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CHEMICAL PROBLEMS 2019 no. 3 (17) ISSN 2221-8688
423
UDC 548.5
REFINEMENT OF PHASE DIAGRAM IN THE CuiS-GeSi SYSTEM
I.J. Alverdiyev
Ganja State University 425, H.Aliyev ave., AZ 2001, Ganja, Azerbaijan, e-mail: ialverdiyev73@gmail.com
Received 10.07.2019
Abstract: Given the inconsistency of the data available in the literature, the Cu2S-GeS2 system was re-studied through the use of DTA and XRD methods. A special emphasis is laid on obtaining equilibrium alloys and preventing deviations of their composition from the stoichiometry of this section. The constructed phase diagram reflects the presence of the two ternary compounds in the system: Cu8GeS6 and Cu2GeS3. The first melts incongruently at 1253 K and has the phase transition at 328 K, and the second melts congruently at 1215 K.
Keywords: Cu-Ge-S system, phase diagram, copper tiogermanates, Cu8GeSg, Cu2GeS3, eutectic. DOI: 10.32737/2221-8688-2019-3-423-428
Introduction
Complex copper chalcogenides are valuable functional materials with thermoelectric, photoelectric, optical, and other properties [1-8]. Recent years marked growth of interest in environmentally friendly thermoelectric materials based on these compounds [1, 9-12]. In addition, many complex copper chalcogenides have mixed ionic-electronic conductivity which makes them very promising for use in the development of photoelectrode materials, electrochemical converters of solar energy, ion-selective sensors, photo-electrochemical visualizers, ionizers, etc. [1, 13-18].
The development of methods for directed synthesis of new complex chalcogenide phases is based on phase equilibriums and thermodynamic data for the corresponding systems [19-22]. Complexes of similar data for a number of complex chalcogenide systems were obtained in [2332].
It should be noted that Cu-Ge-S system is characterized by the formation of ternary compounds with interesting physical properties [1, 8, 12, 13, 15-17] to be used in
practice. In spite of the fact that phase equilibriums in the Cu-Ge-S system and particularly, in the quasibinary section Cu2S-GeS2 were studied in a number of works [5, 33-35], there are some contradictions in the results obtained.
According to [33], this system is characterized by the formation of two ternary compounds Cu8GeS6 and Cu2GeS3, melting incongruently at 1253 and 1213 K, respectively. According to the data of [34], only the Cu2GeS3 compound is formed in the system which melts congruently at 1229 K. In [5], it showed that the Cu2GeS3 melts congruently at 1215 K. The data of [33-35] significantly differ from each other in the coordinates of invariant points. In addition, when studying the
2Cu2S + GeSe2 o 2Cu2Se + GeS2 mutual system [24], we observed some discrepancy between our experimental data and the patterns of phase equilibriums along the Cu2S-GeS2 section.
In view of the foregoing, the objective of this work is to reinvestigate phase equilibriums in the Cu2S-GeS2 system.
Experimental part
To study the Cu2S-GeS2 system, first initial binary compounds (each 20 g) were
synthesized. The syntheses were carried out by direct interaction of elementary components of
2
a high degree of purity in evacuated (~10" Pa) quartz ampoules in a dual-zone inclined furnace. The lower "hot" zone was heated up to 1200 K, and the upper "cold" up to 650 K (boiling point of sulfur - 718 K [35]). In this temperature regime, sulfur completely interacts for two hours with metallic copper, as evidenced by the absence of traces of sulfur (dark brown vapor phase and droplets) on inner walls of the ampoule. Then, to obtain a stoichiometric composition in line with the recommendation [24], the ampoule was sharply cooled in ice water.
Synthesis of the Cu2S was carried out in a similar way. Elementary germanium and sulfur were powdered, transferred into an ampoule which was pumped out and sealed. The temperature of the "hot" zone was raised up to 800 K, and the "cold" to 650 K. Note that in terms of this temperature mode, most of the sulfur was absorbed by germanium in 2530 hours. After this, the ampoule was
Results
completely transferred to the "hot" zone and kept at 800 K for another 10 hours and then chilled.
Both synthesized compounds were identified by DTA and XRD.
Alloys of the Cu2S-GeS2 system (each 0.5 g) were prepared by melting starting compounds in various ratios inside evacuated thick-walled quartz ampoules. The temperature was slowly (over 5 hours) heated up to 1000 K, kept for 10 hours, and then heated up to 1300 K (0-50 mol%) and 1200 K (55-95 mol%). After melting, the temperature was lowered to 800 K where thermal annealing was performed for 500 hours.
All prepared samples were analyzed by means of DTA, XRD, and EMF technique. The XRD data were collected at room temperature using a Bruker D8 ADVANCE diffractometer (with Cu-Ka1 radiation). DTA of the equilibrated alloys was carried out by means of NETZSCH 404 F1 Pegasus system device. The heating rate was 10 K/min.
id discussion
Analysis of powder X-ray diffraction patterns of annealed and slowly cooled alloys with selective compositions showed the presence of two Cu8GeS6 and Cu2GeS3 ternary compounds with practically constant compositions. X-ray diffraction patterns of alloys with intermediate compositions consisted of sets of reflection lines with two phases to confirm the presence of two-phase
mixtures, Cu2S- Cu8GeS6, Cu8GeS6-Cu2GeS3, and Cu8GeS6+GeS2. Cited as an example, Fig. 1 shows powder diffraction patterns of Cu8GeS6, Cu2GeS3 compounds and an alloy with composition 30 mol% GeS2. As you can see, this alloy is a two-phase mixture of these ternary compounds. The diffraction pattern does not contain reflection lines that could be attributed to Cu4GeS4 or another phase.
