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29
ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
A IVtjV
A B 4Te7,
AIVBV2Te4,
UDC 544.344.015.3: 546.81'86'24
PHASE EQUILIBRIA OF THE PbBi2Te4-"PbSb2Te4" SECTION OF THE PbTe-Bi2Te3-Sb2Te3 SYSTEM AND SOME PROPERTIES OF THE SOLID SOLUTIONS
A.I.Aghazade
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
aytenagazade94@gmail. com
Received 08.06.2020 Accepted 17.09.2020
PbTe-Bi2Te3-Sb2Te3 system has been studied by DTA and X-ray diffraction methods along the PbBi2Te4 - "PbSb2Te4" section. It was found that the compound of the composition PbSb2Te4, specified in the literature does not exist. On the basis of the latter a wide (more than 50 mol%) area of solid solution (y) was detected. By using obtained experimental results, a fragment of the solid-phase equilibrium diagram of the PbTe-Bi2Te3-Sb2Te3 system was constructed and boundaries of the heterogeneous areas a+y, a+p, a+p+Y (a- and p- are solid solutions based on PbTe and Sb2Te3, respectively) have been determined.
Keywords: PbTe-Bi2Te3-Sb2Te3 system, phase diagram, solid solutions, tetradymite-like structure, topological insulators.
doi.org/10.32737/0005-2531-2020-4-53-59
Introduction
Ternary tetradymite-like
AIVBV 6Te10 and other compounds formed in AIV-BV-Te systems (AIV - Ge, Sn, Pb; BV - Sb, Bi) are thermoelectric materials with low thermal conductivity [1-4]. Recent studies have shown that these compounds also have three-dimensional topological insulator properties [59] and are perspective for use in spintronics, quantum computing, medicine, and security systems [10-15].
Creating new multicomponent functional materials is based on the phase equilibria data of the appropriate systems [15-18]. Among the functional materials, solid solutions are of particular interest, due possibility to optimize their properties by changing their composition. The investigations of the systems consist of compounds with structural or formula analogs are expedient for obtaining solid solutions [19-23].
The PbTe-Bi2Te3-Sb2Te3 system is interesting in terms of searching for solid solutions with Bi^Sb substitutional based on binary and ternary compounds with a tetradymite-like layered structure. The boundary systems of such concentration triangle have been investigated in several works [24-34].
The PbBi2Te4 formed in the PbTe-Bi2Te3 quasi-binary system melts incongruently at 864 K
[24]. This compound crystallizes in a hexagonal structure (Sp.Gr. R-3m) and has the following lattice parameters: a=4.438(1) A, c=41.773(7) A. Cristal lattice consists of seven-layered blocks (Te-Bi-Te-Pb-Te-Bi-Te) which are repeated along c axis and has Van der Waal's bonds between themselves [25].
A literature data about the PbTe-Sb2Te3 quasi-binary system is contradictory. According to [26] Pb2Sb6Te11 compound with incongruently melting (855 K) is formed in this system. Authors [27, 28] found the layered structure PbSb2Te4 compound in the PbTe-Sb2Te3 system. But the latest studies [29-32] showed the absence of compound PbSb2Te4, and the existence of Pb2Sb6Te11 indicated in the [26]. Using high-temperature microstructural analysis method authors [29-32] obtained that, a Pb2Sb6Te11 sample is a stable in a small temperature range (849-851 K) and is decomposed into PbTe+ Sb2Te3 two-phased mixture in less than 849 K.
According to Roseboom's classification a phase diagram of the Bi2Te3-Sb2Te3 system regards to the I type and is characterized by the formation of continuous solid solutions with layered tetradymite-like structure [33, 34].
Considering the above mentioned, we have undertaken the study a phase equilibria in the PbTe-Bi2Te3-Sb2Te3 quasi-ternary system.
Here, the results of the investigations of the pointed system along the PbBi2Te4-"PbSb2Te4" section are presented.
Experimental part
As a rule, tetradymite-like ternary compounds melt incongruently [15, 24], therefore, for to prepare the alloys of PbTe-Bi2Te3-Sb2Te3 system we used starting binary compounds which melt congruently. For this purpose, firstly initial compounds PbTe, Bi2Te3, Sb2Te3 were synthesized from elementary components with a high degree of purity (99.999 % purity from Alfa Aeser) taken in stoichiometric ratio. The synthesis was carried out by co-melting of elemental components in a vacuum (10-2 Pa) in sealed quartz ampoules at the temperature by 30-350 higher than the melting point of the synthesized compound [35].
The individuality of the synthesized compounds was controlled by differential thermal analysis (DTA) and powder X-ray diffraction technique (XRD).
