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108 AZERBAIJAN CHEMICAL JOURNAL № 1 2023 ISSN 2522-1841 (Online)
ISSN 0005-2531 (Print)
UDC 536:546.222
THERMODYNAMIC PROPERTIES OF THE Cu2GeSe3 COMPOUND
Yu.A.Yusibov1, Z.M.Aliyeva\ M.B.Babanly2
1Ganja State University M.Nagiyev Institute of Catalysis and Inorganic Chemistry, Ministry of Science and Education
of the Republic of Azerbaijan
rektor@gdu.edu.az
Received 15.11.2022 Accepted 13.12.2022
The Cu2GeSe3 compound and phases based on it are of interest as environmentally friendly thermoelectric and photovoltaic materials. This paper presents the results of a thermodynamic study of this compound by the method of electromotive forces. We measured the EMF of concentration cells concerning copper electrode with solid Cu+ conductive electrolyte Cu4RbCl3I2 in the 300-450 K temperature range. Based on the obtained experimental data, the partial molar functions of copper in equilibrium alloys from the three-phase region of the Cu2GeSe3 -GeSe2-Se phase diagram, as well as the standard thermodynamic functions of formation and the standard entropy of the Cu2GeSe3 compound, are calculated. A comparative analysis of the obtained results with the available literature data was carried out.
Keywords: EMF method, solid electrolyte, Cu4RbCl3I2, copper-germanium selenide, thermodynamic functions.
doi.org/10.32737/0005-2531-2023-1-108-114 Introduction
Copper based complex chalcogenides are valuable functional materials. Many of them exhibit functional properties, such as thermoelectric, photoelectric, optical, magnetic, et al. [1-6]. In addition, some of them have ionic conductivity by to Cu+ cations and can be used as electrochemical sensors, electrode materials, solid fuel cells, supercapacitors, electrochromic visualizers, et al. [7-10].
In recent years, the copper germanium selenide, Cu2GeSe3 compound and phases based on it, have been intensively studied as potential environmentally friendly thermoelectric and photovoltaic materials [11-17].
The thermodynamic properties of the Cu2GeSe3 compound were studied in [18-20]. In [18], the standard values of the heat of formation and absolute entropy were determined by the calorimetric method. Later, the authors of [19] determined the partial molar functions of copper, as well as the standard integral ther-modynamic functions of formation and the standard entropy of this compound, using the EMF method with a liquid electrolyte. The data of [18, 19] differ significantly from each other.
The lattice dynamics and thermodynamic properties of this compound are investigated by first-principles calculations in [20]. The authors concluded that the low thermal conductivity in Cu2GeSe3 is due to its low Debye temperature, which originates in the weak covalent Cu-Se bonding.
In this paper, we present the results of an EMF study of the thermodynamic properties of the Cu2GeSe3 compound with a solid Cu+-conducting electrolyte Cu4RbCl3I2. This modification of the EMF method is successfully used for thermodynamic studies of several copper-based chalcogenides [10, 21-29]. It was shown in [26, 27] that cation-conducting electrolytes can be successfully used in the thermodynamic study of copper-containing ternary systems by the EMF method even in the presence of less noble elements than copper.
Experimental part
For experiments, the concentration cells of the type were assembled.
(-)Cu(sol.)Cu4RbCl3I2(sol.)(Cu in alloy)(sol.)(+) (1)
According to the available literature data, in the Cu-Ge-Se system, 2 ternary compounds
are formed - Cu2GeSe3 and Cu8GeSe6, which form stable connodes with elemental selenium in the subsolidus [4]. Taking into account these data, as well as the phase diagrams of the CuSe and Ge-Se [30] systems, we have constructed a diagram of solid-phase equilibria of the Cu2Se-GeSe2-Se subsystem (Figure 1). According to this diagram, for the thermodynamic study of the Cu2GeSe3 compound by the EMF method, equilibrium alloys from the Cu2GeSe3-GeSe2-Se phase region should be used as the right electrodes of cells of type (1). Given that, we prepared 2 alloy electrodes from this area (Figure 1, samples 1 and 2).
The synthesis was carried out by melting of the high purity elementary components in ampules evacuated to ~10-2 Pa. Then, according to the phase diagram [4], they were annealed at 800 K (500 h) and 450 K (300 h), followed by slow cooling to room temperature to reach the equilibrium state.
