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AZERBAIJAN CHEMICAL JOURNAL № 4 2019
ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
UDC 544.344.015.3: 546.56/5723
SOLID-PHASE EQUILIBRIA IN THE RECIPROCAL SYSTEM 6Ag2S+Ag8SnSe6^6Ag2Se+Ag8SnS6
I.J.Alverdiyev
Ganja State University ialverdiyev 73@gmail.com Received 17.07.2019
Solid-state equilibria at 800 and 300 K in the 6Ag2S+Ag8SnSe6o-6Ag2Se+Ag8SnS6 system which is of interest from the point of view of obtaining new mixed ion-electron conductors were studied by DTA, XRD and SEM methods. According to the results obtained it was shown that the system is reversibly reciprocal and is characterized by the formation of continuous (800 K) or wide (300 K) regions of substitutional solid solutions along the boundary systems Ag2S-Ag2Se and Ag8SnS6-Ag8SnSe6.
Keywords: Ag-Sn-S-Se system, silver chalcostannates, phase diagram, Ag8SnS6-Ag8SnSe6 section, solid solutions.
doi.org/10.32737/0005-2531-2019-4-70-75 Introduction
In recent years, complex chalcogenides based on copper and silver have been given great attention as environmentally friendly functional materials with thermoelectric and other properties [1-9]. This makes them promising for use in alternative energy devices and other functional areas of high technology. According to available data, many of these compounds have high ionic conductivity Cu+(Ag+) and can be used as photo-electrode materials, electrochemical converters of solar energy, ion-selective sensors, photoelec-trochemical visualizers, ionistors, etc. [10-15].
Among the most intensively studied such materials, silver chalcostannates and phases based on them can be noted [16-21]. According to the authors of these works, the presence of mixed electron-ion conductivity is one of the significant factors that positively affect their thermoelectric characteristics.
At the initial stage of the development of new multicomponent materials, the study of phase equilibria and thermodynamic properties of the corresponding systems is important [2225]. Similar data sets for complex chalcogenides of copper and silver were obtained in [26-33].
This paper presents the results of a study of solid-state equilibria in the 6Ag2S+Ag8SnSe6 o-6Ag2Se+Ag8SnS6 reciprocal system. This system is an independent part of the reciprocal system 2Ag2S+SnSe2^2Ag2Se+SnS2, which is
of interest from the point of view of obtaining anion-substituted solid solutions based on silver chalcostannates.
Three of the four quasibinary boundary components - Ag2S-Ag8SnS6 Ag2Se-Ag8SnS6 and Ag2S-Ag2Se of the investigated system, were studied in detail [34-37]. The first two are of the eutectic type [34, 35], and the third is characterized by the formation of a continuous series of high-temperature solid solutions [36]. According to [37] at temperatures below 390 K in the Ag2S-Ag2Se system, an intermediate phase of variable composition (s) as well as limited solid solutions based on low-temperature modifications of the starting compounds (a1 and a2) are formed.
A feature of all the starting compounds of the 6Ag2S+Ag8SnSe6o6Ag2Se+Ag8SnS6 system is the presence of polymorphic transitions at relatively low temperatures [5, 34, 35]. This allows us to expect a change in the character of solid-phase equilibria in a studied system with temperature.
Experimental part
Synthesis
The synthesis of the starting compounds was carried out by direct interaction of elementary components of a high degree of purity in evacuated (~10- Pa) quartz ampoules in a dualzone inclined furnace. The lower "hot" zone was heated to temperatures 30-50° - above the
melting point of the synthesized compound, and the upper "cold" zone was 650 K (for sulfides) or 900 K (for selenides) [38]. In such temperature regimes, for two hours, chalcogen completely interacts with metal components, as evidenced by the absence of traces of chalcogen (dark vapor phase and drops) on the inner walls of the ampoule.
All synthesized compounds were identified by differential-thermal analysis (DTA), powder X-ray diffraction method (PXRD) and scanning electron microscopy (SEM).
