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ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
UDC 544.344.015.3: 546.81'87'24
EXPERIMENTAL REINVESTIGATION OF THE PbTe-Bi2Te3 PSEUDO-BINARY
SYSTEM
I.M.Gojayeva\ V.I.Babanly2, A.LAghazade1, E.N.Orujlu1
1M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
2French-Azerbaijani University
aytenagazade94@gmail. com elnur.oruclu@yahoo.com
Received 30.11.2021 Accepted 25.12.2021
A newly refined version of the phase diagram of the PbTe-Bi2Te3 pseudo-binary system was constructed using differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques. It was established that the system forms three tetradymite-type layered ternary compounds, namely PbBi2Te4, PbBi4Te7, and PbBi6Te10, and all these phases melt with peritectic decomposition. However, earlier reported Pb3Bi4Te9, Pb2Bi6Te11, and PbBi8Te13 compounds were not confirmed in our XRD studies. The existence of homogeneity areas based on all ternary compounds and the starting phases PbTe and Bi2Te3 was detected. The crystal structure of the ternary compounds was solved by Rietveld refinement. The newly constructed phase diagram provides very valuable information for choosing appropriate conditions for the synthesis of bulk samples or single crystals growth.
Keywords: lead-bismuth tellurides, tetradymite-type layered structure, phase diagram, ternary compound, topological insulator.
doi.org/10.32737/0005-2531-2022-2-47-53
Introduction
Ability to design and realize materials with desired properties is necessary for the development of multifunctional novel devices and innovations. A new class of inorganic functional materials - topological insulators (TI) that behaves as an insulator in bulk, while the edge or surface contains gapless conducting states hold great promise in spintronics, quantum computing, medicine, security systems, etc. [1 -6]. The synthesis, design, and processing of materials with Tl properties requires reliable information about the phase equilibria of corresponding systems since a better understanding of material properties is important for advanced material selection [7, 8].
An analysis of literature data shows that ternary tetradymite-type compounds with the formula of AIVBV2Te4, A^B^Te^ A^B^Te^, etc. in AIV-BV-Te systems (AIV - Ge, Sn, Pb; BV - Sb, Bi) are well-known thermoelectric materials with low thermal conductivity and have topologi-cal insulating properties as well [9-19]. However, the available information about phase equilibria in these systems is often incomplete and contradictory. Therefore, an experimental reinvestigation of phase diagrams seems necessary. The objective of
this work is to contribute new experimental data on the PbTe-Bi2Te3 pseudobinary system and determine its phase equilibria.
To our best knowledge, the first phase diagram of the PbTe-Bi2Te3 pseudobinary system was reported in [20], where the formation of in-congruently melted (850 K) one compound -PbBi4Te7 was mentioned. The latter authors proposed that it melts congruently at 858 K [21]. The authors of [22] reported the existence of only one compound in the system with Pb3Bi4Te9 composition. It decomposes by eu-tectoid reaction to form PbTe- and Bi2Te3-based solid solutions at 668 K. The existence of ternary compounds PbBi2Te4 and PbBi4Te7 had been reported by [23]. These compounds melt peritectically at 856 and 843 K, respectively. At the same time, Ref. [24] reported PbBi2Te4 to melt congruently at 864 K.
According to the literature [25-27], five ternary compounds - PbBi2Te4, Pb2Bi6Te11, PbBi4Te7, PbBi6Te10, and PbBi8Te13 are formed in the PbTe-Bi2Te3 system. The first compound melts incongruently at 856 K, whereas PbBi4Te7 melts congruently at 858 K. The authors did not specify melting character and temperatures of
other three phases. The coordinates of the two eutectic points are ~65 mol% Bi2Te3/850 K and ~91 mol% Bi2Te3/848 K according to the reported phase diagram in [26].
Experimental part
High purity (99.999%) elemental components supplied by Alfa Aesar and Sigma Al-drich were used to synthesize binary starting compounds PbTe and Bi2Te3. Stoichiometric amounts of Pb, Bi, and Te were sealed inside quartz ampules and melted at temperatures above their melting point. The phase purity of both starting compounds was checked using DTA and XRD.
Alloys of the PbTe-Bi2Te3 system were prepared in various ratios using pre-synthesized PbTe and Bi2Te3. A stoichiometric amount of each binary compound was weighed, then sealed in evacuated quartz ampules. All alloys were synthesized in the 900-1200 K temperature range depending on their compositions and water quenched. To ensure good homogeneity, obtained alloys were annealed at 750 K for 1000 h.