Figure 2 shows the powder diffraction pattern of alloy 2. As can be seen, it consists of reflection lines of the Cu2GeSe3 and GeSe2 compounds and elemental selenium, which is in accordance with the phase diagram.
The right electrodes were prepared by pressing the powdered annealed alloys into pel-
lets with a diameter of ~0.8 cm and a thickness of 0.5 cm. A high-purity copper plate with a diameter of ~1 cm and a thickness of 0.1 cm was used as the left electrode.
The solid electrolyte (Cu4RbCl3I2) was synthesized by the procedure described in [21]
by melting of chemically pure anhydrous CuCl,
_2
Cul, and RbCl in ampules evacuated to ~10 Pa at 900 K followed by cooling to 450 K and homogenizing annealing at this temperature for 100 h. Pellets with a thickness of ~0.4 cm were cut from the obtained cylindrical ingot with a diameter of ~0.8 cm, which were used as a solid electrolyte in cells (1).
The methods for preparing electrodes and assembling an electrochemical cell are described in detail [10, 21]. The EMF was measured in the temperature range 300-450 K using a Keithley 2100 6 ^ digital multimeter with an input resistance of 1014 Q and an accuracy of ±0.1 mV. In this temperature range, the studied alloys were in a solid-state, and the compositions of the equilibrium phases were almost independent of temperature [4]. The temperature of the electrochemical cell was measured using the chromel-alumel thermocouple and a mercury thermometer with an accuracy of ±0.5 K.
Fig. 1. The solid-phase equilibria diagram of the Cu2Se-GeSe2-Se subsystem.
Diffraction Angle [°20]
Fig. 2. Powder diffraction pattern of alloy 2 in Figure 1.
The first equilibrium EMF values were obtained after holding the electrochemical cell at ~400 K for ~60 h, and the subsequent ones every 4-6 h after a certain temperature was established. The EMF values that did not differ from each other during repeated measurements at a given temperature by more than 0.2 mV, regardless of the direction of temperature change, were recorded as equilibrium values. The reversibility of the composed concentration chains was controlled by the constancy of the masses and phase compositions of the electrodes.
Results and discussion
An analysis of the temperature dependences of EMF (Figure 3) showed that they were almost linear. Therefore, the results of the EMF measurements were processed by using Microsoft Excel computer program in an approximation of their linear temperature dependence
by the least-squares method, and following type linear equation was obtained [31]:
E = a + bT ± t
S2 -
^ + Sb(T - t)2
(2)
In equation (2), a and b are coefficients, n is the number of pairs of values E and T; T -average temperature in K, t is Student's test, and SE and Sb are the variances of individual EMF values and the constant b. With the number of experimental points n = 30, and the confidence level equal to 95%, the Student's test is t < 2.
The experimental data of Ti and Ei and steps of calculation are presented in Table 1. Follow equation is received he relative partial thermodynamic functions of copper at 298 K in Cu2GeSe3-GeSe2-Se phase area were calculated from the obtained equation (3) of the temperature dependences of EMF by the equations
E,mV = 371.01 + 0.0636 T ± 2
2 8
—+4.5-10-5(T - 376.2)2 39
n
280 300 320 340 360 380 400 420 440 460
Fig. 3. EMF values for the alloys from the Cu2GeSe3-GeSe2-Se three-phase area at 300-450 K.
