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CHEMICAL PROBLEMS 2020 no. 3 (18) ISSN 2221-8688
315
UDC 544.31:546.8724
THERMODYNAMiC PROPERTiES OF THE Bi2Te3 AND Bi4Te5 COMPOUNDS 1G.S. Hasanova, 2G.B. Dashdiyeva, 1Y.A. Yusibov, 3M.B. Babanly
1Ganja State University H.Aliyev ave, 426, Ganja, Azerbaijan 2Baku Engineering University, H.Aliyev ave., 120, Absheron, Azerbaijan 3Institute of Catalysis and inorganic chemistry, NAS Azerbaijan H.Javid ave., 113, Baku, Azerbaijan e-mail: babanlymb@gmail.com
Received 09.06.2020 Accepted 02.09.2020
Abstract: The thermodynamic properties of the Bi-Te system were studied in the 55-65 at% Te compositions interval and temperatures 300-450 K by using the electromotive forces method. Relative partial molar functions of bismuth in the alloys were calculated. Based on the potential-forming reactions for Bi2Te3 and Bi4Te5 compounds, a mutually consistent set of standard thermodynamic functions and standard entropies is obtained. A comparative analysis of the data obtained for Bi2Te3 with literature is performed, for Bi4Te5 thermodynamic functions were obtained for the first time.
Keywords: bismuth tellurides, thermodynamic functions, EMF method, ionic liquid DOI: 10.32737/2221-8688-2020-3-315-324
Introduction
Bismuth tellurides, especially Bi2Te3, phases, and composites based on it have long been in the field of view of researchers as valuable thermoelectric materials [1-3]. After the discovery of a special quantum state of matter — a topological insulator (TI) [4], it was found that Bi2Te3 and many other layered compounds of tetradimite-like structures are TI which are rather promising for their varied applications [5-11]. In particular, the unique optical properties make them promising for the use in broadband optoelectronics as photodetectors [12-15].
The thermodynamic functions of compounds are their fundamental characteristics and, together with phase diagrams, form the basis for the synthesis and growth of crystals, as well as optimizing the development conditions for new materials based on them [10, 16-18].
Despite a growing interest in bismuth tellurides as the most innovative functional materials, no reliable picture of phase equilibria of this system in the Bi-Bi2Te3 composition interval has so far been established. Various versions of the T-x diagram given in [19-23] are contradictory. Even the results of recent
studies [24, 25] do not agree on the character of the crystal structure and the homogeneity regions of the intermediate phases formed in it.
The thermodynamic properties of bismuth tellurides have also been studied extremely insufficiently. An analysis of the literature shows that modern handbooks and electronic databases contain thermodynamic data only for Bi2Te3 [26-29]. That's why we undertook a comprehensive study of phase equilibria in the Bi-Te system and the thermodynamic properties of bismuth tellurides. In [30], a new refined phase diagram of this system was presented in the composition range <60 at% Te, where the formation of a series of bismuth tellurides with incongruent melting is shown.
This work presents results of a thermodynamic study of the Bi2Te3 and Bi4Te5 compounds by using the electromotive force method (EMf).
Various modifications of the EMF method are widely used to study binary and complex metal chalcogenides [31-41]. In high-temperature studies, as a rule, eutectic melts of alkali metal salts are used as an electrolyte. In the study of solid metal chalcogenides, it is
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CHEMICAL PROBLEMS 2020 no. 3 (18)
advisable to take measurements at temperatures below solidus. For this purpose, the most suitable electrolytes were glycerol solutions of alkali metal salts, first used in [41] in the study
of amalgam systems. In our recent studies [42, 43], an ionic liquid (a mixture of morpholine and formic acid) was successfully used as a liquid electrolyte.
Experimental
To study the thermodynamic properties of the Bi-Te system by the EMF method, the concentration cells of the type
(-) Bi (solid) | ionic liquid + Bi3+ | Bi-Te (solid) (+) (1)
were assembled and their EMF were measured in the temperature range 300-450 K.
Elemental bismuth was used as the left electrode, and alloys from the two-phase regions Bi2Te3+Te, Bi4Te5+Bi2Te3 were used as the right electrodes.