Identified compounds were used for the preparation of samples along the PbBi2Te4-"PbSb2Te4" section. Samples were heated at 1000 K in the furnace and then quenched in ice water. For achieving maximum equilibrium
state the samples were annealed at 730-750 K for 800 hours.
The obtained alloys were studied by DTA and XRD. DTA was carried out on a Linseis DSC400 differential scanning calorimeter using the Linseis TA V 2.3.1 software. XRD was performed on a Bruker D2 PHASER diffract-tometer, with CuKa1-radiation. PXRD patterns were indexed by using TOPAS 4.2 software.
Results and discussion
Annealed alloys with compositions 20, 30, 40, 50, 60, 80 mol% PbBi2Te4 of the PbBi2Te4-"PbSb2Te4" section were studied by the PXRD technique. X-ray diffraction patterns of some alloys are presented in Figure 1. As can be seen, alloys with compositions 50 and 80 mol% PbBi2Te4 have qualitatively similar diffraction patterns. Diffraction peaks of these alloys are slightly shifted relative to the diffraction peaks of the PbBi2Te4 compound that is associated with a change in the crystal lattice parameters during the formation of solid solutions.
Diffraction patterns of the samples containing 20 and 40 mol % PbBi2Te4 (Figures 2b, 2a) show their non-homogeneity. They were two- and three-phase mixtures, respectively.
Diffraction angle [2d]
Fig. 1. Powder XRD patterns of some alloys of the PbBi2Te4-"PbSb2Te4" section.
]}irlhu.-Lii>n Amk I 201
Fig. 2. Powder XRD patterns of the alloys containing 40 (a) and 20 (b) mol% PbBi2Te4.
The diffraction peaks of a-phase (solid solution based on PbTe) coincide with diffraction lines for pure PbTe, while the peaks of the P-phase (solid solution based on Sb2Te3) slightly shifted relative to pure Sb2Te3 towards small angles. This is due to the formation of a continuous solid solution in the Bi2Te3-Sb2Te3 system [33, 34].
Comparative analysis of diffraction patterns (Figures 1 and 2) with the literature data on PbTe-Bi2Te3 [24, 25], PbTe-Sb2Te3 [26, 29-32], Bi2Te3-Sb2Te3 [33, 34] systems allowed us to construct a certain fragment the
solid-phase equilibria diagram of the PbTe-Bi2Te3-Sb2Te3 quasi-ternary system. According to the phase diagram (Figure 3), a wide area (more than 50 mol%) of solid solutions (y-phase) is formed in this system along the PbBi2Te4-"PbSb2Te4" section. At lower concentrations of PbBi2Te4, the alloys are three-phase (a+P+Y) and two-phase (a+P).
In order to construct a complete scheme of phase equilibria in PbTe-Bi2Te3-Sb2Te3 system, additional experimental researches are required.
Fig. 3. Fragment of the solid phase equilibria diagram of PbTe-Bi2Te3-Sb2Te3 system. The dots indicate the composition of the samples studied by X-ray.
Crystal lattice parameters of the solid solutions were determined based on the powder X-ray patterns using TOPAS 4.2 software (Table 1). Based on these data, of the lattice parameters dependence on composition was constructed (Figure 4). As seen from Figure 4, both lattice parameters are linear depend vs composition, and a breakpoint was observed at a limit concentration of the solid solution (~48 mol% PbBi2Te4).
According to the DTA and XRD results (Table 1, Figure 5), the phase diagram of the PbBi2Te4-"PbSb2Te4" polythermal section of the phase diagram of the PbTe-Bi2Te3-Sb2Te3 system was constructed (Figure 6).
As is seen in Figure 6 in the PbBi2Te4-"PbSb2Te4" section of the PbTe-Bi2Te3-Sb2Te3 system, the a-phase based on PbTe is primarily
crystallized from the liquid in the entire range of compositions. This means that this section is located entirely in the primary crystallization field of the a-phase. Crystallization below liquidus over a wide range of compositions (60100 mol% PbBi2Te4) most likely occurs by the L + a ^ y peritectic reaction and finishes with the formation of a homogeneous y-phase in the 55-100 mol% PbBi2Te4 composition. It is not possible to accurately determine the characters of the phase equilibria in the 0-45 mol% PbBi2Te4 composition area. Taking into account the decomposition of Pb2Sb6Te11 compound into PbTe and Sb2Te3 lower 849 K [29-32] and as well as the obtained XRD results, we determined the phase regions in the subsolidus in the indicated composition region.
