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730 East European Scientific Journal #10(74), 2021 ...av,...,..
ХИМИЧЕСКИЕ НАУКИ
UOT546.863'24 '+665 ' 24
Mamedova N.Sh.
doctoral student Baku State University, Baku
PHASE EQUILIBRIUM IN THE Sb2Te3-HoTe3 SYSTEM
DOI: 10.31618/ESSA.2782-1994.2021.3.74.141
Summary: The phase equilibrium of the Sb2Te3-HoTe3 system was studied by means of physical and chemical analysis methods DTA, RFA, MQA, as well as density and microhardness measurements, and its phase diagram was constructed. It has been determined that the Sb2Te3-HoTe3 system is a partial quasi-binary cross section of the ternary Bi-Ho-Te system. The system undergoes a process of eutectic equilibrium and peritectic transformation. In the Sb2Te3-HoTe3 system at room temperature, of the based Sb2Te3 solid solutions extend to 4.5 mol % and of the based HoTe3 solid solutions have practically not been established.
Keywords: phase, solid solution, eutectic, syngony, microhardness.
The ternary system of antimony and holmium chalcogenides has not been studied in detail in the literature. It should be noted that the physicochemical and physical properties of holmium chalcogenides have not been studied in detail. It is noted that HoTe, Ho2Te3, Ho2Te5 and HoTe3 compounds are obtained in the HoTe system. Only the crystallographic properties of these compounds have been studied. It is known that chalcogenides of rare earth elements and triple compounds and solid solution alloys based on them are magnetic, photoelectric, thermoelectric and luminescent materials and are widely used in semiconductor technology [1-10].
Unlike holmium chalcogenides, a large number of systems involving antimony chalcogenides have been studied. Sulfide and selenide compounds of antimony and ternary compounds based on them and solid solution alloys show high photoelectric properties [1115]. Antimony telluride compounds are materials with medium resistance thermoelectric properties [16-19]. From this point of view, the study of the chemical interaction of Sb2Te3 and HoTe3 chalcogenides is of scientific and practical importance. The Sb2Te3-HoTe3 system is being studied for the first time.
The aim of this work is to construct the phase diagram of the Sb2Te3-HoTe3 system by studying its phase equilibrium. The following information is available on the initial components of the system: The Sb2Te3 compound melts congruently at 622°C and crystallizes in the rhombohedral-hexagonal system of the Bi2Te2S type, lattice parameters: ah = 4.264 A, ch = 30.42 A, p = 23o34/, sp. gr. R3m-D53d [20]. The HoTe3 compound melts incongruently at 780°C and crystallizes in the tetragonal system, lattice parameters: a = 4.29; c = 25.40 A, sp.qr. Bmmb [10].
Experimental part
Alloys of the Sb2Te3-HoTe3 system were synthesized by fusing the components Sb2Te3 and HoTe3 in a quartz ampoule evacuated to 0.133 Pa. The
synthesis was carried out in the temperature range 800-1000°C. Taking into account the peritectic nature of the formation of the HoTe3 compound, annealing was carried out below 20°C at the peritectic temperature to obtain its full composition. The samples were heat treated at 400°C for 350 hours to achieve equilibrium. Homogenized samples were investigated by methods of physicochemical analysis (DTA, XRD, MSA, as well as by measuring density and microhardness).
Differential thermal analysis (DTA) of the alloys was carried out on an HTR-73 low-frequency pyrometer. Al2O3 was used as a standard, the heating rate was 10°C.
X-ray diffraction patterns of the alloys were taken on a D2 PHASER X-ray device using CuKa radiation. The microstructural analysis of the alloys was carried out on an MIM-8 metallographic microscope. To reveal the microstructure of the alloys, we used an etchant with the composition HNO3 conc.: H2O2 = 2: 1, etching time was 10 s.
The microhardness of each phase was measured on a PMT-3 device at a load of 0.15 N. The density for the samples was determined by the pycnometric method.
Results and its discussion
The synthesized alloys of the Sb2Te3-HoTe3 system are in a compact mass. Rich samples with Sb2Te3 are silver, the rest are gray. The system alloys are resistant to air, water and organic solvents. They are readily soluble in acids HNO3, H2SO4 and strong alkalis (NaOH, KOH). After the homogenization of the samples was completed, a physicochemical analysis was carried out. According to the results of differential thermal analysis of the samples, it was found that the thermograms of alloys of the Sb2Te3-HoTe3 system contain two and three endothermic effects. High thermal effects in the system are due to the decomposition of HoTe3 as well as complex interactions.
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Fig. 1. Microstructures of alloys of the Sb2Te3-HoTe3 system. 1-4, 2-30, 70 mol % HoTe3.
The results of microstructure analysis (MSA) of the alloys show that the samples in the concentration range of 0-4.5 mol % HoTe3 are single-phase, and the rest of the samples are two-phase. HoTe3-based solid solutions are practically undefined. In fig. 1 shows the microstructures of alloys of the Sb2Te3-HoTe3 system. The 4 mol % HoTe3 sample is a solid solution based on the Sb2Te3 compound. Samples with 30 and 70 mol % HoTe3 are two-phase.
