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CHEMICAL PROBLEMS 2023 no. 3 (21) ISSN 2221-8688
229
UDC [541.123.6+536]:546.22/23
PHASE EQUILIBRA IN THE Ag2Se-AgsGeSe6-AgsSiSe6 SYSTEM AND CHARACTERIZATION Of the AgsSi1.xGexSe« SOLID SOLUTIONS
1G.M. Ashirov, 1K.N. Babanly, 1L.F. Mashadiyeva, 2Y.A. Yusibov, 1M.B. Babanly
1Acad. M. Nagiyev Institute of Catalysis and Inorganic Chemistry, 113, H.Javid ave, AZ1143, Baku, Azerbaijan 2Ganja State University, 429, Heydar Aliyev ave., Ganja, Azerbaijan e-mail: [email protected]
Received 07.06.2023 Accepted 10.08.2023
Abstract: Phase equilibria in the Ag2Se-Ag8SiSe6-Ag8GeSe6 system were studied by differential thermal analysis and X-ray diffraction thechnique. Based on the experimental results and literature data, the projection of the liquidus surface of the Ag2Se-Ag8SiSe6-Ag8GeSe6 system, the isothermal section at 300 K and some polythermal sections of the phase diagram were constructed. It was determined that continuous series of solid solution are formed based on high-temperature cubic modification and limited solid solution areas are formed based on low-temperature modification of initial compounds on the Ag8SiSe—Ag8GeSe6 side of concentration triangle. The formation of solid solutions leads to a sharp decreasing ofpolymorphic transition temperatures of ternary compounds and stabilization of high-temperature phases at room temperature. The liquidus surface of the Ag2Se-Ag8SiSe6-Ag8GeSe6 system consists of 2 areas reflecting the initial crystallization of a'-phase based on HT-Ag2Se and HT-Ag8Sii-xGexSe6 solid solutions. The obtained new phases are of interest as environmentally safe materials with thermoelectric properties and mixed ion-electron conductivity.
Keywords: Argyrodite family compounds, silver-germanium selenide, silver-silicon selenide, phase equilibria, liquid surface, solid solutions, T-x diagram, crystal lattice parameters. DOI: 10.32737/2221-8688-2023-3-229-241
1. Introduction
Complex copper and silver chalcogenides with germanium subgroup elements are valuable functional materials [1-3]. Among these compounds, synthetic analogues of the mineral argyrodite with the formula A8BX6 (A-Cu, Ag; BIV-Si, Ge, Sn; X-S, Se, Te) are widely studied as environmentally safe materials with high thermoelectric figure of merit at medium temperatures [4-12]. Some of these compounds exhibit both photovoltaic and optical properties [13-17]. Most of the argyrodite family compounds have ionic conductivity due to the high mobility of Cu+ (Ag+ ) cations, which makes them very promising for use in preparing of photoelectrode materials, electrochemical solar energy converters, and ion-selective sensors [18-22].
It is known that the search and study of new multicomponent materials is based on information about the phase equilibrium of the corresponding systems and the thermodynamic properties of the phases formed in them [23-29]. Most compounds of the argyrodite family have polymorphic transitions at low temperatures. As a rule, their high-temperature modifications are crystallized in a cubic structure, while their low-temperature phases have lower symmetry. Low-temperature modifications of some representatives of this class are also isostructural. This increases the possibility of the formation of solid solutions of different structures in the systems based on argyrodite analogues. In a series of works [30-37], phase equilibria in systems consisting of argyrodite
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CHEMICAL PROBLEMS 2023 no. 3 (21)
phases were studied and new phases with variable composition were discovered in them.
The present study aimed to obtain a picture of phase equilibria in the the Ag2Se-Ag8GeSe6-Ag8SiSe6 composition of the Ag2Se-GeSe2-SiSe2 quasiternary system.
