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CHEMICAL PROBLEMS 2023 no. 2 (21) ISSN 2221-8688
UDC 546.86'87'24'22
PHASE RELATIONS IN THE Sb2Te2S-Bi2Te2S SYSTEM AND CHARACTERIZATION OF
Sb2-xBixTe2S SOLID SOLUTIONS
JF.R. Aliyev, JE.N. Orujlu, 1,2D.M. Babanly, 3A.L. Mustafayeva
Azerbaijan State Oil and Industry University, French - Azerbaijani University, AZ1010, Azadlig ave. 20, Baku, Azerbaijan 2Institute of Catalysis and Inorganic Chemistry of MSE Azerbaijan Republic, AZ 1143, H. Javid ave. 113, Baku, Azerbaijan
3Baku State University, AZ 1148, Z. Xalilov str. 23, Baku, Azerbaijan e-mail: _ fariz_ar@hotmail. com
Received 25.03.2023 Accepted 01.06.2023
Abstract: The phase equilibria of the Sb2Te2S-Bi2Te2S system was studied by powder X-ray diffraction (PXRD), differential thermal analysis (DTA), scanning electron microscope (SEM) and energy dispersive X-ray (EDX) microanalysis. It was shown that due to incongruent melting of the Sb2Te2S compound the system is non-quasibinary, but it is stable below the solidus and characterized by the formation of a continuous series of solid solutions with a tetradymite-like hexagonal structure. The lattice parameters were determined from powder diffraction patterns. It was established that the crystal lattice parameters of solid solutions vary linearly with composition.
Keywords: phase diagram, solid solutions, tetradymite-like structure, topological insulators. DOI: 10.32737/2221-8688-2023-2-132-139
Introduction
Van der Waals (VdW) materials based on bismuth and antimony chalcogenides have received great interest thanks to their thermoelectric properties [1-5]. Thermal conductivity of such alloys is reduced due to the effective scattering of phonons, which causes an increase in thermoelectric properties. Such materials include in sodium-ion batteries, solar panels, new-generation generators, and refrigerators [6-10]. Recently, tetradymite-like layered phases have been confirmed as 3D topological insulator (TI) [11-13] phases of quantum matter, thereby giving a boost of interest to the layered tetradymite mineral -Bi2Te2S and its analogues, as well as solid solutions and doped phases based on them [1418]. The last investigations demonstrated that these materials have wide potential applications in optoelectronics, spintronics, quantum computing etc. [19-24].
The Bi2Te2S tetradymite mineral melts congruently at 898 K and crystallizes in rhombohedral space group R-3m with the unit cell parameters a = 4,2648 A and c = 29,5882 A [25]. In its crystal structure, Te-Bi-S-Bi-Te atoms are covalently bonded to form five-layer slabs, alternating along the c axis so that there are three five-layer slabs per unit cell. Due to strong intra-stack cation-anion bonds and the electroneutrality of the stack as a whole, these five-layer slabs are stable structural elements and bonded with each other by the VdW bonds along the c axis.
Literary data on phase equilibria of the Sb2Te3-Sb2S3 system [26] show that the Sb2Te2S compound melts incongruently at 758 K and has a very narrow region of primary crystallization from about 66.5 to 69 mol.% Sb2Te3. The results of [26] confirm that this phase has a tetradymite-like structure in
CHEMICAL PROBLEMS 2023 no. 2 (21)
www.chemprob.org
hexagonal form with parameters a=4.1675A and c=29.483A.
An analysis of the structural features of tetradymite and other tetradymite-like compounds [14, 18, 27-30] shows that, in a five-layer package, the substitution of atoms by related atoms in both cationic and anionic positions is possible. Primary compounds of the system have close lattice parameters and the same space group. This allows them mix to the solid state and form a continuous series of solid
solutions below the solidus.
One of the important approaches to the development of new advanced materials is the study of phase diagrams composed of known compounds possessing desired functional properties [31, 32]. The Sb2Te2S-Bi2Te2S system is interesting in terms of search for solid solutions with Bi ^ Sb substitution based on ternary compounds possessing tetradymite like layered structure.
Experimental part
High-purity (99.999%) elements from Alfa Aesar were used for the synthesis of the initial phases and the alloys of different compositions along the Sb2Te2S-Bi2Te2S section. Stoichiometric amounts of elemental components corresponding to 1 g of samples were sealed under the vacuum (10-2 Pa) in quartz ampoules and heated up to 1000 K and kept at this temperature. After the 3 h synthesis process, all alloys were quenched in ice water and further annealed at 600 K for 1000 h in order to reach full homogenization. The PXRD results of the synthesized alloys showed that samples containing >70 mol % Sb2Te2S contained a small amount of Sb2Te3 phase, that reaching a maximum amount for Sb2Te2S. Taking it into account, these non-equilibrium
alloys were ground into powder, pressed into tablets, and additionally annealed at 600 K for 500 hours. The latter powder XRD data confirmed their single-phase nature.
