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CHEMICAL PROBLEMS 2019 no. 4 (17) ISSN 2221-8688
565
UDC 544.344.3:546.289.24
SOLID-PHASE EQUILIBRIA IN THE TlBiTe2-TlTbTe2 SYSTEM
G.I. Alakbarzade
National Aerospace Agency of the Azerbaijani Republic e-mail: alakbarzadegi@gmail.com
Abstract: The solid-phase equilibria in the TlBiTe2-TlTbTe2 system were studied by using the powder X-ray diffraction method. Despiteisostructural character of the starting compounds (hexagonal structure, Sp. Gr. R-3m), it was shown that the system is characterized by a limited mutual solubility of the initial components. The solubility based on TlBiTe2 reaches ~45 mol%, and base on TlTbTe2 is about 22 mol%. The lattice parameters of the intermediate solid solutions were calculated using the powder roentgenograms. The solid solutions obtained are of great interest as potential magnetic topological insulators.
Keywords: TlBiTe2-TlTbTe2system, solid solutions, powder X-ray diffraction, topological insulator.
DOI: 10.32737/2221-8688-2019-4-565-570
Introduction
Multicomponent heavy metal composition phases: Tl4PbTe3-Tl9GdTe6-
chalcogenides, including thallium, are of great interest as prospective functional materials with different properties such as magnetic, optical, thermoelectric[1-14]. Moreover, new characteristics of topological effects have been realized experimentally in chalcogenides such as PbBi6Teio, TlBiX2, Pbi-xSnxTe [14-17].
Purposeful changes in the composition of the above compounds can lead to optimization of functional properties of the above materials. For this, it is necessary to study systems consisting of structural analogs, since it can be expected that they form wide areas of solid solutions [18-20].
Earlier, we investigated some systems, characterized by formation of variable
Tl9BiTe6 [21], Tl9SbTe6-Tl9Gd(Tb)Te6[22]. The aim of the present work is the investigation of the mutual solubility of components in the TlBiTe2-TlTbTe2. TlBiTe2 melts congruently at 820 K [23] and crystallizes in a hexagonal structure (Sp. Gr.R-
3 m) with parameters:a=4.526; c=23.12 Â; z = 3 [24, 25].
TlTbTe2 compound is structural analogue of TlBiTe2with lattice parameters: a = 4.416; c = 24.27 Â; z = 3 [26].
The isostructural character of initial compounds makes it possible to expect the formation of wide areas of substitutional solid solutions between them.
Experiments and results 1. Materials and synthesis high purity elements. The provenance
Initial compounds of TlBiTe2 and purity of the elements used in TlTbTe2were synthesized by direct melting of investigation are shown in Table 1.
and this
Tab e 1. Provenance and purity of the materials usee in this investigation
Chemical Mass fraction of Purity Source CAS No Form Purification and purity analysis methods
Bismuth 0.99999 Alfa Aesar (Germany) 7440-69-9 broken ingots as stated by the supplier
Thallium 0.99999 Alfa Aesar (Germany) 7440-28-0 rod - -
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CHEMICAL PROBLEMS 2019 no. 4 (17)
Tellurium 0.9999 Alfa Aesar (Germany) 13494-80-9 broken ingots
Terbium 0.999 Alfa Aesar (Germany) 7440-27-9 pieces
The synthesis of initial components was carried out in an evacuated (~ 10 Pa) quartz ampoule. To achieve an equilibrium state, after synthesis, the intermediate ingot of the TlBiTe2 was subjected to heat treatment 700 K for 500 h and cooled in the furnace.
The synthesis of the TlTbTe2 was carried out by means of the ceramic method. First, the quartz tube was heated slowly to 1000 K in a furnace, allowed to remain at 1000 K for 100 h, and then slowly cooled down to room temperature. An intermediate ingot of TlTbTe2 was crushed in an agate mortar, pressed into pellets and then, the heating procedure was repeated at 900 K for 500 h.
Intermediate alloys of the TlBiTe2-TlTbTe2system were also synthesized by the ceramic method by melting of the pre-
synthesized ternary compounds at 1000 K in evacuated quartz tubes with subsequent thermal annealing at 750 K (1000h.). Then the samples were ground, thoroughly mixed, pressed into tablets and annealed under the same conditions. 2. Methods
We used the X-ray diffraction analysis (Bruker D8 diffractometer, CuKa radiation) in order to control the purity of the synthesized compounds. The analysis was carried out between 10°< 20 < 70° at a room temperature. The lattice parameters of initial compounds and intermediate alloys were calculated by indexing powder patterns using Topas V3.0 software (Table 2).Calculated lattice parameters of TlBiTe2 and TlTbTe2 were close to the corresponding literature data [24, 26].
Results and discussion
Fig. 1 presents powder X-ray diffraction patterns of some annealed intermediate alloys. The diffraction patterns of alloys containing 60, 80 and 90 mol% TlBiTe2 are qualitatively similar to those for pure TlBiTe2 while alloy
20 mol.% TlBiTe2 has a diffraction pattern similar to TlTbTe2. X-ray diffraction patterns of alloys with compositions of 30 mol% TlBiTe2 consist of a set of diffraction lines of hexagonal phases with both initial compounds.