DTA was performed using the LINSEIS HDSC PT1600 system (accuracy ±2 K) with a heating rate of 10 K/min. It is worth noting that close thermal effects in DTA curves were detected in samples with a weight of 0.03 g. Phase identification of each phase was carried out by XRD on Bruker D2 PHASER diffractometer using CuKai radiation. The scanning range was from 5 to 750. FullProf Suite and Match 3! Crystal Impact software was used for the calculation of lattice parameters and pattern indexing.
Results and discussion
The results of the study of synthesized and annealed alloys by DTA and XRD methods allowed us to determine a new picture of phase equilibria in the PbTe-Bi2Te3 system.
The XRD results confirm the existence of previously reported three ternary compounds in the system with the composition of PbBi2Te4, PbBi4Te7, and PbBi6Tei0. As can be seen in Figure 1, samples of all three compositions have a qualitatively identical diffraction pattern and are fully indexed in a tetradymite-type layered structure. Besides, there is a wide range of solid solutions based on both starting binary com-
pounds. However, the solubility based on them does not exceed 10 mol% at low temperatures.
Figure 2 presents diffraction patterns of samples containing 10, 60, 80, and 90 mol% Bi2Te3. It is seen that alloys having 10 and 90 mol% Bi2Te3 compositions are phase-pure and have a set of diffraction reflections corresponding to cubic and tetradymite-type rhombohedral structures, respectively. However, powder dif-fractograms of alloys with the composition of 60 and 80 mol% Bi2Te3 were found to be a biphasic mixture of the two closest phases. It should be noted that the two-phase equilibrium of PbBi2Te4 and PbBi4Te7 phases in an alloy containing 60 mol% Bi2Te3 indicates the absence of a previously reported ternary compound Pb2Bi6Te11 [25] in the PbTe-Bi2Te3 system. Similarly, the fact that a sample having 80 mol% Bi2Te3 is biphasic, its phase composition [(PbBi6Te10) + (Bi2Te3)] indicates no other (n = 4, 5 ...) representatives of the homologous series PbTe-nBi2Te3 are formed in the system as well.
The structural parameters of all three ternary compounds shown in Figure 1 were refined by the Rietveld technique and obtained results are summarized in Table 1. It is seen that calculated parameters for ternary compounds are in good agreement with literature data.
The DTA results of annealed alloys are shown in Figure 3 and Table 2, and the newly constructed T-x phase diagram of the PbTe-Bi2Te3 system based on these results is present in Figure 4. It can be seen from Figure 4 that all three intermediate compounds present in the system are formed by the following peritectic reaction: L+a^PbBi2Te4; L+PbBi2Te4^ PbBi4Te7; L+ PbBi2Te4^PbBi6Te10. Along with the system, the coordinates of the peritectic points were found to be at 62 (p0, 70 (p2), and 78 mol% Bi2Te3 (p3), respectively.
The phase diagram also shows that the homogeneity area of a-solid solutions based on PbTe reaches 18 mol% at a peritectic temperature of 864 K. This is confirmed by the Tammann triangle as shown in Figure 4. At the eutectic equilibrium temperature, the P-phase based on Bi2Te3 has a homogeneity area of ~12 mol%.
The temperature of L^PbBi6Te10+P eu-tectic transformation was measured as 848 K at 85 mol% Bi2Te3.
Fig. 1. XRD patterns of alloys with the composition of PbBi2Te4, PbBi4Te7, and PbBi6Te1
Fig. 2. XRD patterns of alloys with the composition of 10, 60, 80, and 90 mol% Bi2Te3.
'Table 1. Determined crystal structure parameters of some phases in the PbTe-Bi2Te3 with the comparison of literature data
Phase Syngony Space group Lattice parameters, A Ref.
a c
PbTe Cubic Fm-3m 6.46 - [28]
a (10 mol% Bi2Te3) Cubic Fm-3m 6.4543(2) - This work
PbBi2Te4 Rhombohedral R-3m 4.436 4.4382(5) 41,77 41.775(6) [26] This work
PbBi4Te7 Trigonal P-3m1 4.426 4.4234(5) 23.892 23.876(8) [25] This work
PbBi6Te!Q Rhombohedral R-3m 4.4Q6 4.4012(5) 102.54 102.538(2) [26] This work
ß (90 mol% Bi2Te3) Rhombohedral R-3m 4.4028(4) 30.511(2) This work
Bi2Te3 Rhombohedral R-3m 4.3837(4) 30.482(3) [26]
Table 2. DTA results of alloys in the PbTe-Bi2Te3 system
Composition, mol% Bi2Te3 Thermal effects, K
Isothermal Polythermal
0 1198 -
10 - 1040-1145
20 865 865-1097
30 864 864-1041
40 863 863-973
50 864 864-905
55 857;864 -
60 856;863 -
65 856 856-861
66,7 856 856-860
70 852 852-855
75 851 851-853
80 848 -
85 848 -
90 - 850-855
95 - 854-857
100 858 -
Fig. 3. DTA curves of alloys containing 10, 20, 66.7, 75, and 85 mol% Bi2Te3.