Table 1. Results of computer processing of EMF measurements of concentration cells of type (1)
Ti, K Ei, mV t - t Ei( t - t ) (ti - t)2 e e - e (ei - e)2
301.3 354.53 -74.90 -26555.48 5610.51 352.87 1.66 2.76
305.8 355.22 -70.40 -25008.67 4956.63 353.17 2.05 4.18
310.1 352.31 -66.10 -23288.87 4369.65 353.47 -1.16 1.34
314 356.27 -62.20 -22161.18 3869.25 353.73 2.54 6.43
319.9 354.98 -56.30 -19986.56 3170.07 354.14 0.84 0.71
326.2 352.44 -50.00 -17623.17 2500.33 354.56 -2.12 4.51
331.1 351.86 -45.10 -15870.06 2034.31 354.90 -3.04 9.23
335.3 354.69 -40.90 -14508.00 1673.08 355.18 -0.49 0.24
340.8 355.97 -35.40 -12602.52 1253.40 355.56 0.41 0.17
345.4 355.02 -30.80 -10935.80 948.85 355.87 -0.85 0.73
353.6 355.03 -22.60 -8024.86 510.91 356.43 -1.40 1.96
359.2 357.76 -17.00 -6083.11 289.11 356.81 0.95 0.90
364.5 357.45 -11.70 -4183.36 136.97 357.17 0.28 0.08
370.2 355.25 -6.00 -2132.68 36.04 357.56 -2.31 5.34
374.7 358.13 -1.50 -538.39 2.26 357.87 0.26 0.07
378.3 359.58 2.10 753.92 4.40 358.11 1.47 2.15
385.5 358.42 9.30 3332.11 86.43 358.60 -0.18 0.03
390.2 359.99 14.00 5038.66 195.91 358.92 1.07 1.14
394.1 361.58 17.90 6471.08 320.29 359.19 2.39 5.72
399.8 358.66 23.60 8463.18 556.80 359.58 -0.92 0.84
406 358.17 29.80 10672.27 887.84 360.00 -1.83 3.35
411.7 357.69 35.50 12696.80 1260.01 360.39 -2.70 7.27
418.4 359.12 42.20 15153.67 1780.56 360.84 -1.72 2.97
422.5 363.33 46.30 16820.97 2143.38 361.12 2.21 4.87
426.7 360.11 50.50 18184.35 2549.91 361.41 -1.30 1.69
430.2 362.83 54.00 19591.61 2915.64 361.65 1.18 1.40
436.9 363.52 60.70 22064.45 3684.09 362.10 1.42 2.01
440.3 362.18 64.10 23214.53 4108.38 362.33 -0.15 0.02
443.6 362.88 67.40 24456.90 4542.31 362.56 0.32 0.10
449.8 364.12 73.60 26798.02 5416.47 362.98 1.14 1.30
t =376.2 E =357.97
He relative partial thermodynamic functions of copper at 298 K in Cu2GeSe3-GeSe2-Se phase area were calculated from the obtained equation (3) of the temperature dependences of EMF by the equations
AGCu = -zFE
aHou = -zF
E + TI^E 5T
aSou = zF| jE i = zFb
(4)
= -zFa (5)
(6)
where z is the charge on the current-forming Cu+ cation, F is the Faraday number, and a and b are the constants in the equation (2).
And the following values are received aGCu = -37.63±0.06 kJ-mol-1
AHou = -35.80±0.25 kJ-mol-1
This makes it possible to calculate the standard integral thermodynamic functions of the compound based on these data by the method of potential-forming reactions
According to the diagram of solid-phase equilibria, these partial molar functions of copper are thermodynamic functions of the following potential-forming reaction:
ASou = 6.13±0.65 J-mol-1-K-1
2Cu + GeSe2 + Se = Cu2GeSe3 From this equation it follows that the standard Gibbs free energy of the formation and the entropy of the Cu2GeSe3 compound can be calculated by the relation: AfG°(Cu2GeSe3) = 2AGCu + AfG°(GeSe2) (7) AfG°(Cu2GeSe3) = 2AGCu + AfG°(GeSe2) (8) The enthalpy of formation was calculated in a similar way. The standard entropy can be calculated by the equation:
S°(Cu2GeSe3) = 2ASCu + S°(Ge) + 3S°(Se) + ASf(CuGe2Se3) (9)
In calculations (7)-(9), we used the standard entropies for copper (S0 (Cu) = 33.15±0.08 J-mol-1-K-1), germanium (S0(Ge) = 31.13±0.30 J-mol-1-K-1), and selenium (S0 (Se) =42.13±0.21 J-mol-1-K-1) [32] and also the standard thermodynamic functions of GeSe2 (Table 2). The heat of formation of this compound was determined by the calorimetric method [33], and the standard entropy was taken from the handbook [32]. The Gibbs free energy of formation of the GeSe2 compound was calculated based on the data of [32, 33] using the Gibbs-Helmholtz equation. Our results and the literature data are given in Table 2.