20000 27000
2eooo
25000 24000
9000 8000
20 mol. % TIBiTej + 80 mol. % TITbTej
30 mol. % [ llli l ej + 70 mol. % TIThTei
6ft mol. % TIBÍTe¡ + 40 mol. °/o TITbTei
SO mol. % I lili I tj + 20 mol. % TITbTei
00 mol. % TIBiTe; + 10 mol. % TITbTe:
2-Theta - Scale
Fig. 1. Powder X-ray diffraction patterns of some annealed intermediate alloys
of the TlBiTe2-TlTbTe2 system
In order to determine the mutual solubility of initial compounds, we constructed concentration dependences of the hexagonal lattice parameters (Table 2, Fig. 2). These dependencies have fracture points at compositions of ~22 and ~45 mol% TlBiTe2 which correspond to limiting compositions of a- and P-solid solutions based on TlTbTe2
andTlBiTe2, respectively. In the a + P two-phase region, the lattice periods of the two coexisting phases have constant values regardless of alloys' overall composition. Within the homogeneity region of the a- and P phases, the lattice constants is a linear function of the composition.
Table 2. Phase compositions and crystallographic parameters of TlBiTe2-TlTbTe2phases
Compositions, % TlBiTe2 Phase compositions Hexagonal lattice parameters, A
0 (TlTbTe2) a a =4.416; c = 24.27 A; z = 3
10 a a=4.431; c= 24.140
20 a a=4.443; c= 24.043
30 a+ß a - phase: a =4.446; c =24.001 P - phase: a =4.478; c =23.441
40 a+ß a - phase:a = 4.448; c =23.999 P - phase: a = 4.483; c =23.501
50 a+ß a - phase:a =4.447; c =24.025 P - phase: a =4.479; c =23.482
60 ß a=4.488; c=23.581
70 ß a=4.495; c=23.483
80 ß a=4.506; c=23.374
90 ß a=4.518; c=23.542
100 ß a=4.526; c=23.12 A; z = 3
Fig. 2. Concentration dependences of hexagonal lattice parameters of some annealed alloys of the TlBiTe2-TlTbTe2system
It should be noted that, according to the data of [27], in the TlSbTe2-TlTbTe2 system, the solubility based on initial compounds reached 30 and 10 mol%, respectively. Somewhat higher mutual solubility in the
TlBiTe2-TlTbTe2 system seems to be related to closer crystallographic radii of the Tb (1.063Ä) and Bi (1.17Ä) than that between Tb (1.063Ä) and Sb (0.9Ä) [28].
Conclusion
The formation of a wide area of solid solutions based on TlBiTe2 (45 mol%) is established in the TlBiTe2-TlTbTe2system based on the results of the powder X-ray diffraction analysis. The solubility based on
TlTbTe2 does not exceed 22 mol%. The crystal lattices parameters of the obtained solid solutions are calculated. The solid solutions obtained are of great interest as potential magnetic topological insulators.
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).
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TlBiTe2-TlTbTe2 SiSTEMiNDd BdRKFAZA TARAZLIQLARI
Q.i. dfokbarzada
Azdrbaycan milli aerokosmik agentliyi e-mail: alakbarzadegi@ gmail.com
ilk dafa olaraq TlBiTe2-TlTbTe2 sisteminda barkfaza tarazliqlari rentgenfaza analizi usulu ila tadqiq olunmu§dur. Ovuntu rentgenoqramlari asasinda tayin olunmu§ qafas parametrlarinin tarkibdan asililiq qrafirklari asasinda muayyan edilmi§dir ki, TlBiTe2 asasinda hallolma ~45 mol.%, TlTbTe2 asasinda isa 22 mol% ta§kil edir. Alinmi§ bark mahlullar potensial maqnit xassali topoloji izolyator materiallari kimi maraq dogurur.
Agar sozlar: TlBiTe2-TlTbTe2 sistemi, bark mahlullar, rentgenfaza analizi, topoloji izolyator.
ТВЕРДОФАЗНЫЕ РАВНОВЕСИЯ В СИСТЕМЕ TlBiTe2- TlTb Te2
Г.И. Алекберзаде
Национальное аэрокосмическое агентство Азербайджана e-mail:alakbarzadegi@gmail.com
Впервые методом рентгенфазового анализа изучены твердофазные равновесия в системе TlBiTe2-TlTbTe2. Установлено, что несмотря на изоструктурность исходных соединений (гексагональная структура, пр.грЯ-Sm), данная система характеризуется ограниченной взаимной растворимостью компонентов. Растворимость на основе TlBiTe2 составляет ~45 мол%, а на основе TlTbTe2 - 22 мол%. На основании порошковых дифрактограмм рассчитаны параметры кристаллической решетки твердых растворов. Ключевые слова: система TlBiTe2-TlTbTe2, твердые растворы, порошковая рентгенография, топологический изолятор.
