CC BY
38
ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 3 2022 75
ISSN 0005-2531 (Print)
UDC 544.344.015.3: 546.818724
PHASE RELATIONS AND CHARACTERIZATION OF SOLID SOLUTIONS IN THE SnBi2Te4-PbBi2Te4 AND SnBi4Te7-PbBi4Te7 SYSTEMS
A.I.Aghazade
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
aytenagazade94@gmail. com Received 28.01.2022 Accepted 15.02.2022
Phase equilibria in SnBi2Te4-PbBi2Te4 and SnBi4Te7-PbBi4Te7 systems were studied using differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques. It has been shown that both systems are characterized by the formation of a continuous series of solid solutions with a tetradymite-like layered structure. The lattice parameters of solid solutions are linear functions of composition. Keywords: SnBi2Te4-PbBi2Te4 system, SnBi4Te7-PbBi4Te7 system, solid solutions, phase diagram, tetradymite-like structure, topological insulator.
doi.org/10.32737/0005-2531-2022-3-75-80 Introduction
Tetradymite-like layered ternary compounds belonging to (AIVTe)n(Bi2Te3)m (AIV -Ge, Sn or Pb; n and m are integers) homologous series are innovative functional materials that exhibit new effective thermoelectric and topo-logical insulating properties [1-10]. These materials have a complex multilayer structure, consisting of covalently bonded blocks stacked along the c axis, and each of them is connected with weak van der Waals bonds. They also show low thermal conductivity due to several complex properties. Interesting electrical behaviors can be observed for intermediate compositions with a possible enhancement of the thermoelectric power factor (S o) [11-16].
One of the most convenient ways to optimize the functional properties of these compounds is to obtain new multicomponent non-stoichiometric phases based on them. Because in such phases, it is possible to change the composition in a wide range, which opens up opportunities for purposeful change of properties. The formation of substitutional solid solutions in similar systems was investigated in some works [17-19].
In the presented work, the phase equilibria in the SnBi2Te4-PbBi2Te4 and SnBi4Te7-PbBi4Te7 systems were studied and the phase diagram of both systems was constructed. The electrical and thermal transport properties of the Sn1-xPbxBi2Te4 series were studied in [12]. However, the phase equilibria of these systems have not been studied before.
Initial ternary compounds of both systems are well characterized already. SnBi2Te4 and SnBi4Te7 melt with peritectic reaction at 870 and 863 K, while PbBi2Te4 and PbBi4Te7 melt peritectically at 864 and 857 K, respectively [20-23].
Experimental part
Samples of Sn1_xPbxBi2Te4 and Sn1_xPbxBi4Te7 (x=0-1) systems were synthesized at 1000 K by co-melting of a stoichiometric amount of high-purity elements (99.999%, Alfa Aesar) in evacuated quartz ampoules and then quenched in ice water. Obtained polycrystalline samples were thermally treated for 1000 hours at 4500C in order to ensure homogeneity.
Annealed samples were studied by differential thermal analysis (DTA) and X-ray diffraction (XRD) analysis methods. LINSEIS HDSC PT1600 system was used for DTA (100C •min- heating rate), and a Bruker D2 PHASER (with CuKa1 radiation) was used for XRD. X'pert Highscore Plus and Match 3! Crystal Impact software was used for the determination of lattice parameters and pattern indexing.
Results and discussion
Figure 1 shows the XRD results of the annealed samples of SnBi2Te4-PbBi2Te4 and SnBi4Te7-PbBi4Te7 systems. From the comparison of diffractograms, it is clear that the diffraction patterns of all the intermediate compositions are qualitatively similar to the initial ternary compounds, and their crystal
structures are characteristic of the hexagonal unit cell (R-3m). As the concentration of lead increases, the peaks shift to higher angles due to the ionic radii difference, which is characteristic of continuous solid solutions. In the first system, lattice parameters increase as the concentration of lead increases because lead has a larger ionic radius than tin
(r(Pb)=119
pm>r(Sn2+)=118 pm). Interestingly, it is not observed in the second system which structure is composed of two different types of layered blocks. The lattice parameters were refined by the Rietveld technique and obtained results for some chosen Sn1-xPbxBi2Te4 and Sn1-xPbxBi4Te7
alloys are listed in Tables. The calculated values for initial ternary compounds -SnBi2Te4, SnBi4Te7, PbBi2Te4, and PbBi4Te7 are in good agreement with those reported in [21-23]. As can be seen from Figures 2a and b, lattice parameters vary linearly with a composition according to Vegard's law. These results confirm the formation of complete solid solutions in both SnBi2Te4-PbBi2Te4 and SnBi4Te7-PbBi4Te7 systems.
The table summarizes DTA results and calculated lattice parameters of some samples in both systems.
8000
CiOOO
2000
a) 1 l n J j Sni.^PbxBiiTcj |
„ J ^ A.
ll A x=o,e ;
J l A JL x=0>4 -
■ ■ • 1. . . J J LlJ x=0,2 :
: . i . .j x=o :
20 25 30 35 40 4Ï 50 55 (¡0 65 70 75 Diiliactiou Angle L=20|
(¡000
3 WOO
- h) Siii.iPb^BiiTe? -
L l'i _ x=0_75 j
i . A j « :
)
'-•rrr^rrrwJrrrrr^rf hi - . .....»:■» :
10 15 20 25 30 35 40 45 50 55 50 C5 70 75
Diffraction Angle [=20J
Fig. 1. XRD patterns of a) Sn1-xPbxBi2Te4 and b) Sn1-xPbxBi4Te7 alloys.