Fig. 4. Phase diagram of the PbTe-Bi2Te3 pseudo-binary system.
Conclusion
In this work, the PbTe-Bi2Te3 system has been studied using DTA and XRD, and its updated version of the phase diagram was constructed based on obtained results. Although the formation of six ternary compounds was mentioned in different studies, the existence of only three tetradymite-type layered ternary compounds, namely PbBi2Te4, PbBi4Te7, and PbBi6Te10 was confirmed according to our results in the PbTe-Bi2Te3 system. All ternary compounds melt peri-tectically and have a narrow homogeneity field. At the same time, there is a wide range of solid solutions (a- and P-phases) based on starting PbTe and Bi2Te3 compounds. Crystallographic parameters of ternary compounds, as well as solid solutions, were determined by the Rietveld method.
Acknowledgments
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 the Institute of Catalysis and Inorganic Chemistry of ANAS (Azerbaijan) and Donostia International Physics Center (Basque Country, Spain) and partially supported by the Science Development Foundation under the President of the Republic of Azerbaijan - Grant № EIF-GAT-5-2020-3 (3 7)-12/02/4-M-02.
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PbTe-Bi2Te3 KVAZIBINAR SISTEMININ TOKRAR EKSPERIMENTAL TODQIQI
Í.M.Qocayeva, V.LBabanli, A.Í.Agazada, E.N.Oruclu
Differensial termiki va rentgenfaza (RFA) analizlari ils PbTe-Bi2Te3 kvazibinar sistemi tadqiq edilmiçdir, sistemin daqiqlaçdirilmiç yeni faza diaqrami qurulmuçdur. Müayyan edilmiçdir ki, sistemda tetradimitabanzar layli quruluçlu ûç birlaçma - PbBi2Te4, PbBi4Te7 va PbBi6Tei0 amala galir va onlarin har biri peritektik reaksiya üzra parçalanmaqla ariyir. Bir sira içlarda gostarilan Pb3Bi4Te9, Pb2Bi6Te11 va PbBi8Te13 birlaçmalarin movcudlugu bizim RFA naticalarina asasan oz tasdiqini tapmamiçdir. Sistemda amala galan ûçlû birlaçmalar va hamçinin ilkin PbTe va Bi2Te3 binar birlaçmalar asasinda homogenlik sahalari açkar edilmiçdir. Ûçlû birlaçmalarin kristal quruluçlan Rietveld ûsulu ila tayin edilmiçdir. Qurulmuç yeni faza diaqrami sistemda amalagalan fazalann polikristal va monokristal nûmunalarin sintezi zamani uygun sintez çaraitinin seçilmasi ûçûn boyük ahamiyyat kasb edir.
Açar sozlar: Qurgu§un-bismut telluridlari, tetradimitabanzar layli quruluç, faza diaqrami, ûçlû birlaçma, topoloji izolyator.
ПОВТОРНОЕ ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ КВАЗИБИНАРНОЙ СИСТЕМЫ PbTe-Bi2Te3
И.М.Годжаева, В.И.Бабанлы, А.И.Агазаде, Э.Н.Оруджлу
Использованием методов дифференциального термического и рентгенфазового (РФА) анализов построена новая уточненная версия фазовой диаграммы квазибинарной системы PbTe-Bi2Te3. Установлено, что система образует три тройных тетрадимитоподобных соединения PbBi2Te4, PbBi4Te7 и PbBi6Te10 со слоистой структурой, плавящихся с разложением по перитектической реакции. В наших РФА исследованиях не были подтверждены ранее указанные в литературе соединения Pb3Bi4Te9, Pb2Bi6Te11 и PbBi8Te13. Обнаружено наличие областей гомогенности по всем тройным соединениям и исходным фазам PbTe и Bi2Te3. Кристаллическая структура тройных соединений была определена методом Ритвельда. Построенная новая фазовая диаграмма дает очень ценную информацию для выбора подходящих условий для синтеза и роста поликристаллов и монокристаллов образующихся в системе фаз.
Ключевые слова: теллуриды свинца-висмута, слоистая структура типа тетрадимита, фазовая диаграмма, тройные соединения, топологический изолятор.