Alloys'composition, mol. % from "PbSb2Te4" Thermal effects, K Phase composition Crystal lattice parameters, Â
a c
PbBi2Te4 864, 940 4.4382(5) 41.775(6)
80 858,943 Y 4.4183(4) 41.735(3)
60 852-857, 950 Y 4.3971(5) 41.6974(7)
50 850, 950 Y 4.3847(5) 41.6731(8)
40 848, 947 a+P+Y *4.3843(5) *41.6693(9)
30 848, 945 a+P — —
20 853, 937 a+P — —
"PbSb2Te4" 854-860, 927 a+P — —
Note: The * - indicated parameters belong to the y-phase.
mol%:
Fig. 4. Concentration dependence of lattice parameters for some alloys of the PbBi2Te4-"PbSb2Te4" section.
Fig. 5. DTA results for some alloys of the PbBi2Te4-"PbSb2Te4" section. TJ
950
900
850
L
L+a
L+cx+y
Y i a+ß+7 i 849 K a+ß i
PbBi,Te, 20
40
60
80 "PbSbJeJ
mol% "PbSb,Te/
Fig. 6. T-x diagram of the PbBi2Te4-"PbSb2Te4" section of the PbTe-Bi2Te3-Sb2Te3 system.
Conclusion
PbTe-Bi2Te3-Sb2Te3 system was studied along the PbBi2Te4-"PbSb2Te4" section. It has been shown that a large (~55 mol%) solid solution area with a layered tetradymite-like structure based on PbBi2Te4 is formed in this system. It is not possible to accurately determine the phase equilibria in the 0-45 mol% PbBi2Te4 composition area of the T-x diagram. However, it was established that in subsolidus, the samples from this composition field consist of heterogeneous mixtures of a+P (0-20 mol% PbBi2Te4) and a+P+Y (20-45 mol% PbBi2Te4). The obtained experimental results allowed us to construct a fragment of the of solid-phase equilibria diagram of the PbTe-Bi2Te3-Sb2Te3 system at room temperature, and to determine the areas of homogeneity of the a-, P- and Y-phases, as well as to define the boundaries of the heterogeneous areas a+P, a+Y, and a+P+Y. The obtained y- and P-solid solutions are of interest as topological insulator materials.
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PbTe-Bi2Te3-Sb2Te3 SISTEMININ PbBi2Te4-"PbSb2Te4"KOSiYi ÜZRO FAZA TARAZLIQLARI УЭ BORK
MOHLULLARIN BOZi XASSOLORi
A.i.Agazad3
DTA уэ RFA üsullan ib PbTe-Bi2Te3-Sb2Te3 sistemi PbBi2Te4-"PbSb2Te4" kasiyi üzra öyrenilmiijdir Müayyan edilmiijdir ki, эdэbiyyatda göstэrilэn PbSb2Te4 tэrkibli Ыг1э§тэ mövcud deyil, PbBi2Te4 birlэ§mэsi эsaslnda geni§ (50 mo1%-dэn artiq) Ьэгк mэhlul (у) sahэsi mövcuddur. Alinmi§ tэcrübi nэticэ1эr эsaslnda PbTe-Bi2Te3-Sb2Te3 sisteminin bэrkfaza tarazliqlari diaqraminin fraqmenti qurulmu§, sistemdэ a+y, a+ß, a+ß+y (a vэ ß müvafiq olaraq PbTe vэ Sb2Te3 эsaslnda Ьэгк mэhlullardlr) heterogen sahэ1эrinin sэrhэd1эri tэyin edilmi§dir
Agar sözlar: PbTe-Bi2Te3rSb2Te3sistemi, faza diaqrami, Ьэгкm3hlullar, tetradimit3b3nz3r qurulu§, topoloji izolyatorlar.
ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ PbTe-Bi2Te3-Sb2Te3 ПО РАЗРЕЗУ PbBi2Te4-"PbSb2Te4" И НЕКОТОРЫЕ СВОЙСТВА ТВЕРДЫХ РАСТВОРОВ
А.И.Агазаде
Методами ДТА и РФА изучена система PbTe-Bi2Te3-Sb2Te3 по разрезу PbBi2Te4-"PbSb2Te4". Выявлено, чтосоединение с составом PbSb2Te4, указанное в литературе не существует. На основе соединения PbBi2Te4 выявлена широкая область (более 50 мол %) твердых растворов (у). На основе полученных результатов построен фрагмент диаграммы твердофазных равновесий системы PbTe-Bi2Te3-Sb2Te3, определены границы гетерогенных областей a+y, a+ß, a+ß+y (a- и ß-твердые растворы на основе PbTe и Sb2Te3) соответственно. Ключевые слова: система PbTe-Bi2Te3-Sb2Te3, фазовая диаграмма, твердые растворы, тетради-митоподобная структура, топологические изоляторы.