Table 2 shows that the data set we obtained differs significantly from the results of [19] and is closer to the data of [18]. We believe that the values of the standard thermodynamic functions presented in [19] are greatly underestimated. This, in particular, is evidenced by the fact that the absolute value of -AfG° according to [19] is much lower than the sum of the standard free Gibbs energies of formation of Cu2Se and GeSe2, which is thermodynamically impossible. The analysis of work [19] shows that the EMF values used by the authors in thermody-namic calculations are significantly lower than expected. This may be due to the occurrence of side processes in the cell, in particular, to the inconstancy of the charge of copper cations. In addition, the equation of the potential-forming reaction, compiled by the authors [19], does not correspond to the nature of phase equilibria in the Cu-Ge-Se system [4].
p
Table 2. Standard integral thermodynamic functions of the Cu2GeSe3
Compound -afG0 -afH0 S0 J-mol"1 -K"1 Notes
kJ-mol"1
GeSe2 101.5±2.9 102.3±2.6 112.6±3.4 [32]
Cu2GeSe3 176.8±3.1 173.9±3.1 233.3±5.1 The present work. EMF method with solid electrolyte
80.7±1.5 86.7±6.9 - [19]
- 206.3 228.1 [18], calorimetry method
Conclusion
In this paper, we present a new mutually agreed set of thermodynamic data for Cu2GeSe3 compound, which is of great interest as a base compound for obtaining environmentally friendly thermoelectric materials. For this purpose, we used the EMF method with a solid Cu+ conducting electrolyte Cu4RbCl3I2. The data obtained are significantly different from the available results obtained by the same method with a liquid electrolyte. In our opinion, they are greatly underestimated due to side processes occurring in the electrochemical cells. The results of our work confirm the advantage of the EMF method with a solid electrolyte, which is free from many disadvantages with a liquid Cu+ conducting electrolyte.
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Cu2GeSe3 BiRLO§MOSiNiN TERMODiNAMiK XASSOLORl
Y.O. Yusibov, Z.M.Oliyeva, M.B.Babanli
Cu2GeSe3 birla§masi va onun asasinda fazalar ekoloji tahlükasiz termoelektrik va fotovoltaik materiallar kimi böyük maraq kasb edirlar. Maqalada bu birla§manin elektrik harakat qüvvasi (EHQ) üsulu ila termodinamik tadqiqinin naticalari taqdim olunur. Cu+ kegiriciliya malik Cu4RbCl3I2 bark elektrolitindan istifada etmakla tartib olunmu§ mis elektroduna nazaran qatiliq dövralarinin elektrik harakat qüvvasi 300-450 K temperatur intervalinda ölgülmü§dür. Ahnmi§ tacrübi naticalara asasan faza diaqraminin Cu2GeSe3-GeSe2-Se ügfazali sahasindan götürülmü§ tarazliq arintilarinda misin parsial molyar funksiyalari, hamginin Cu2GeSe3 birla§masinin standart amala galma termodinamik funksiyalari va standart entropiyasi hesablanmi§dir. Olda edilmi§ naticalarin mövcud adabiyyat malumatlari ila müqayisali tahlili aparilmi§dir.
Agar sözlzr: EHQ üsulu, bark elektrolit, Cu4RbCl3I2, mis-germanium selenidi, termodinamik funksiyalar.
ТЕРМОДИНАМИЧЕСКИЕ СВОЙСТВА СОЕДИНЕНИЯ Cu2GeSe3
Ю.А.Юсибов, З.М.Алиева, М.Б.Бабанлы
Соединение Си^е8е3 и фазы на его основе представляют интерес как экологически безопасные термоэлектрические и фотоэлектрические материалы. В данной работе преставлены результаты термодинамического исследования этого соединения методом электродвижущих сил. Были измерены ЭДС концентрационных относительно медного электрода цепей с твердым Си+ проводящим электролитом Си^ЬС1312 в интервале температур 300-450 К. На основании полученных экспериментальных данных рассчитаны парциальные молярные функции меди в равновесных сплавах из трехфазной области Си^е8е3 -GeSe2-Se фазовой диаграммы, а также стандартные термодинамические функции образования и стандартная энтропия соединения Си^е8е3. Проведен сравнительный анализ полученных результатов с имеющимися литературными данными.
Ключевые слова: метод ЭДС, твердый электролит, Си4ЯЬС1312, селенид меди-германия, термодинамические функции.
