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AZERBAIJAN CHEMICAL JOURNAL No 3 2020 ISSN 2522-1841 ^riin^
ISSN 0005-2531 (Print)
UDC 546.86872415
PHYSICO-CHEMICAL INTERACTION IN THE SbTeI-Bi2Te3 SYSTEM
E.J.Ahmadov
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
elvin.ehmedov.2014@mail.ru
Received 10.03.2020 Accepted 08.06.2020
The SbTeI-Bi2Te3 polythermal section of the phase diagram of the Sb2Te3+2BiI3-^Bi2Te3+2SbI3 reciprocal system was constructed using DTA and XRD methods. It was established that Bi2-xSbxTe3 solid solutions (a-phase) primarily crystallize from the liquid phase along this section. Subsequently, crystallization proceeds by a number of non- and monovariant reactions, a+y1, a+y2, and a+y1+y2 (y1 and y2 are solid solutions based on SbTeI, and BiTeI) heterogeneous regions of are formed in subsolidus.
Keywords: SbTel-Bi2Te3 system, phase diagram, solid solutions, topological insulators, Rashba semiconductors.
doi.org/10.32737/0005-2531-2020-3-46-49 Introduction
Chalcogenides and chalcohalogenides of heavy p-elements are in the center of attention as materials exhibiting 3D topological insulating properties and 3D Rashba spin splitting [1-5]. These compounds are considered to be promising materials for spintronics, quantum computing, and energy converters due to their unique optical, photoelectric, thermoelectric, magnetic, and other properties [6-8].
The search and design of multi-component chalcogenide and chalcohalogenide materials are based on the study of phase equilibria in relevant systems [9-11].
Systems composed of Sb and Bi chalco-genides and halogenides are of particular interest in terms of obtaining new chalcohalide phases. For example, the study of the Bi2Se3-Bi2Te3-BiI3 system revealed wide solutions based on BiTeI and BiSeI compounds and continuous solid solutions along with the Bi2Se3-Bi2Te3 boundary system [12]. The formation of Sb-^-Bi substitute solid solutions is also expected in the Sb2Te3+ 2BiI3^Bi2Te3+2SbI3 system. In previous studies, we presented the results of solid-phase equilibria and the T-x phase diagram of the SbTeI-BiTeI cross-section in this system [13, 14].
Presented work is devoted to the study of the character of the physico-chemical interaction in the Sb2Te3+2BiI3^Bi2Te3+2SbI3 reciprocal system along the SbTeI-Bi2Te3 section.
Primary compounds of this system have been studied in detail. SbTeI melts peritectically by decomposition at 645 K and crystallizes in a monoclinic structure: a=13.701, b=4.2418, c= 9.201 A, P=128.63° (F.gr. C2/m) [15, 16]. The complete phase diagram of the Sb-Te-I system was constructed and the thermodynamic properties of the SbTeI compound were studied [17].
Bi2Te3 melts congruently at 858 K and crystallizes in a tetradymite-type hexagonal lattice a=4.395, c=30.44 A, Z=3 (F.gr. R3m) [18, 19].
Experimental part
Synthesis of the starting compounds was carried out by co-melting of simple substances with high purity (99.999% purity from Alfa Ae-sar Company) in quartz ampoules under vacuum (10-2 Pa). Bi2Te3 compound was synthesized in a single-zone mode at 900 K. The SbTeI compound was synthesized in a two-zone furnace due to the high vapor pressure of iodine. The temperature of the "cold" zone was 400 K (the sublimation temperature of iodine was 386 K) [20], whereas the temperature of the "hot" zone was kept at a temperature 30-50 K higher than the melting temperature of the SbTeI compound. At the next stage, the SbTeI compound was powdered, pressed as pellets, and annealed at 600 K for 1000 hours for homogeneity.
Samples of the studied system were prepared in different proportions by co-melting of synthesized and identified primary compounds in quartz ampoules. Samples were annealed at 620 K for ~1000 hours to bring them closer to the equilibrium state.
Experimental studies were conducted by differential-thermal analysis (DTA) and X-ray diffraction analysis (XRD). DTA was carried out using the differential-scanning calorimeter "NETZSCH 404 F1 Pegasus system" (heating rate 10 K/min), and XRD - in the Bruker D8 diffractometer (CuXa-radiation) at the 20 = 5075 range.
Conclusions and discussions
Co-analyzing T-x diagrams of the Sb2Te3-Bi2Te3 [21] and SbTeI-BiTeI [14] systems and solid-phase equilibrium diagram of the Sb2Te3+2BiI3~Bi2Te3+2SbI3 reciprocal system [13] with DTA and RFA results of annealed samples, the physico-chemical interaction in the SbTeI-Bi2Te3 system was determined and its phase diagram was constructed (Figure 1).