The NETZSCH 404 F1 Pegasus system with 10 K/min of heating and cooling rate was used for DTA. X-ray phase analysis was performed at room temperature in the range of 29 = 5-75 degrees on a Bruker D2 PHASER diffractometer with CuKai radiation. Lattice parameters were calculated using the TOPAS 4.2 program. SEM analyses were performed by Tescan Vega 3 SBH Scanning Electron Microscope. For elemental analysis of samples by the EDX method, the P/B ZAF quantification method was applied.
Results and discussion
Annealed alloys of the Sb2Te2S-Bi2Te2S compositions were studied by the PXRD system with 20, 40, 60, and 80 mol. % Bi2Te2S method.
- ......1.........................■ SbiTejS
; . 1 _Ji_ Shi « Kit ( 1 c;S
: 1 . „ LÀ A-A shnBlnlejS
: . 1 | _Jl_ - A À ^ A. ,'uv
1 SbuliuItiS
L- 4,,,,,— -JL BitTviS
Hl.no 20.(10 MuM 40.00 50.DO Ml
nun ai lion MiyU- |"ÏB]
Fig.1. PXRD patterns of the alloys along the Sb2Te2S-Bi2Te2S system.
Obtained X-ray diffraction patterns of the alloys are presented in Fig.1.
As can be seen, all intermediate Sb2-xBixTe2S alloys, as expected, are individual phases without any traces of impurities. Diffraction peaks of Sb2Te2S are slightly shifted towards larger angles. This is due to the formation of a continuous solid solutions in this system.
Crystal lattice parameters of all alloys were determined from PXRD patterns using the
TOPAS 4.2 program (Table 1). Based on the results obtained, a graph of the dependence of lattice parameters on composition was constructed (Fig.4). As can be seen, the lattice parameters have a linear dependence on the composition, which indicates that Vegard's rule is observed.
Based on the DTA and PXRD results (Table 1, Fig. 3), the phase diagram of the Sb2Te2S-Bi2Te2S system was constructed.
Table 1. DTA results and lattice parametres of the Sb2-xBixTe2S alloys
Composition, mol % Bi2Te2S Thermal effects, K Parameteres of the crystal lattice, Â
Sb2Te2S 758, 853 a = 4.1675(2), c = 29.483(1)
10 770-791-848 —
20 783-824-844 a = 4.1877(1), c = 29.503(2)
30 798-841 —
40 812-849 a = 4.2068(2), c = 29.527(3)
50 824-863 —
60 839-873 a = 4.2284(1), c = 29.548(2)
70 854-878 —
80 868-883 a = 4.2462(2), c = 29.569(3)
90 878-890 —
Bi2Te2S 893 a = 4.2648(1), c = 29.588(2)
Figure 2 a demonstrates the DTA heating thermogram for non-homogenized as-cast Sb16Bi04Te2S alloy. In order to reach the equilibrium state, alloy was grounded into powder and pressed into pellet. Further, this pellet was annealed at 600 K for 500 hours.
Thermal analysis of the homogenized pellet illustrates the achievement of equilibrium state. Thermal effects at 740 K and 773 K were transformed into one peak at 783 K illustrated in the figure 2b.
Fig. 2. DTA heating thermograms for the alloys: a) as-cast Sb16Bi04Te2S; b) annealed Sb16Bi0.4Te2S.
