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AZERBAIJAN CHEMICAL JOURNAL No 1 2020
ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
UDC 546.87.22/24.15
PHYSICO-CHEMICAL INTERACTION IN THE BiSI-BiTeI SYSTEM
E.J.Ahmadov
Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
Received 21.07.2019 Accepted 23.10.2019
Based on the results of the DTA and X-ray phase analysis, the phase equilibria in the BiTeI-BiSI system were studied and the T-x phase diagram was constructed. It was established that the system is stable in the subsolidus area and there are ~7-8 mol % and ~5 mol% solid solution areas based on BiTeI and BiSI compounds at room temperature, accordingly. In2016 general, the system is non-quasibinary. Bi19 S27I3 (0-33 mol % BiTeI ) and tetradymite-Bi2Te2 S (33-83mol% BiTeI ) phases primarily crystallize from the liquid phase in the wide composition ranges.
Keywords: BiTeI-BiSI system, phase diagram, solid solutions, Rashba semiconductors.
doi.org/10.32737/0005-2531-2020-1-36-40
Introduction
Due to their unique properties, BVXHal (BV - As, Sb, Bi; X - S, Se, Te; Hal - Cl, Br, I) type compounds have been in the center of the researchers' attention from the middle of the last century [1-3]. These compounds exhibit ferroelectric, thermoelectric, photovoltaic, magnetic, optical etc. properties and are considered a promising material for solar-powered devices [4-12]. In recent years, the topological insulator properties and 3D Rashba spin splitting have been observed in these compounds [13-17], that made these alloys even more attractive.
The phase equilibria in BV-X-Hal systems were studied, the projection of liquidus surfaces was constructed, melting character and primary crystallization regions of existing phases were determined [18-23].
Obtention of phases of variable compositions on basis of the BVXHal type compounds is of great importance to find materials with superior functional properties. The BiTeI-BiSeI and BiTeI-SbTeI systems which are interesting in this regard, have been studied by authors [24, 25] and a wide range of solid solutions based on the Rashba semiconductor BiTeI have been detected. Hence, the purpose of the present contribution is to investigate the equilibria in the BiTeI-BiSI system and to search for new phases.
Initial compounds of the BiTeI-BiSI system have been studied in detail. BiTeI melts con-gruently at 828 K [26]. It has a hexagonal struc-
ture with lattice parameters: a = 4.3392 (1), c = 6.854 (1) A, z = 1 [27]. BiSI melts incongruently by decomposition at 808 K [28] along the quasi binary Bi2S3-BiI3 system. This compound has orthorhombic crystal structure with: a = 8519 (5), b = 10177 (8), c = 4172 (6) A, z = 4 [29].
Experiments and analyses
The syntheses of the initial BiTeI and BiSI compounds were carried out by co-melting the stoichiometric proportions of high purity (99.999% by weight) elements in an inclined two-zone furnace under vacuum condition (10-2 Pa). Initially, the temperature of the "hot" zone was 900 K, whereas the "cold" zone temperature kept at 400 K (the sublimation temperature of iodine is 386 K) considering the high vapor pressure of iodine. After much of the iodine was reacted, the ampoule moved to the "hot" zone. After stirring the homogeneous liquid in the ampoule at this temperature, the furnace cooled gradually. At the next stage, the incongruently melting BiSI compound was annealed at 750 K for 300 h and the congruently melting BiTeI was annealed at 780 K for 20 h, to achieve complete homogenization.
Two sets of samples of different composition (0.5 g each) of the BiTeI-BiSI system were synthesized in evacuated quartz ampoules using initially prepared and identified compounds and annealed at 700 K for 2 weeks. Experimental studies were conducted by differential-thermal analysis (DTA) and X-ray phase analysis (XRD).
PHYSICO-CHEMICAL INTERACTION IN THE BiSI-BiTel SYSTEM
37
The DTA was carried out using the differential-scanning calorimeter "NETZSCH 404 F1 Pegasus system" (speed of heating 10 K/min), and XRD-in the Bruker D8 diffractometer (Cu^-radiation) at the 29 = 50-750 range.
Discussion of the experimental results
Figure 1 represents the powder X-ray diffraction patterns of thermally treated alloys. As can be seen, the diffraction pattern of the 90 mol% BiSI sample consists mainly of the BiSI diffraction peaks. The lines of the most intense peaks of the solid solutions based on BiTeI are also observed (12.8, 26.9, 35.5 and 46.30). Similarly, the diffraction pattern of the 10 mol% BiSI sample consists mainly of the BiTel diffraction peaks and lines of the most intense peaks (18, 20.1, 22.5 and 28.50) of the solid solutions based on BiTeI. From this result, we can predict that the solubility regions based on BiTel and BiSI are ~7-8 and 5 mol% respectively. The powder X-ray diffractograms of the 10-90 mol% BiSI-containing samples consist of peaks of both mutually saturated solid solutions.
T-x phase diagram of the system was constructed using DTA results (Table 1). The system is non-quasibinary due to the peritectic melting character of BiSI. However, it is stable in subsolidus and characterized by the for-
mation of limited solid solutions on the basis of initial compounds (Figure 2).