Comparative analysis of this diagram with the phase diagrams of the Sb2Te3-Bi2Te3 [21], Sb-Te-I [17], and Bi-Te-I [22] systems shows that initially, the Sb2-xBixTe3 (a-phase) solid solutions crystallize in the entire composition range along the section. Crystallization processes are similar to the non- and monovariant processes in the SbTeI-BiTeI system below the liquidus [14]: crystallization in the 0-10, 10-80 and 80-97 mol % Bi2Te3 composition ranges continues with the L+a^y1 peritectic (P1P2-curve), L+a^-y2 peritectic (KP2-curve) and L2^a+y2 eutectic (eK-curve) reactions, respectively. The horizontal line at 665 K represents the nonvariant peritectic equilibrium L+a+y2^y1. In contrast to the SbTeI-BiTeI system, these peritectic processes on the SbTeI-Bi2Te3 section end with complete depletion of the liquid phase and partial depletion of the primary crystalline phases. Therefore, in the ~15-28 mol% Bi2Te3 compositional range the system pass to the a+y1+y2 three-phase state, crystallization in the range of 0-15 mol% Bi2Te3 ends with the formation of a+y1, and in the range of 28-97 mol% Bi2Te3 ends with the formation of a+y2 mixture.
The phase compositions of the SbTeI-Bi2Te3 alloys were confirmed by the XRD method. The powder diffractograms of the two selective samples are shown in Figure 2.
As can be seen, the diffraction pattern of the Example A consists of the reflection lines of the a, y1, and y2-phases and the Example B consists of the reflection lines of the a and y2-phases. This indicates that they consist of a+y1+y2 and a+y2 heterogeneous mixtures respectively, as shown in the phase diagram. Finally, note that the composition of all three phases in heterogeneous areas is outside of the plane of this section. Their location at room temperature is visually reflected by the isothermal section of the Sb2Te3+2BiI3^-Bi2Te3+2SbI3 reciprocal system at 300 K [13].
Fig. 1. Phase diagram of the SbTeI-Bi2Te3 system.
48
E.J.AHMADOV
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I I I I < 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 —
► ♦ • — a ■ Yi ♦ - 72 -
► —
• ♦ • ♦ i J I 1 ,L Á J • ♦ I* #2
* ■ 1___________-jj • i ■ * 71 1 ** • _ I 1 - * #1
35.00 40.00 45.00
Diffraction Angle [°20]
Fig. 2. Diffractograms of some samples of the SbTeI-Bi2Te3 section: 1 - 20 mol% Bi2Te3, 2 - 40 mol% Bi2Te3.
Conclusion
For the first time, the nature of physi-cochemical interactions in the SbTeI-Bi2Te3 polythermal section of the Sb2Te3+2BiI3^-Bi2Te3+2SbI3 reciprocal system was determined and the T-x diagram was constructed. It has been shown that this section is non-quasi-binary and entirely located in the initial crystallization area of the a-phase. In the subsolidus, it passes through the a+y1, a+y1+y2 and a+y2 heterogeneous areas.
Acknowledgment
The work has been carried out within the framework of the international joint research laboratory "Advanced Materials for Spintronics and Quantum Computing" (AMSQC) established between Institute of Catalysis and Inorganic Chemistry of ANAS (Azerbaijan) and Donostia International Physics Center (Basque Country, Spain) and partially funded by the grant EIF/MQM/Elm-Tehsil-1-2016-1(26)-71/01/4-M-33.
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SbTeI-Bi2Te3 SISTEMINDO FIZIKI-KIMYOVI QAR§ILIQLI TOSIR
E.C.Ohmadov
DTA va RFA üsullan ils Sb2Te3+2BiI3^Bi2Te3+2SbI3 qarçiliqli sisteminin faza diaqraminin SbTeI-Bi2Te3 politermik kasiyi qurulmuçdur. Müayyan olunmuçdur ki, maye fazadan ilkin olaraq Sb2-xBixTe3 bark mahlullari (a-faza) kristallaçir. Sonra kristallaçma bir sira non- va monovariant reaksiyalar üzra davam edir va subsolidusda sistemda a+yb a+y2 va a+y1+y2 (y1 va y2, SbTel va BiTel asasinda bark mahlullardir) heterogen sahalari amala galir. Alinmiç yeni bark mahlullar topoloji izolyator materiallari va Raçba yarimkeçiricilari kimi praktiki maraq kasb edir.
Açar sözlar: SbTel-Bi2Te3 sistemi, faza diaqrami, bark mahlullar, topoloji izolyatorlar, Ra§ba yarimkeçiricilari.
ФИЗИКО-ХИМИЧЕСКОЕ ВЗАИМОДЕЙСТВИЕ В СИСТЕМЕ SbTeI-Bi2Te3
Э.Дж.Ахмедов
На основании результатов дифференциально-термического и рентгенофазового анализов построен политермический разрез SbTeI-Bi2Te3 фазовой диаграммы взаимной системы Sb2Te3+2BiI3^Bi2Te3+2SbI3. Установлено, что по данному разрезу из расплава первично кристаллизуются твердые растворы Bi2-xSbxTe3 (a-фаза). Процесс кристаллизации продолжается по ряду нон- и моновариантных реакций, в результате которых в субсолидусе образуются гетерогенные области a+уь a+y2 и a+y1+y2 (у1 и у2 твердые растворы на основе SbTeI и BiTeI).
Ключевые слова: система SbTel-Bi2Te3, фазовая диаграмма, твердые растворы, топологические изоляторы, полупроводники Рашба.