The started compounds of the studied Ag2Se-Ag8GeSe6-Ag8SiSe6 system have been sufficiently studied. The Ag2Se compound melts congruently at 1173K and undergoes a polymorphic transition at 406K [38]. Ag8SiSe6 compound also melts congruently. Different authors give sharply different values of its melting point. Authors of [39, 40] define melting temperature at 1203 K. In [41] and [42] showed temperture 1258 K and 1268 K, respectively. The polymorphic transition temperatures of this compound are 315, 354 K, respectively [43]. The low-temperature modification is tetragonal (Sp.gr. I-4m2, a = 0.7706, b =1.10141 nm) [39, 40], the
intermediate modification (IT) is simple cubic (Sp.gr. P4232, a = 1.087 nm) [20] and high-temperature modification has a face-centered cubic structure (Sp.gr. F-43m, a = 1.09413 nm) [39, 40]. The compound Ag8GeSe6 melts congruently at 1176 K and has a polymorphic transformation at 321 K [40]. The low-temperature modification has an orthorhombic (Sp.gr. Pmn2h a = 0.78235, b = 0.77126, c = 1.08854 nm) [44] and the high-temperature modification has a cubic (Sp.gr. F-43m, a = 1.09931 nm) structure [9].
Ag2Se-Ag8GeSe6 and Ag2Se-Ag8SiSe6 sides of the Ag2Se-Ag8GeSe6-Ag8SiSe6 solids triangle are quasi-binary and form a eutectic diagram. In the Ag2Se-Ag8GeSe6 system, the eutectic crystallizes at 1103 K [40, 45, 46], and in the Ag2Se-Ag8SiSe6 system at 1113 K [39], 1123 K [40], 1073 K [41]. Another boundary system Ag8GeSe6-Ag8SiSe6 has not been studied.
2. Experiment part
2.1. Synthesis
Ag2Se, Ag8SiSe6 and Ag8GeSe6 compounds were first synthesized for conducting research. The synthesis was carried out by melting stoichiometric mixtures of the corresponding simple substances with high purity in quartz ampoules under vacuum conditions (10-2Pa). Since the saturated vapor pressure of selenium (Tboil.=958 K) at the melting temperature of all three compounds is high, their synthesis was carried out in a two-zone mode in an inclined furnace. The temperature of the furnace was gradually raised and heated to a temperature of 40-50 K above the melting point of the synthesized compound. A part of the ampoule outside the oven is constantly cooled. Due to the process of cooling with water for 2-3 hours, the selenium accumulated in the form of steam at the end of the ampoule was condensed and sent to the reaction zone, and after the absorption of most selenium, the ampoule was completely inserted into the furnace. After keeping in the oven for 4-5 hours, it was gradually cooled by disconnecting the oven from the power source.
The synthesized compounds were identified by differential thermal analysis (DTA) and X-ray diffraction analysis (XRD).
Experimental DTA results for Ag2Se and Ag8GeSe6 compounds showed that their temperatures of polymorph transition and melting corresponde to the above literature data [38, 40]. Three endothermic effects were detected on the heating DTA curve of Ag8SiSe6 compound. Small peaks corresponding to 315 K and 355 K represent polymorphic transformations, and an intense peak at 1278 K reflects the melting point. These results agree with the data in [43]. X-ray phase analysis confirms the homogeneity of the synthesized samples and coincides with the diffraction patterns given in the literature for RT-modification of compounds [9, 20, 39, 30, 44].
Alloys of the Ag2Se-Ag8GeSe6-Ag8SiSe6 system were prepared by melting the mixtures of the primary compounds in different proportions in vacuumed quartz ampoules. To bring the samples to equilibrium, they were thermally treated for a long time (500 h) at 900 K. Two samples were prepared for each composition in the Ag8GeSe6-Ag8SiSe6 system, one of which was gradually cooled in a furnace disconnected from the current source after thermal treatment, and the other was annealed by dropping the ampoule in cold water below 900 K.
2.2. Analysis
All the alloys were analyzed using powder XRD and DTA techniques. Powder XRD analysis was performed in a Bruker D2 PHASER diffractometer using CuKa1 radiation within the scanning range of 29=5^75. DTA measurements were recorded with a "Netzsch 404 F1 Pegasus system" differential scanning
calorimeter (under flowing argon atmosphere) and a multichannel device based on the electronic "TC-08 thermocouple data logger" (in sealed quartz tubes). The measurement results were processed using the NETZSCH Proteus Software. The temperature measurement accuracy was within ±2 K.
3. Results and discussion
Based on the obtained experimental results and literature data on the Ag2Se-Ag8GeSe6 and Ag2Se-Ag8SiSe6 systems [39-41, 45, 46], we obtained a detailed description of the phase equilibrium in the Ag2Se-Ag8GeSe6-Ag8SiSe6 system.