Figure 3 shows the phase diagram of the Sb2Te2S-Bi2Te2S system. The system is partially non-quasibinary due to the incongruent melting of one of the initial compounds, Sb2Te2S. The a-phase based on Sb2Te3, initially
crystallizes from the solution in the <30 mol. % Bi2Te2S composition range. Below the liquidus of the a-phase, crystallization continues according to the monovariant peritectic reaction
L+a-—-y, leading to the formation of an L-—7. Excess melt crystallizes with the
intermediate three-phase region L+a+y, which formation of a stable sub-solidus y-phase and
could not be detected by means of DTA. In the forms Bi — Sb cation-substituted continuous
>30 mol. % Bi2Te2S compositions field, solid solutions. crystallization proceeds according to the scheme
Fig. 3. The phase diagram of the Sb2Te2S-Bi2Te2S system
Fig. 4. Dependence of lattice parameters on composition along the Sb2Te2S-Bi2Te2S system
The SEM image of the sample with 40 by the EDX method of same sample are given in mol. % Bi2Te2S composition is given in the the table 2 which shows that elemental
Fig.5. As can be clearly seen from the image, composition corresponds to the stoichiometry of the sample is monophasic and has a layered the given alloy. structure. Results of the elemental microanalysis
Fig.5. SEM image of the alloy containing 40% mol Bi2Te2S
Table 2. Elemental microanalysis results of the 40% mol Bi2Te2S sample
Element Weight % Atom % Error %
Antimony 23.78 22.43 0.98
Bismuth 27.45 17.37 2.78
Tellurium 41.95 43.91 1.27
Sulphur 6.82 16.29 4.10
100 100
Conclusion
From results of DTA, XRD, SEM and EDX investigations the phase diagram of the Sb2Te2S-Bi2Te2S system was constructed. It has been established that the system is non-quasibinary as a whole, however, is stable in the subsolidus region and characterized by the formation of continuous solid solutions with a
tetradymite-like structure. Lattice parameters of solid solutions were defined to be linearly dependent upon composition. Obtained solid solutions of the Sb2.xBixTe2S composition have great practical interest as potential thermoelectrics and topological insulators.
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ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ Sb2Te2S-Bi2Te2S И ХАРАКТЕРИСТИКА ТВЕРДЫХ РАСТВОРОВ Sb2-xBixTe2S
Ф.Р. Алиев, Э.Н. Оруджлу, А.Л. Мустафаева, Д.М. Бабанлы
1 Азербайджанский университет нефти и промышленности, Французско-азербайджанский университет Пр. Азадлыг, 20, Баку, AZ 1010 2 Институт катализа и неорганической химии Пр. Г.Джавида, 113, Баку, AZ1143 3Бакинский государственный университет Ул. З.Халилова, 23, Баку, AZ1148, e-mail: _ fariz_ar@hotmail. com
Аннотация: Фазовые равновесия в системе Sb2Te2S-Bi2Te2S были исследованы методами ренгенфазового анализа, дифференциального термического анализа, сканирующего электронного микроскопа и энергодисперсионного рентгеновского микроанализа. Показано, что из-за инконгруэнтного плавления соединения Sb2Te2S система в целом неквазибинарна, но устойчива ниже солидуса и характеризуется образованием непрерывного ряда твердых растворов с тетрадимитоподобной гексагональной структурой. По результатам порошковых дифрактограмм были определены параметры решетки. Установлено, что параметры кристаллической решетки твердых растворов линейно зависят от состава. Ключевые слова: фазовая диаграмма, твердые растворы, тетрадимитоподобная структура, топологические изоляторы.
\Sb2Te2S-Bi2Te2S SiSTEMiNDa FAZA TARAZLIQLARI УЭ Sb2-xBixTe2S B9RK
M9HLULLARININ XASSaLaRi
F.R. aliyev, E.N. Oruclu, A.L. Mustafayeva, D.M.Babanli
1Azsrbaycan Dovlst Neft vs Ssnaye Universiteti, Azsrbaycan-Fransiz Universiteti
AZ 1010, Baki, Azadliq pr., 20 2Kataliz vs Qeyri-uzvi Kimya institutu AZ 1143, Baki, H.Cavidpr., 113
3Baki Dovlst Universiteti AZ 1148 Baki, Z.Xslilov kug., 23 e-mail: fariz_ar@hotmail. com
Xulasa: Sb2Te2S-Bi2Te2S sisteminin faza tarazhqlan toz rentgenfaza analizi, differensial termiki analiz, skanaedici elektron mikroskopiyasi va enerji-dispersiv rentgen mikroanalizi ila tadqiq edilmi§dir. Gostarilmi§dir ki, Sb2Te2S birla§masinin inkonqruent arimasi sababindan sistem butovlukda qeyri-kvazibinar olsa da solidusdan a§agida stabildir va tetradimitebanzar heksoqonal qurulu§a malik fasilasiz bark mahlullarin amala galmasi ila xarakteriza olunur. Toz difraktoqramlari asasinda numunalarin qafas parametrlari tayin olunmu§ va muayyan edilmi§dir ki, bark mahlullarin kristal qafas parametrlari tarkibdan asililigi xattidir.
A?ar sozlar: faza diaqrami, bark mahlullar, tetradimitabanzar qurulu§, topoloji izolyatorlar.