Alloys-right electrodes were synthesized by fusion of elementary bismuth and tellurium of high purity in vacuum (~ 10-2 Pa) quartz ampoules. After fusion at 900 K, the samples were quenched into cold water, followed by thermal annealing at 750 K (500 h) and 400 K (20 h). The phase composition of the obtained
alloys was confirmed by XRD. As an example, in Fig. 1. presents the powder X-ray powder diffraction pattern of an alloy with a composition of 57 at% Te. As can be seen, the diffraction pattern of this alloy consists of a set of reflection lines Bi2Te3 and Bi4Te5 which is in accordance with the phase diagram of the Bi-Te system [21, 30].
An ionic liquid (morpholine formate) with the addition of BiCl3 was used as electrolyte. To synthesis of the ionic liquid, the morpholine, formic acid, and anhydrous BiCl3 purchased from Alfa Aesar were used. The ionic liquid was obtained following the procedure described in [44]:
The assembly of electrochemical cells of type (1) and the method for EMF measurements are described in detail in [33, 43].
Fig. 1. Powder X-ray of a two-phase alloy Bi4Te5+Bi2Te3
The first equilibrium EMF values were obtained after maintaining the electrochemical cell at ~350 K for 40-60 h., while subsequent ones every 3-4 h. after a certain temperature.
The EMF values were considered equilibrium, which did not differ from each other when repeatedly measured at a given temperature by
more than 0.2 mV, regardless of the direction of the temperature change.
Results
As a result of the EHQ measurements, it was established that the EMF has a constant value in each of the phase areas of Bi2Te3+Te and Bi4Te5+Bi2Te3 of the Bi-Te system, and linearly depends on the temperature (Fig.2). This confirms the existence of two-phase areas in the phase diagram and allows us to use this for thermodynamic calculations.
The obtained experimental data were
processed using the Microsoft Office Excel 2003 computer program using the least-squares method and linear equations of E = a + bT type were obtained. The experimental data of Ti and Ei and steps of calculation for both regions are given in Table 1. The linear equations obtained during the calculations are shown in Table 2 in a form recommended by modern handbooks [32, 33]
E = a + bT ± t
S-2 _
^L + 5 b(T - T)2 n
1/2
(2)
Fig. 2. Temperature dependence of the EMF type (1) cells in the Bi2Te3+Te (a) and Bi4Te5+
Bi2Te3 (b) two-phase regions
In equation (2), a and b are coefficients, n is the number of pairs of values of E and T; T-average temperature, K; Student's t-test, 5 E and 52- are dispersions of individual values of EMF
and constant b. Given that the number of experimental points is n = 30, at a confidence level of 95%, the Student test is t<2.
Table 1. Experimentally obtained data for temperature (Ti) and EMF (Ei) and data associated with the calculation steps for the Bi2Te3+Te phase region of the Bi-Te system
Ti, K Ei, mV tj - t Ei(ti - t) (ti - t)2 e E; - E (Ei - E)2
