечению SnTe-Sb2Te3.
Ключевые слова: система Sn-Sb-Te, фазовая диаграмма, твердые растворы, метод ЭДС, термодинамические свойства.