Alloys of the boundary system Ag8SnS6-Ag8SnSe6 and inside the concentration plane 6Ag2S+Ag8SnSe6^6Ag2Se+Ag8SnS6 (each weighing 0.5 g) were prepared by fusion of the starting compounds in various ratios in evacuated quartz tubes. After melting, the temperature was lowered to 800 K, at which thermal annealing was performed for 500 hours. A number of alloys after annealing were quenched in cold water.
Analysis
DTA, XRD and SEM techniques were employed to check the purity of the synthesized starting compounds and analyze the samples.
DTA was performed using a NETZSCH 404 F1 Pegasus differential scanning calorime-
ter. Heating rate was of 10 K/min and accuracy about ±20.
X-ray powder diffraction data were collected at room temperature using a Bruker D8 ADVANCE powder diffractometer utilizing CuXa - radiation within 20 = 10 to 70°. The unit cell parameters were calculated by indexing of powder patterns using Topas V3.0 software.
The microstructure and equilibrium compositions of the phases were determined by FEI Quanta™ 250 scanning electron microscope.
Results
Powder X-ray diffraction patterns of alloys of the Ag8SnS6-Ag8SnSe6 system, slowly cooled after annealing, are shown in Figure 1. As can be seen, alloys with compositions of 40100 mol% Ag8SnSe6 have diffraction patterns qualitatively similar to the diffraction pattern of the low-temperature orthorhombic modification of Ag8SnSe6 (ô2-phase) (Table). An alloy with a composition of 20 mol% Ag8SnSe6 is also single-phase and its diffraction pattern is similar to the diffraction pattern of pure Ag8SnS6 (low-temperature modification). The diffraction pattern of the 30 mol% Ag8SnSe6 alloy consists of a set of reflection lines of 51- and ô2- phases.
5000
4000
.5
•o
3000
2000
1000
AgsSnSe6
! . I , . j I 20 mol° oAgsSnSi «JÜÜUUwli-x^uLÀ.WLv^'.l______________ ________ .
LjiLU^juu^^ .....^™1%Ag8SnS6
60 mol%AgsSnSi
j I . 80 mol%AgsSnSs
W^U^ma-A-^LA._v. .......
J AggSnSe
10 20 30 40 50 60
2-Theta-Scale
Fig. 1. Powder XRD patterns for some compositions in the Ag8SnS6-Ag8SnSe6 system.
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I.J.ALVERDIYEV
The XRD results of alloys quenched after annealing at 800 K showed that they are all single-phase and have diffraction patterns characteristic of cubic syngony.
The crystallographic data of ôi, ô2, and ô-phases are presented in Table. The period of the cubic lattice of high-temperature ô-solid solutions has a linear dependence on the composition, i.e. obeys Vegard's rule.
The XRD results were confirmed by SEM (Figure 2). As can be seen, alloys with compositions of 20 and 40 mol% Ag8SnSe6 are single-phase, while the alloy with a composition of 30 mol% Ag8SnSe6 is two-phase.
The results of SEM and XRD analysis of selected alloys of the 6Ag2S+Ag8SnSe6 o 6Ag2Se+Ag8SnS6 system, quenched after annealing at 800 K, showed that they are all two-phase and consist of a mixture of a- and ô-phases. The symbol a refers to a continuous series of solid solutions of the Ag2S-Ag2Se boundary system. According to these data, the solid-phase diagram of the reciprocal system
6Ag2S+Ag8SnSe6^6Se+Ag8SnS6 at 800 K has been constructed (Figure 3a).
The character of solid-phase equilibria in this system at room temperature (Figure 3b) differs sharply from Figure 3a. As can be seen from Figure 3b at 300K, this system consists of a two-phase (ai+8i, a2+8i, a2+S2) and three-phase (ai+a2+8i, a2+S1+S2) fields. The formation of these phase regions is caused by solid-phase transformations in the boundary systems Ag2S-Ag2Se and Ag8SnS6-Ag8SnSe6. Our data indirectly indicate that the intermediate s-phase does not exist in the boundary system Ag2S-Ag2Se system. And the region of homogeneity of the a2 phase is much wider than indicated in [37] and covers the composition range of 45-i00 mol% Ag2Se. According to Figure 4, the diffraction pattern of alloy #i consists of the reflection lines of a2- and S2-phases. If a two-phase region a2+s existed in the boundary system, this would lead to the formation of a three-phase region a2+S2+s and alloy #i would be in this phase region.