Fig.1. Powder diffraction patterns Cu8GeS6, Cu2GeS3 compounds and alloy with composition of 30 mol% GeS2
The DTA data are given in the Table, and the diagram based thereon is shown in Fig. 2. As can be seen, two ternary compounds are formed in the system: Cu8GeS6 and Cu2GeS3. The first melts by peritectic decomposition at 1253 K and undergoes a polymorphic transformation at 328 K, while the second melts congruently at 1215 K. The peritectic
point (P) has a composition of 25 mol%. There are two eutectics (ei and e2) in the system that have compositions of 43 and 90 mol% and are crystallized at 1205 K and 1092 K, respectively. The horizontal lines at 375 and 328 K correspond to polymorphic transitions of Cu2S and Cu8GeS6, respectively.
Table 1. DTA data for the Cu2S-GeS2 alloys
Composition, mol % GeS2 Thermal effects, К
Isothermal Polythermal
CU2S 377; 1403 -
5 375;1255 1255-1385
10 328; 375; 1252 1252-1330
20 328;1253 1253-1285
25 330;1250 1208-1250
30 328; 1205 1205-1247
40 326;1205 1205-1222
45 1205 -
50 1215 -
52 1090 1090-1212
60 1090 1090-1206
70 1095 1095-1185
80 1092 1092-1165
90 1092 -
95 1090 1090-1113
100 1120 -
Fig.2. Phase diagram of the Cu2S-GeS2 system
Comparison of the constructed phase diagram with the literature data [5, 33-34] shows that in many respects it is in accord with [33]. The main difference concerns the melting character of Cu2GeS3. According to our data, this compound melts congruently and forms an eutectic with Cu8GeS6, whereas according to [33], the horizontal at 1213 K extends to 70 mol% GeS2 and corresponds to
peritectic equilibrium. According to Fig. 2, this horizontal corresponds to a temperature of 1205 K and refers to eutectic equilibrium. We have not confirmed the existence of ternary compounds of the compositions Cu4GeS4 and Cu2Ge2S5 indicated in [5, 36]. Perhaps they are metastable phases and are absent in the equilibrium phase diagram.
Conclusion
Cu2S-GeS2 system was examined in in the system. Cu8GeS6 melts incongruently at
detail by DTA and XRD methods and phase 1253 K and has a phase transition at 328 K
diagram was constructed. Two ternary while Cu2GeS3 melts congruently at 1215 K. Cu8GeS6 and Cu2GeS3 compounds were found
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Cu2S-GeS2 SiSTEMiNiNFAZA DiAQRAMININDdQiQLd^DiRiLMdSi
i. C. Alverdiyev
Ganca Dövlat Universiteti AZ2001, Ganca, H.dliyevpr., 425, e-mail: ialverdiyev73@gmail.com
Cu2S-GeS2 sistemina aid dddbiyyat malumatlarinin ziddiyydtli olmasini nazara alaraq, bu sistemda faza tarazliqlari DTA va RFA üsullari ila takrar tadqiq edilmiçdir. Tadqiqatlar zamani nümunalarin tarazliq halina gatirilmasina va onlarin tarkiblarinin baxilan sistemin T-x müstavisi üzarinda olmasina xüsusi diqqat verilmiçdir. Qurulmuç faza diaqramina asasan, sistemda 2 ûçlû birlaçma mövcuddur: CusGeSe va Cu2GeS3. Birinci 1253 К-da inkonqruent ariyir va 328 К-da polimorf çevrilmaya maruz qalir, ikinci isa 1215 К-da konqruent ariyir.
Açar sözlw. Cu-Ge-S sistemi, faza diaqrami, mis-tiogermanatlari, CusGeS6, Cu2GeS3, evtektika.
УТОЧНЕНИЕ ФАЗОВОЙ ДИАГРАММЫ СИСТЕМЫ Cu2S-GeS2
И.Дж. Алвердиев
Гянджинский Государственный Университет AZ 2001, Гянджа, пр. Г.Алиева, 425, e-mail: ialverdiyev73@gmail.com
Учитывая противоречивость имеющихся в литературе данных, в работе методами ДТА и РФА повторно изучена система Cu2S-GeS2. Особое внимание уделено получению равновесных сплавов и предотвращению отклонения их состава от стехиометрии данного разреза. Построенная фазовая диаграмма отражает наличие в системе двух тройных соединений: CusGeSв и Cu2GeS3. Первое плавится инконгруэнтно при 1253 К и имеет фазовый переход при 328 К, а второе плавится конгруэнтно при 1215 К. Ключевые слова: система Cu-Ge-S, фазовая диаграмма, тиогерманаты меди, CusGeSв и Cu2GeSз, эвтектика.