Liquidus consists of three curves. Based on T-x diagrams of the Bi2S3-BiI3 [18] and Bi2Te3-BiI3 [19] systems, we can assume that s (based on Bi19S27I3), 5 (based on Bi2Te2S) and y2 (based on BiTel) phases crystallize in the 033 mol% BiTel, 33-83 mol% BiTel and >83 mol% composition interval, respectively. Crystallization in the L+s field continues with mo-novariant L+s^y1 peritectic (PU1 curve) and L^s+y2 eutectic (15-40 mol% BiTel) reactions. The horizontal line at 775 K conforms the transition reaction (U1):
L + s ~ 5 + Y1 (1)
Crystallization in the 40-53 mol% BiTeI composition intervals continues with monovari-ant L^y1+5 and in the 53-83 mol% BiTel composition intervals - with eutectic L^y2+5 reactions. At about 8-83 mol% BiTel composition interval crystallization ends by the transition reaction (U2) (Figure 2):
L + ô ^ yi + Y2.
(2)
At the transition reaction temperature, the homogeneity area of the y1 phase is ~7 mol%, and the homogeneity area of the y2 phase is ~13 mol%.
Fig. 1. X-ray images of some alloys of the BiTeI-BiSI system.
-
L
L+e \
L+8 jP^1 ы/ I J
775 \ L+7,+S L+T-+8 I
Y. 1 1 753 Yi+7; 1 \ Y" 1 1
BiSI 20 40 60 80 BiTeI mol% BiTeI
Fig. 2. Phase diagram of the BiTeI-BiSI system.
DTA results for the system BiTeI-BiSI
Composition, mol% BiTeI Thermal effects, K
0 (BiSI) 808; 895
5 780-800-873
10 753; 775-788-848
15 753; 775-828
20 753; 775-780-811
25 753; 775-782-801
30 753; 775-785-791
40 753; 775-798
50 733; 758-805
60 753; 766-809
70 753; 782-810
80 753; 796-806
90 788-813
100 (BiTeI) 828
Fig. 3. DTA curves of some alloys of the system BiTeI-BiSI.
PHYSICO-CHEMICAL INTERACTION IN THE BiSI-BiTeI SYSTEM
39
Let us explain the phase diagram (Figure 2) in the context of heating curves (Figure 3) of some alloys in the BiTeI-SbTeI system. As it can be seen, a diffused endo effect is observed in the heating thermogram of 90 mol% BiTeI containing sample. This reflects the melting of the y2 phase based on BiTeI at 788-813 K. The strong endo effect observed at 753 K in a sample of 80 mol% BiTeI is appropriate to the U2 transition reaction, subsequent thermal effects reflect the monovariant eutectic L^y2+S process and the end of melting. On the thermogram of 40 mol% BiTeI containing sample, the sharp peak at 753 K represents U2 transition reaction, while the peaks at 775 K and 798 K belong to the melting of L+y1+5 and L+S eutectic mixtures, accordingly. Similarly, in 10 mol% BiTeI containing sample the intensive peaks at 753 K and 775 K belong to U2 and U1 transition reactions, and the peaks at 788 K and 848 K demonstrate the melting of L+s+y1 and L+s eutectic mixtures.
Conclusion
For the first time, the existence of BiTe1-xSxI solid solutions was detected using DTA and XRD methods and the phase diagram of the system was constructed. It was established that, although the system is non-quasibinary, it is stable in subsolidus and is characterized by the formation of limited solid solutions on the of basis initial compounds. Maximum solubility was observed at 750 K: ~ 6 mol% on the basis of BiSI and more than ~ 10 mol% based on BiTeI. New solid solutions based on the BiTeI are of great practical interest as potential Rash-ba semiconductors and topological insulator materials.
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BiSI-BiTeI SISTEMINDO FIZIKI-KIMYOVI QAR§ILIQLI TOSIR
E.C.Ohmadov
DTA va RFA naticalarina asasan BiTel-BiSI sisteminda faza tarazliqlan ôyranilmiç va T-x faza diaqrami qurulmuçdur. Müayyan olunmuçdur ki, sistem subsolidusda stabildir va otaq temperaturunda BiTel asasinda 7-8 mol%, BiSI asasinda isa 5 mol%-a yaxin bark mahlul sahasi amala gatirir. Bütóvlükda isa sistem qeyri-kvazibinardir. Geniç tarkib intervallarinda maye fazadan ilkin olaraq Bii9S27I3 (0-33 mol% BiTel) va tetradimit - Bi2Te2S (33-83 mol% BiTel) kristallaçir.
Açar sozlar: BiTel-BiSI sistemi, faza diaqrami, Ьэгк шэЫиПаг, Ra§bayarimkeçiricibri.
ФИЗИКО-ХИМИЧЕСКОЕ ВЗАИМОДЕЙСТВИЕ В СИСТЕМЕ BiSI-BiTeI
Е.Дж.Ахмедов
На основании результатов ДТА и рентгенофазового анализов были изучены фазовые равновесия в системе BiTel-BiSI и построена фазовая диаграмма T-x. Установлено, что система стабильна в области субсолидуса. На основе соединений BiTel и BiSI образуются твердые растворы, которые при комнатной температуре занимают области ~7-8 и ~5 мол% соответственно. Однако в общем система - неквазибинарная. В широком диапазоне составов из жидкости кристаллизуются Bi19S27I3 (0-33 мол% BiTel) и тетрадимит-B^Te^ (33-83моль% BiTel).
Ключевые слова: система BiTel-BiSI, фазовая диаграмма, твердые растворы, полупроводники Рашбы.