3.1. Border section of Ag8SiSe6 -
Ag8GeSe6
Based on DTA and XRD results, the phase diagram of Ag8SiSe6-Ag8GeSe6 system was constructed (Figure 1). As can be seen, in this system there is a continuous solid solution (5-phase) between the high-temperature modifications of the initial compounds.
Fig. 1. Phase diagram of Ag8SiSe6-Ag8GeSe6 system and composition dependence of crystal lattice
parameters of HT-Ag8Sii-xGexSe6 solid solutions.
Based on the low-temperature (P) and medium-temperature (y) modifications of the Ag8SiSe6 compound, as well as the low-temperature modification (s) of the Ag8GeSe6 compound, limited solid solution areas are formed. It can be seen from the phase diagram
that the formation of solid solutions is accompanied by a decrease in the polymorphic transformation temperatures of the primary compounds. This leads to the widening of the temperature range in which the high-temperature ion-conducting 5-phase exists and
its stability at room temperature and below in the range of 15-70 mol% Ag8GeSe6.
Table 1. Crystal lattice parameters of HT-Ag8Si1-xGexSe6 solid solutions
Composition, mol% Ag8GeSe6 Lattice parameters, 900 K ; (Cubic, F-43m) a, Â
Ag8SiSe6 10.9405
10 10.9485
20 10.9528
40 10.9665
60 10.9772
80 10.9812
90 10.9884
Ag8GeSe6 10.9939
Fig. 2. X-ray diffraction patterns of Ag8SiSe6-Ag8GeSe6 system alloys (room temperature).
Fig. 2 shows the powder diffraction patterns of slowly cooled samples after thermal treatment. As can be seen, the diffraction patterns of the samples containing 90 and 80 mol% Ag8GeSe6 are qualitatively the same as pure RT-Ag8GeSe6, while the diffractograms of the samples belonging to the 20-70 mol% Ag8GeSe6 interval are similar to the cubic HT-modifications of the original compounds. Finally, the sample with 10 mol% Ag8GeSe6 has the same diffraction pattern as IT-Ag8SiSe6. These results confirm the phase diagram.
XRD results of samples quenched from 900 K are shown in Figure 3. It is clear that the diffractograms of the initial compounds and all intermediate samples are qualitatively the same
and have a characteristic diffraction pattern for cubic structure. This confirms the formation of continuous 5-solid solutions in the system.
Lattice parameters of high-temperature modifications of ternary compounds and high-temperature solid solutions formed between them were calculated using the TOPAS 3.0 computer program, and the results are listed in Table 1.
Fig. 1 also shows the composition dependence graph of lattice parameters of HT-Ag8Sii_xGexSe6 solid solutions. As can be seen, the lattice parameters of solid solutions increase linearly with Ge substitution and follow Vegard's rule.
Fig. 3. X-ray diffraction patterns of Ag8SiSe6-Ag8GeSe6 samples quenched at 900K
3.2 Solid phase equilibria of the
Ag2Se-Ag8GeSe6-Ag8SiSe6 system.
In Figure 4, the solid solutions formed in the Ag8GeSe6-Ag8SiSe6 system form connode lines with RT-Ag2Se(a-phase) and with each other. Boundary connode lines divide the
system into 10 heterogeneous areas. Sevenareas are two-phase (a+P, a+y, a+S, a+s, P+y, y+S, S+s), and three fields are three-phase (a+P+y, a+y +S, a+s+S ). The noted phase fields were confirmed by the XRD. Figure 5 shows the powder diffractograms of several mixtures.
5Ag.Se
Ag.SiSe. 20 40 mQ| % 60 80 Ag.GeSe,
Fig. 4. isothermal section at 300K
Fig. 5. Powder XRD patterns (300 K) of Ag2Se-Ag8SiSe6 - Ag8GeSe6 alloys: (a) alloy - #1, (b)
alloy #2, (c) alloy #3 and (d) alloy #4 in Fig.4
Fig. 6. Projection of the liquidus surface and studied polythermal sections of the Ag2Se-Ag8GeSe6-Ag8SiSe6 system. Primary crystallization area: 1 (a'), 2 (5). *Broken lines indicate studied vertical
sections.
