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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 2 2021 ISSN 0005-2531 (Print)
UDC 241. 123.2
PHASE EQUILIBRIUM IN THE Ga2Se3-CoSe SYSTEM
N.M.Allazova\ T.R.Qurbanova2, R.F.Abbasova2, M.R.Allazov3
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
2Baku State University 3 Azerbaijan Technical University
nigar.allazova@gmail.com
Received 15.01.2021 Accepted 30.03.2021
Using differential thermal, X-ray phase, microstructural analyzes and microhardness measurements, we studied the phase equilibrium in the Ga2Se3-CoSe system in the entire concentrated region and constructed its phase diagram. It has been established that the phase diagram refers to eutectic types with monotectics. The eutectic crystallizes at 10000C and 10 mol% CoSe. Liquid phase delamination occurs in the concentration range from 15 to 85 mol% CoSe and 10300C. There are no intermediate compounds formed in the system.
Keywords: system, phase equilibrium, delamination, phase transition, cobalt selenide.
doi.org/10.32737/0005-2531-2021-2-59-62 Introduction
Ga2Se3 is a defective compound formed with an open maximum at 10200C. In the crystal lattice of this sphalerite-type compound, 1/3 of the space occupied by gallium is empty, and its crystallochemical formula is written as Ga0,666[]0,333Se [1, 2]. The large number of own defects leads to a high concentration of charge
20 21 3
carriers in this compound (~1020-1021) cm , and therefore alien impurity atoms in it are not electroactive. On the other hand, due to a large number of specific defects its thermal conductivity reduces, that leads to an increase in the cost of its thermoelectricity [2]. Defective structure of the compound may affect the nature of its interaction with other chalcogenides and the kinetics of the process. It is accidental because on the basis of Ga2Se3 and other similar compounds type A i,11 B there has been obtained a number of compounds of more complex composition, as well alien elements [1, 3-6].
Finally, structural defects have a significant effect on many mechanical and physical parameters of solids. For example, the effect of defects on the electrical conductivity, photoconductivity, plasticity, density, etc. of a substance. varies. In fact, Ga2Se3 and such compounds with a special structural defects are insensitive to many elements in terms of electrical
conductivity. However, it is known that transition elements, especially 3d-transition elements, have a significant effect on their electrophysical and magnetic properties. It should be noted that the nature of the interaction of various 3d-transition elements with indium and gallium chalcogenides of formula A111B varies sharply [6]. For example, the presence of a FeGa2Se4 compound in the Ga2Se3-FeSe system has been confirmed in the corresponding phase diagrams [7, 8]. However, a compound with a similar composition in the presence of nickel was not found in the Ga2Se3-NiSe system, it was confirmed that the phase diagram of this system is monotectic-eutectic and layered in liquid alloys over a wide concentration range [6, 9]. It should be noted that a similar situation is repeated in similar systems in the presence of sulfur. That is, the presence of FeGa2S4 is confirmed in the corresponding phase diagram [10-12] and it is noted that alloys containing 50 mol% Ga2S3 and 50 mol% CoS (NiS) are two-phase [6, 13, 14]. However, in many studies such two-phase compounds have been presented as compounds [1517]. Thus, FeGa2S4 (Se4) compounds have dont cobalt and nickel analogues. In contrast, in Ga2S3 (Se3)-CoS (NiS) systems, stratification is observed in liquid alloys containing 50 mol% CoS (NiS).
The authors of [18] also show the presence of CoGa2Se4 compound in the Ga2Se3-CoSe system. The authors of [19] show that intermediate phase is not formed in the Ga2Se3-CoSe system. Unfortunately is found in sometimes contradictory information the literature.
Experimental part
The purpose of this article is to reexamine the Ga2Se3-CoSe system and determine the true existence of a compound containing CoGa2Se4 by the classical methods of physical- chemical analysis.
Samples of the system were synthesized by melting of highly pure elements (gallium grade Ga-000, cobalt grades are carbonyl, selenium grade OSCH 17-3) in evacuated (~ 0.1 Pa) and sealed quartz ampoules. During synthesis, the temperature was gradually raised to 11000C and maintained by stirring periodically for 6 hours. Samples were cooled slowly (~6 deg/min) to room temperature and kept to heat treatment processing at 8000C for 100 hours.
As a result of the synthesis, completely melted gray, monolithic samples that do not adhere to the quartz ampoule are obtained. The samples are divided into two approximately equal parts. The first part is taken for X-ray diffraction and MSA. The second part is crushed into small pieces, filled into special quartz ampoules for DTA, vacuumed and sealed.
Thermograms of the samples were taken
on a two-coordinate potentiometer H 307/1 with the help of a chromel/alumel thermocouple. Calcined Al2O3 powder was used as a standard. Thermograms mainly show two series of thermal effects (1030 and 1000°C). The thermogram of the stoichiometric CoSe compound has only one thermal effect, its melting point (10550C), that corresponds to the thermal analysis of cobalt monoselenide made of carbonyl cobalt [20].