Types and crystal parameters of solid solutions of the Ag8SnS6-Ag8SnSe6 system
Composition, mol% Ag8SnSe6 Syngony, Sp.Gr., crystal parameters, Â
Room temperature Quenched at 900 K
Orthorombic Cubic, F-34m
a b c a
0 (Ag8SnS6) 15.334 7.562 10.724 10.858
20 15.425 7.618 10.795 10.911
40 7.769 7.656 10.823 10.953
60 7.819 7.712 10.898 11.018
80 7.865 7.768 10.973 11.073
100 7.913 7.825 11.047 11.122
a b c
Fig. 2. SEM images for some alloys in the Ag8SnS6-Ag8SnSe6 system: 20 mol% Ag8SnSe6 (a); 30 mol%. Ag8SnSe6 (b); 40 mol% Ag8SnSe6 (c).
a
b
Fig. 3. Solid-phase diagram of the 6Ag2S+Ag8SnSe6 ^6Se+Ag8SnS6 system at 800 K (a) and 300 K (b). #i alloy, XRD of which is shown in Figure 4.
2Theta (Coupled TwoThetan"heta) WL=1.54060 Fig. 4. Powder diffraction pattern of alloy #1 which is shown in Figure 3.
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Conclusion
We have constructed solid-phase equilibrium diagrams of the 6Ag2S+Ag8SnSe6o-6Ag2Se+Ag8SnS6 system at 800 and 300 K. It is shown that this reciprocal system is adiagonal. At 800 K, it is characterized by the formation of continuous areas of substitution solutions along boundary systems Ag2S-Ag2Se and Ag8SnS6-Ag8SnSe6. At 300 K it is characterized by a more complex interaction leading to the formation of a number of two- and three phase regions. Obtained solid solutions based on both modifications of the starting compounds are of practical interest as functional materials with mixed ion-electron conductivity.
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6Ag2S+Ag8SnSe6o6Ag2Se+Ag8SnS6 QAR§ILIQLI SISTEMINDO BORKFAZA TARAZLIQLARI
i.C.Alverdiyev
DTA, RFA va SEM üsullan ils qançiq ion-elektron keçiriciïarinin alinmasi baximindan maraq kasb edan 6Ag2S+Ag8SnSe6^6Ag2Se+Ag8SnS6 sisteminda 800 va 300 K temperaturlarda barkfaza tarazliqlari ôyranilmiçdir. Müayyan edilmiçdir ki, sistem qarçiliqh dönar olub, Ag2S-Ag2Se va Ag8SnS6-Ag8SnSe6 sistemlari boyunca fasilasiz (800 K) va ya geniç (300 K) bark mahlul sahalari amala galmasi ila xarakteriza olunur.
Açar sözlar: Ag-Sn-S-Se sistemi, gümü§ xalkostannatlari, faza diaqrami, Ag8SnS6-Ag8SnSe6kasiyi, bark mahlullar. ТВЕРДОФАЗНЫЕ РАВНОВЕСИЯ ВО ВЗАИМНОЙ СИСТЕМЕ 6Ag2S+Ag8SnSe6^6Ag2Se+Ag8SnS6
И.Дж.Алвердиев
Методами ДТА, РФА и СЭМ изучены твердофазные равновесия в при 800 и 300 К в системе 6Ag2S+ Ag8SnSe6-o6Ag2Se+Ag8SnS6, которая представляет интерес с точки зрения получения новых смешанных ионно-электронных проводников. На основании полученных данных показано, что система взаимно -обратимая и характеризуется образованием непрерывных (800 К) или широких полей (300 К) твердых растворов замещения вдоль граничных систем Ag2S-Ag^ и Ag8SnS6-Ag8SnSe6.
Ключевые слова: система Ag-Sn-S-Se, халкостаннаты серебра, фазовая диаграмма, разрез Ag8SnS6-Ag8SnSe6, твердые растворы.