301.3 132.63 -74.87 -9930.01 5605.52 132.72 -0.09 0.01
305.8 132.18 -70.37 -9301.51 4951.94 132.66 -0.48 0.23
310.1 132.81 -66.07 -8774.76 4365.24 132.60 0.21 0.04
314 132.22 -62.17 -8220.12 3865.11 132.55 -0.33 0.11
319.9 132.95 -56.27 -7481.10 3166.31 132.47 0.48 0.23
326.2 132.13 -49.97 -6602.54 2497.00 132.39 -0.26 0.07
331.1 132.95 -45.07 -5992.06 2031.30 132.32 0.63 0.39
335.3 131.87 -40.87 -5389.53 1670.36 132.27 -0.40 0.16
340.8 132.92 -35.37 -4701.38 1251.04 132.19 0.73 0.53
345.4 132.21 -30.77 -4068.10 946.79 132.13 0.08 0.01
353.6 132.82 -22.57 -2997.75 509.40 132.02 0.80 0.63
359.2 131.89 -16.97 -2238.17 287.98 131.95 -0.06 0.00
364.5 131.54 -11.67 -1535.07 136.19 131.88 -0.34 0.11
370.2 132.21 -5.97 -789.29 35.64 131.80 0.41 0.17
374.7 131.52 -1.47 -193.33 2.16 131.74 -0.22 0.05
378.3 131.11 2.13 279.26 4.54 131.69 -0.58 0.34
385.5 130.86 9.33 1220.92 87.05 131.60 -0.74 0.55
390.2 131.25 14.03 1841.44 196.84 131.54 -0.29 0.08
394.1 130.84 17.93 2345.96 321.48 131.48 -0.64 0.42
399.8 131.25 23.63 3101.44 558.38 131.41 -0.16 0.03
406 131.74 29.83 3929.80 889.83 131.33 0.41 0.17
411.7 131.58 35.53 4675.04 1262.38 131.25 0.33 0.11
418.4 131.92 42.23 5570.98 1783.37 131.16 0.76 0.58
422.5 130.48 46.33 6045.14 2146.47 131.11 -0.63 0.39
426.7 130.71 50.53 6604.78 2553.28 131.05 -0.34 0.12
430.2 131.27 54.03 7092.52 2919.24 131.00 0.27 0.07
436.9 131.52 60.73 7987.21 3688.13 130.91 0.61 0.37
440.3 131.08 64.13 8406.16 4112.66 130.87 0.21 0.04
443.6 130.93 67.43 8828.61 4546.80 130.83 0.10 0.01
448.8 130.31 72.63 9464.42 5275.12 130.76 -0.45 0.20
T =376.17 E =131.7233
Table 2. Experimentally obtained data for temperature (Ti) and EMF (Ei) and data associated with the calculation steps for the Bi4Te5+ Bi2Te3 phase region of the Bi-Te system
Ti, K Ei, mB t - t Ei(t - t) (ti - t)2 e Ei - E (Ei - E)2
301.3 102.44 -74.87 -7669.68 5605.52 102.04 0.40 0.16
305.8 101.81 -70.37 -7164.37 4951.94 102.23 -0.42 0.18
310.1 102.75 -66.07 -6788.69 4365.24 102.42 0.33 0.11
314 101.91 -62.17 -6335.74 3865.11 102.58 -0.67 0.45
319.9 102.19 -56.27 -5750.23 3166.31 102.83 -0.64 0.42
326.2 103.62 -49.97 -5177.89 2497.00 103.10 0.52 0.27
331.1 103.14 -45.07 -4648.52 2031.30 103.31 -0.17 0.03
335.3 103.86 -40.87 -4244.76 1670.36 103.49 0.37 0.13
340.8 102.85 -35.37 -3637.80 1251.04 103.73 -0.88 0.77
345.4 104.74 -30.77 -3222.85 946.79 103.92 0.82 0.66
353.6 103.97 -22.57 -2346.60 509.40 104.28 -0.31 0.09
359.2 104.81 -16.97 -1778.63 287.98 104.51 0.30 0.09
364.5 105.25 -11.67 -1228.27 136.19 104.74 0.51 0.26
370.2 104.76 -5.97 -625.42 35.64 104.99 -0.23 0.05
374.7 105.72 -1.47 -155.41 2.16 105.18 0.54 0.29
378.3 105.46 2.13 224.63 4.54 105.33 0.13 0.02
385.5 104.93 9.33 979.00 87.05 105.64 -0.71 0.50
390.2 105.75 14.03 1483.67 196.84 105.84 -0.09 0.01
394.1 106.24 17.93 1904.88 321.48 106.01 0.23 0.05
399.8 106.91 23.63 2526.28 558.38 106.25 0.66 0.43
406 106.05 29.83 3163.47 889.83 106.52 -0.47 0.22
411.7 107.02 35.53 3802.42 1262.38 106.76 0.26 0.07