The reference literature on binary systems [21] states that there is a polymorphic transition at ~ 640.50C. In this study, the stoichiometric CoSe thermogram shows no thermal effect other than the melting process.
Results and its discussion
Diffractograms of CoSe and intermediate samples of the system were drawn on a D2 PHASER diffractogram using monochromatic CuKa rays. X-ray reflexes determined from a diffractogram of CoSe with a stoichiometric composition gradually cooled to room temperature are the same as those shown in [20]. The reflexes observed in the diffractograms of the intermediate components show that the samples of the system are two-phase (consisting of primary compounds) (Table 1). Reflexes belonging only to Ga2Se3 are observed to move to a slightly larger angle, that may be due to the solution based on Ga2Se3. In intermediate samples, CoSe-specific reflexes remain unchanged in angle.
Ga2Se3 50 mol% Ga2Se3 + 50 mol% CoSe CoSe
da, nm* Inis. da, nm Inis. da, nm Inis.
0.508 6 0.508 13
0.314 35 0.314 20
0.300 4
0.271 6 0.271 2
0.268 45 0.268 100
0.238 5 0.238 12
0.202 2
0.192 100 0.192 60
0.184 7 0.184 15
0.180 21 0.180 46
0.174 3 0.174 9
0.164 10 0.164 6
0.159 5 0.159 2
0.145 10 0.145 22
0.133 7 0.133 17
PHASE EQUILIBRIUM ÎN THE Ga2Se3-CoSe SYSTEM
61
H|,MPa 5000 -
3000-
t°,C
1000
900
800 -
700
4850
<3160 0
Li+a
Ga2Se3 20
b)
-I-1-1-r-
L1+L2
\Li+CoSe
1030
1000 -O——-o-
L2 + CoSe -o----
a + CoSe
a)
-1-1-1-1-1-T"
-1-T"
1055
40 60
mol%
80
CoSe
Ga2Se3 - phase equilibrium of the CoSe system (a) and dependence of microhardness on composition (b).
a
Microstructure analysis and microhardness measurements were performed on a PMT-3 device. Phase liquefaction is observed in the samples in the liquid state, the heavier phase accumulates in the lower part of the samples. Here, the microhardness of the CoSe phase is 4850 MPa under load 50 g and the microhardness of the Ga2Se3 phase under load 20g was set equal to 3160MPa.
Based on the results of the study, a phase diagram of the Ga2Se3-CoSe system was constructed (Figure).
Conclusion
As can be seen, the system is a monotectic eutectic type. Eutectic crystallizes at 10 mol% CoSe and at 1000°C. The monotectic process takes place at 85 mol% CoSe and 10300C, and stratification in the liquid alloy covers a range of 15-85 mol% concentration. The area of a solid solution based on Ga2Se3 can be up to 5 mol% CoSe at eutectic temperature. However, at room temperature, the limit of a-solid solution is less
than 1 mol% CoSe. Cobalt monoselenide-based solubility is practically non-existent.
Thus, the results of our research confirm the conclusions of the authors of the work [19]. That is, the Ga2Se3-CoSe system does not form a new intermediate phase, including a compound containing CoGa2Se4.
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Ga2Se3-CoSe SISTEMINDO FAZA TARAZLIGI
N.M.Allazova, T.R.Qurbanova, R.F.Abbasova, M.R.Allazov
Differensial termiki (DTA), rentgen faza, mikroqurulu§ va mikrobarkliyin ólgülmasi üsullari ila Ga2Se3- CoSe sisteminin bütün qurulu§ sahalarinda faza tarazligi tadqiq edilmi§ va sistemin faza diaqrami qurulmu§dur. Malum olmuijdur ki, sistemin faza diaqrami monotektikali evtektik tiplidir. Sistemin evtektikasi 10000C va 10 mol% CoSe tarkibda kristalla§ir. Maye fazada tabaqala§ma geni§ qatiliq intervalinda (15-85 mol% CoSe) va 10300C temperaturda ba§ verir. Sistemda yeni birla§ma a§kar edilmami§dir.
Agar sozlar: sistem, faza tarazligi, tsbsqsls^ms, faza kegidi, kobalt selenid.
ФАЗОВОЕ РАВНОВЕСИЕ В СИСТЕМЕ Ga2Se3-CoSe
Н.М.Аллазова, Т.Р.Гурбанова, Р.Ф.Аббасова, М.Р.Аллазов
Методами дифференциально-термического, рентгенофазового, микроструктурного анализов и измерением микротвердости исследовано фазовое равновесие в системе Ga2Se3-CoSе во всей концентрированной области и построена её фазовая диаграмма. Установлено, что фазовая диаграмма относится к эвтектическим типам с монотектикой. Эвтектика кристаллизуется при 10000C и 10 мол% CoSe. расслаивание фаз в жидком состоянии происходит в интервале концентраций от 15 до 85 мол.% CoSe и 10300C. В системе не образуются промежуточные соединение.
Ключевые слова: система, фазовое равновесие, расслаивание, фазовый переход, селенид кобальта.