To determine the accuracy of the DTA results and analyze the microstructure, an X-ray phase analysis of
the alloys of the system was carried out. For this, Fig. 2 shows the results of X-ray phase analysis of HoTe3 samples with 4 and 80 mol %. As seen from Fig. 2, diffraction patterns of samples with 30 and 70 mol % HoTe3 consist of diffraction lines of the initial components. The diffraction lines in the diffraction pattern of the alloy containing 4 mol % HoTe3 are identical to the diffraction lines of the SbtTe3 compound and differ insignificantly in interplanar distances. This sample is a solid solution based on SbtTe3 (Fig. 2).
2. Diffraction patterns of alloys of the Sb2Te3-HoTe3 system. 1-Sb2Te3, 2-4, 3-80, 4-100 mol% HoTe3.
The phase diagram of the Sb2Te3-HoTe3 system was constructed based on the results of physicochemical methods of analysis (Fig. 3). The Sb2Te5-HoTe3 system was found to be, as expected, a partial quasi-binary section of the Sb-Ho-Te ternary
system. The liquidus of the system is surrounded by monovariant curves of the equilibrium of an a-solid solution based on SbtTe3 and a HotTes compound in liquid equilibrium. Eutectic equilibrium and peritectic transformation take place in the system.
32 East European Scientific Journal #10(74), 2021
t°C
®+H02Te5 :+HoTe3+ H02Te5
a+HoTe3
_l_I_L
J_I_L.
800 780
Sb2Te3 20
40 60
MOfl. %
80 H0Te3
Fig.3. Phase diagram of the Sb2Te3-HoTes system.
Since the HoTe3 compound is peritectic, splitting in this region leads to the formation of three-phase regions above the solidus line. There are three-phase fields in the concentration range 12-40 mol % HoTe3 (M + a + Ho2Tes) and in the range 40-100 mol % HoTe3 (M + Ho2Te5 + HoTe3). The intersection of the a and Ho2Te5 liquidity curves in the system is 20 mol % HoTe3 and 550oC. At this moment, a three-phase equilibrium M-^-a + Ho2Te5 is formed. Primary crystals of the Ho2Te5 compound are separated from the liquid in the concentration range of 20-100 mol % HoTe3. In
the system, the four-phase peritectic transformation M + Ho2Te3^a + HoTe3 occurs on an isothermal line with a temperature of 400°C. At room temperature, solid solutions up to 4.5 mol % are formed in a system based on Sb2Te3, while solid solutions based on HoTe3 are practically not established. Two-phase alloys a + HoTe3 crystallize in the range of 4.5 - 100 mol % HoTe3 below the solidus line. Some physicochemical properties of alloys of the Sb2Te3-HoTe3 system are given in Table 1.
Table 1.
Results of DTA, measurements of microhardness and determination of the density
Composition, mol% Thermal effects , °C Density, 103 kq/m3 Microhardness, MPa
Sb2Te3 HoTe3
a HoTe3
P=0,15 H
100 0,0 622 6,51 1200 -
95 5,0 540,615 6,53 1250 -
90 10 450,600 6,56 1260 -
80 20 400,560 6,60 1260 -
70 30 400,520,575 6,62 1260 -
60 40 400,640 6,65 1260 -
50 50 400,500,680 6,67 - -
40 60 400,575,710 6,69 - 1190
30 70 400,650,750 6,72 - 1190
20 80 400,700,770 6,76 - 1190
10 90 400,725,790 6,80 - 1180
0,0 100 780,800 6,82 - 1160
o
o
As can be seen from the table, two different values microhardness of alloys in the system. The value of of microhardness were determined when measuring the microhardness (1200-1260) MPa corresponds to the
ив
BBgSB
microhardness of the a-solid solution formed on the basis of Sb2Te3, the value of (1160-1190) MPa corresponds to the microhardness of the new HoTe3 compound. The density of the alloys increased monotonically in the two-phase field between the solid solution area.
Conclusion
By studying the chemical interactions in the Sb2Te3-HoTe3 system, its phase diagram is constructed. It was found that the Sb2Te3-HoTe3 system is a partial quasi-binary region of the Sb-Ho-Te ternary system. The system is undergoing a process of eutectic equilibrium and peritectic transformation. It was found that, at room temperature, solid solutions based on Sb2Te3 reach 4.5 mol % while solid solutions based on HoTe3 are practically not found. Since the HoTe3 compound is peritectic, it decomposes at a higher peritectic temperature, which leads to the formation of three-phase regions (M+a+Ho2Tes) and (M+Ho2Te5+HoTe3) above the solidus line. In the concentration range 4.5-100 mol.% HoTe3 below the solidus line, two-phase alloys consisting of a + HoTe3 crystallize.
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