3.3. Projection of the liquidus surface of the Ag2Se-Ag8GeSe6-Ag8SiSe6 system
The projection of the liquidus surface of this system consists of two areas (Fig. 6). One of them corresponds to the initial crystallization of the a' solid solution based on the high-
temperature modification of the Ag2Se compound, and the second to the 5 phase. These areas are bounded by the e1e2 curve reflecting the monovariant eutectic equilibrium (eutectic coordinates are 60 mol% Ag8GeSe6 and 21 mol% Ag8SiSe6, respectively):
L O a'+ 5 ( T = 1120 - 1103K) (1)
3.4. Some polythermal sections of the phase diagram
Isopleth sections Ag8SiSe6-[A] (Fig. 7) and 5Ag2Se-[B] (Fig. 8) (where [A] is two-phase alloy in the Ag2Se-0.2Ag8GeSe6 section with 50 mol% Ag2Se; [B] is Ag8Si0.5Ge0.5Se6 solid solution) were studied in order to determine the crystallization sequence of the phases and the exact position of the monovariant line e1e2 and liquidus surface of the system.
The Ag8SiSe6-[A] section. The liquidus of this section consists of two curves (Figure 7). These curves correspond to the primary crystallization of the a' and 5 solid solutions. At the point of intersection of those curves (5 mol% Ag8SiSe6), the a'+5 eutectic mixture crystallizes from the liquid. The monovariant
process (1) occurs in a very small temperature range and has a sharp peak in the DTA curves. Therefore, the L+a'+5 three-phase area formed during the reaction (1) is delimited by a broken line. The reaction (1) ends with the formation of the two-phase field a'+5. The horizontal line at 403K below the solidus corresponds to the polymorphic transformation of a' solid solutions based on the high-temperature modification of the Ag2Se. The temperature of this transformation is constant and is the same with the corresponding transition temperature for the pure Ag2Se. Near this transition temperature, both modifications of Ag2Se indicate negligible solubility. A decrease in polymorphic transformation temperatures of Ag8SiSe6 and Ag8GeSe6 compounds is observed.
Fig.7. Isopleth section Ag8SiSe6-[A] ([A] is two-phase alloy in the Ag2Se-0.2Ag8GeSe6 section
with copmostion 50 mol% Ag2Se)
Fig. 8. Isolepth section Ag2Se-[B] ([B] is Ag8Si05Ge05Se6 solid solution)
The Ag2Se-[B] section. The picture of phase equilibria in this section is similar to the previous polythermal section (Fig. 8). This section also passes through the primary crystallization areas of a' and S solid solutions.
A monovariant eutectic equilibrium (1) is observed in the system below the liquidus, and a two-phase field a'+S is formed. The horizontal line at 403K corresponds to the polymorphic transformation of Ag2Se.
Conclusion
Here, a new picture of phase equilibria in the Ag2Se-Ag8SiSe6-Ag8GeSe6 system was obtained. The diagram of the solid phase equilibria of the system at 300 K, the projection of the liquidus surface, as well as the T-x diagrams of the Ag8SiSe6-Ag8GeSe6 boundary system and two internal polythermal sections were constructed. In the Ag8SiSe6-Ag8GeSe6 system, continuous solid solutions (S-phase) were found between the high-temperature modifications of the primary compounds with a cubic structure. It was determined that the formation of solid solutions is accompanied by a
decrease in the polymorph transition temperatures of both compounds. This extends the lower limit of the homogeneity region of the ion-conducting S-phase below room temperature. The fact that the liquidus surface of the S-phase is very wide creates a good opportunity for growing single crystals of this phase by the directional crystallization method. It is also shown that the liquidus surface of the Ag2Se-Ag8SiSe6-Ag8GeSe6 system consists of two areas corresponding to the initial crystallization of the (HT-Ag2Se) and S-phase.