418.4 106.93 42.23 4515.65 1783.37 107.05 -0.12 0.01
422.5 107.38 46.33 4974.92 2146.47 107.22 0.16 0.03
426.7 107.91 50.53 5452.69 2553.28 107.40 0.51 0.26
430.2 107.05 54.03 5783.91 2919.24 107.55 -0.50 0.25
436.9 108.16 60.73 6568.56 3688.13 107.84 0.32 0.10
440.3 107.08 64.13 6867.04 4112.66 107.98 -0.90 0.81
443.6 107.95 67.43 7279.07 4546.80 108.12 -0.17 0.03
448.8 108.57 72.63 7885.44 5275.12 108.35 0.22 0.05
T =376.17 E =105.24
Table 3. Relations between the EMF and the temperature for type (1) cells in some phase regions of the Bi-Te system in the 300-450 K temperature interval
№ Phase area E, mV=a+bT± t- 5e(T)
1 Bi2Te3 + Te 136.73 - 0.0133T ± 2 0 21 ' + 3.4 • 10-6(T 376.17)2 _ 30 _ /2
2 Bi4Te5 +Bi2Te3 89.16 + 0.0428T ± 2 ~ 0 23 ' + 3.7 • 10-6(T 376.17)2 _ 30 -i 1/2
From obtained equations (Table 3) by using the thermodynamic expressions
A
ASb,
lBi
Bi = - zFE
( 'dE }
--zF I — I = zFb
I dT J „
" (oE^ "
-zF E - tI — I
_ lorJp _
= -zFa
(3)
(4)
(5)
the partial molar Gibbs free energy, enthalpy, calculated (Table 4). and entropy of bismuth in the alloys were
Table 4. Relative partial functions of bismuth in the alloys of the Bi-Se system at T = 298.15 K
Phase area -AGBi -AHBi ASBi
kJ/mol J/(molK)
Bi2Te3 + Te 38,429±0,064 39,58±0,27 -3,85±0,71
Bi4Te5 +Bi2Te3 29,497±0,067 25,81±0,28 12,38±0,74
The areas of homogeneity of the compounds thermodynamic characteristics of the following Bi2Te3 and Bi4Te5 are very small [21, 30], so potential-forming reactions (the substances in these partial molar values are the the crystalline state) [32, 33]
Bi+1.5Te=0.5Bi2Te3 (6)
Bi+2.5Bi2Te3=1.5Bi4Te5 (7) From relations (6) and (7) by using relations
A fZ°(Bi2Te3) = 2AZBi (8)
2 — 5 —
AfZ0 (Bi4Te5) = - AZBi + -AZBi2Te3 (9)
3 3 2 3
S0 (Bi2Te3) = 2ASBi + 2S0(Bi) + 3S0(Te) (10)
2—2 5
S0 (Bi4Te5) = 3ASBi + jS0(Bi) + 3S0(Bi2Te3) (11)
the standard thermodynamic functions of the formation of Bi2Te3 and Bi4Te5 were calculated. In relations (8) and (9) Z = G or H. For the thermodynamic calculations, we used the literature data [29] on the standard entropies of elementary bismuth (56.7 ± 0.5 J-mol-1-K-1) and
tellurium (49.5±0.2 J-mol"1-K"1). The obtained values are summarized in Table 5. In all cases, the estimated standard uncertainties were calculated by accumulating the errors. In Table 5, in addition to our experimental results, literature data are also presented.
Table 5. Standard integral thermodynamic functions of bismuth tellurides
Compound -A/G0 (298K) -A/ H° (298K) S° (298K) Source, method
kJmol-1 J mol -1K-1
Bi4Te5 147.8±0.8 149.2+1.1 375.2+5.1 Present work, EMF
Bi2Te3 76.9±0.2 79.2+0.5 254.2+3.0 Present work, EMF
77.9+0.6 80.0+4.4 [45], EMF
89.5+0.9 99.5+9.5 [46], EMF
82.8 87.0 [47] , EMF
78.5+0.2 [48], calorim.
84.1+3.2 [49], calorim.
80.5+5.0 [50], tenzim.
77.1 77.4 260.9 [26], recomend.
77.3+1.7 78.2+0.5 260.8 [27], recomend.
78.7+2.1 261.1+8.4 [28], recomend.
75.3+1.7 78.6+0.2 251.0+8.4 [29], recomend.