Alloy mole (%) Thermal effects (K) Lattice parameters (A)
a c
SnBi2Te4 870;895 4.40388(3) 41.6015(4)
80mol% SnBi2Te4 867;912 4.4102(2) 41.625(4)
60mol% SnBi2Te4 865; 869; 926 4.4198(3) 41.655(2)
40mol% SnBi2Te4 865-868; 934 4.4263(2) 41.673(4)
20mol% SnBi2Te4 864; 941 4.4304(3) 41.695(3)
PbBi2Te4 863;948 4.4385(6) 41.733(7)
SnBi4Te7 863;870 4.3998(2) 23.981(3)
75mol% SnBi4Te7 860-862; 869 4.4052(3) 23.964(2)
50mol% SnBi4Te7 858-861; 867 4.4107(3) 23.938(4)
25mol% SnBi4Te7 857-859; 864 4.4152(4) 23.914(2)
PbBi4Te7 856;860 4.4233(4) 23.879(7)
SnBi/Te, 80 60 40 20 PbBi;Te4 SnBi/le. 80 60 40 20 PbBiJe. mol% mol%
Fig. 2. Concentration dependence of lattice parameters of a) Sn1-xPbxBi2Te4 and b) Sn1-xPbxBi4Te7 alloys.
Fig. 3. T-x diagram of the SnBi2Te4-PbBi2Te4 system.
Fig. 4. T-x diagram of the SnBi4Te7-PbBi4Te7 system.
The constructed phase diagram of the SnBi2Te4-PbBi2Te4 system based on the obtained results is given in Figure 3.
It is shown that continuous series of solid solutions are formed in this system. As can be seen from the phase diagram, the considered system is entirely located in the primary crystallization area of a-solid solutions. Therefore, the a-phase primarily crystallizes from the liquid. This process leads to the formation of the L+a two-phase field. Crystallization below the liquidus goes by the L+a^-y monovariant reaction along the peritectic curve, and as a result, both initial phases - liquid (L) and asolid solution are completely consumed at the same time which leads to the formation of homogeneous y-phase in the subsolidus.
On the other hand, according to the Gibbs phase rule, the three-phase field on polythermal sections cannot be directly bordered by the single-phase field. As we mentioned above, violations of this rule are possible in quasi-stable sections in some cases. However, to eliminate phase rule violation, the L+y area is separated by a dashed line in the lower part of the L+a+y area on the phase diagram. This reflects the direct crystallization of the y-phase from the liquid at the final stage of the process.
Figure 4 shows the constructed T-x diagram of the SnBi4Te7-PbBi4Te7 system based on DTA and XRD results. As can be seen, the y-phase primarily crystallizes from the liquid. The thermal effects belong to the L+y two-
phase area from below referring to the L+y^-S (T=870-863 K) monovariant peritectic reaction. As a result of this reaction, the L+y+S three-phase field is first formed, and crystallization finishes with the formation of a homogeneous S-phase field. As mentioned in the previous system, to eliminate the direct contact of single-and three-phase fields, the L+S field is separated with a dashed line.
Conclusion
Phase equilibria in SnBi2Te4-PbBi2Te4 and SnBi4Te7-PbBi4Te7 systems were studied based on DTA and XRD methods. Both systems are non-quasi-binary and characterized by the formation of a continuous series of solid solutions with a tetradymite-like structure. In both systems, lattice parameters of solid solutions vary linearly depending on composition according to Vegard's law. The obtained solid solutions are of great interest as topological insulators and thermoelectric materials.
Acknowledgments
Author thanks M.B. Babanly for useful discussions and V.A.Gasymov for assistance with XRD examinations.
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SnBi2Te4-PbBi2Te4 va SnBi4Te7-PbBi4Te7 SÍSTEMLORÍNDO FAZA TARAZLIQLARI УЭ BORK
MOHLULLARIN XARAKTERÍS TIKALARI
A.LAgazada
Differensial termiki analiz va rentgenfaza analiz metodlarindan istifada etmakla Sni.xPbxBi2Te4 va Sni_xPbxBi4Te7 sistemlarinda faza tarazliqlari öyranilmi§dir. Göstarilmi§dir ki, har iki sistem tetradimitabanzar layli quruluíjlu fasilasiz bark mahlullar sirasi amala galmasi ila xarakteriza olunur. Bark mahlullarin elementar qafas parametrlari tarkibin xatti funksiyasidir.
Agar sözlzr: SnBi2Te4-PbBi2Te4 sistemi, SnBi4Te7-PbBi4Te7 sistemi, Ьэгк шэЫиПаг, faza diaqrami, tetradimit3b3nz3r qurulu§, topoloji izolyator.
ФАЗОВЫЕ РАВНОВЕСИЯ И ХАРАКТЕРИСТИКА ТВЕРДЫХ РАСТВОРОВ В СИСТЕМАХ
SnBi2Te4-PbBi2Te4 И SnBi4Te7-PbBi4Te7
А.И.Агазаде
Методами дифференциального термического анализа (ДТА) и рентгеноструктурного анализа (РФА) исследованы фазовые равновесия в системах Snj-xPbxBi2Te4 и Snj-xPbxBi4Te7. Показано, что обе системы характеризуются образованием непрерывного ряда твердых растворов со слоистой тетрадимитоподобной структурой. Параметры решетки твердых растворов являются линейными функциями состава.
Ключевые слова: Система SnBi2Te4-PbBi2Te4, система SnBi4Te7-PbBi4Te7, твердые растворы, фазовая диаграмма, тетрадимитоподобная структура, топологический изолятор.