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ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ AgiSe-AgsGeSee-AgsSiSee И ХАРАКТЕРИСТИКА ТВЕРДЫХ РАСТВОРОВ Ag8Si1-xGexSe6
*Г.М. Аширов, *К.Н. Бабанлы, *Л.Ф. Машадиева, 2Ю.А. Юсибов, *М.Б. Бабанлы
1 Институт Катализа и Неорганической Химии им. акад. М.Нагиева AZ1143 Баку, пр.Г.Джавида, 113, Баку, Азербайджан 2Гянджинский Государстенный Университет Пр. Г.Алиева, 429, Гянджа, Азербайджан e-mail: [email protected]
Аннотация: Методами дифференциально-термического и рентгенофазового анализов изучены фазовые равновесия в системе Ag2Se-Ag8SiSe6-Ag8GeSe6. На основании экспериментальных результатов и литературных данных построены проекция поверхности ликвидуса системы Ag2Se-Ag8SiSe6-Ag8GeSe6, изотермический разрез при 300 К и некоторые политермические сечения фазовой диаграммы. Установлено, что на боковой системе Ag8SiSe6-Ag8GeSe6 концентрационного треугольника образуются непрерывный ряд твердых растворов на основе высокотемпературной кубической модификации исходных соединений, и ограниченные твердые растворы - на основе их низкотемпературных модификаций. Образование твердых растворов приводит к резкому снижению температур полиморфных переходов тройных соединений и стабилизации высокотемпературных фаз при комнатной температуре. Поверхность ликвидуса системы Ag2Se-Ag8SiSe6-Ag8GeSe6 состоит из 2 областей, отражающих первичные кристаллизации а'-фазы на основе HT-Ag2Se и твердых растворов HT-Ag8Sii-xGexSe6. Полученные новые фазы представляют интерес как экологически безопасные материалы с термоэлектрическими свойствами и смешанной ионно-электронной проводимостью.
Ключевые слова: соединения семейства аргиродита, селенид серебра-германия, селенид серебра-кремния, фазовые равновесия, поверхность ликвидуса, твердые растворы, Т-х диаграмма, параметры кристаллической решетки.
Ag2Se-Ag8GeSe6-Ag8SiSe6 SISTEMIND9 FAZA TARAZLIGI УЭ Ag8Sii_xGexSe6 B9RK MЭHLULLARIN XARAKTERÎSTÎKASI
1 G.M. Э^Ыго^ 1 K.N. Babanli, 1 L.F. Maçadiyeva, 2Y.A. Yusibov, 1 M.B. Babanli
1Akad. M.Nagiyev adina Kataliz va Qeyri-üzvi Kimya institutu AZ 1143, Baki, H.Cavid pr., 113 2Ganca Dövlat Universiteti Ganca §., H .dliyevpr., 429 e-mail : [email protected]
Xülasa: Ag2Se-Ag8SiSe6 - Ag8GeSe6 sisteminda faza tarazliqlari diferensial termiki analiz va rentgen faza analizi üsullari ila tadqiq edilmi§dir. Eksperimental naticalar va adabiyyat malumatlari asasinda Ag2Se-Ag8SiSe6 - Ag8GeSe6 sisteminin likvidus sathinin proyeksiyasi, faza diaqraminin 300 K-da izotermik kasiyi va bazi politermik kasiklari qurulmuçdur. Müayyan edilmi§dir ki, qatiliq ûçbucaginin Ag8SiSe6 - Ag8GeSe6 yan tarafinda ilkin birlaçmalarin yüksak temperaturlu kubik modifikasiyalan arasinda fasilasiz, açagi temperaturlu modifikasiyalari asasinda isa mahdud bark mahlul sahalari amala galir. Bark mahlullarin amala galmasi birlaçmalarin polimorf keçid temperaturlarinin kaskin azalmasi va yüksak temperaturlu fazalarin otaq temperaturunda stabillaçmasina gatirib çixarir. Ag2Se-Ag8SiSe6 - Ag8GeSe6 sisteminin likvidus sathi HT- Ag2Se asasinda a'- fazanin va HT-Ag8Sii-xGexSe6 bark mahlullarinin ilkin kristallaçmasini aks etdiran 2 sahadan ibaratdir. Alinmiç yeni fazalar termoelektrik xassalara va qariçiq ion-elektron keçiriciliyina malik ekoloji tahlükasiz materiallar kimi maraq kasb edir.
Açar sozlar: arqirodit ailasi birlaçmalari, gümü§-germanium selenidi, gümü§-silisium selenidi, faza tarazliqlari, likvidus sathi, bark mahlullar, T-x diaqrami, kristal qafas parametrlari.