As can be seen from the Table 5, the determined by us is in good agreement with the standard enthalpy of of Bi2Te3 formation data [45, 48-50] obtained by the methods of
EMF, calorimetry, and tensimetry. Very close A/H° values are recommended in handbooks [26-29]. The data obtained by high-temperature EMF measurements [46, 47] are somewhat overestimated. The value of the standard Gibbs
energy of formation obtained by us also agrees with the data of [45] and the values recommended in [26-29]. The thermodynamic functions of Bi4Te5 were determined by us for the first time.
Conclusion
The thermodynamic properties of the alloys of the Bi-Te system were studied by EMF measurements of the concentration cells relative to the bismuth electrode in the 55-65 at% Te compositions and temperatures in 300-450 K intervals. Relative partial molar functions of bismuth in the alloys, standard thermodynamic
formation functions, and standard entropies of Bi2Te3 and Bi4Te5 compounds were calculated. Results obtained for Bi2Te3 supplement and refine the literature data, while the thermodynamic functions for Bi4Te5 are determined for the first time.
Acknowledgment
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 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, a grant EiF/MQM/Elm-Tehsil-1-2016-1(26)-71/01/4-M-33.
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Bi2Teз VЭ Bi4Te5 BiRLЭ§MЭLЭRiMN TERMODiNAMiKXASSЭLЭRi
1G.S. Hэsэnova, 2Q.B. Da§dlyeva, 1Y.A. Yusibov, 3M.B. Babanh
1Одпеэ Во\Ы Universiteti, Н. ЭИуеу рг., 425, Оэпеэ, Azдrbaycan 2БакгMuhдndislik Universiteti, Н.ЭИуеург. , 120, АЬ^егоп, Azдrbaycan 3АМЕА Kataliz vэ Qeyri-uzvi Kimya ^ШыШ, H.Cavidрк,113, Bakl, Azдrbaycan
ЫТ sisteminin termodinamik xassэlэri Из 55-65 at% Te tэrkib vэ 300-450 К temperatыr intervallnda tдdqiq edilmi§dir. ХэШэЬЫэ bismыtыn nisbi parsial molyar fыnksiyalarl hesablanmцdlr. Potensiabmэbgэtirici reaksiyalar эsaslnda Bi2Teз vэ Bi4Te5 ^гЬ^зЬИШП standart дmдlдgдlmд termodinamik fыnksiyalarlnln vэ standart entropiyalarlnln qar§lllqll tдnzimlдnmi§ qiymэtlэri komplekslдri allnmцdlr. Bi2Te3 щШ termodinamik xassэlэri allnmц nдticдlдr эdэbiyyatla muqayisэli tэhlil edilmi§, Bi4Te5 щШ isд termodinamik fыnksiyalar ^ dдfд tдyin olыnmы§dыr.
Адаг sдzfar: termodinamik fыnksiyalar, Bi-Te sistemi, elektrik hдrдkдt quvvэsi шыЫ
ТЕРМОДИНАМИЧЕСКИЕ СВОЙСТВА Bi2Teз И Bi4Te5 СОЕДИНЕНИЙ
1Г.М. Гасанова, 2 Г.Б. Дашдиева, 1Ю.А. Юсибов, 3М.Б. Бабанлы
1Гянджинский Государственный Университет Пр.Г.Алиева, 425, Гянджа, Азербайджан 2Бакинский Инженерный Университет, Пр.Г.Алиева, 120, Абшерон, Азербайджан 3Институт Катализа и Неорганической химии, Пр.Г.Джавида,113, Баку, Азербайджан
Методом электродвижущих сил изучены термодинамические свойства системы Bi-Тe в интервале составов 55-65 ат% Тe и температур 300-450 К. Рассчитаны относительные парциальные молярные функции висмута в сплавах. На основании потенциалобразующих реакций получены взаимосогласованные комплексы значений стандартных термодинамических функций образования и стандартных энтропий для соединений Bi2Te3 и Bi4Te5. Проведен сравнительный анализ полученных для Bi2Тeз данных с литературными, для Bi4Te5 термодинамические функции определены впервые. Ключевые слова: теллуриды висмута, термодинамические функции, метод ЭДС, ионная жидкость
