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Condensed Matter and Interphases
Kondensirovannye Sredy i Mezhfaznye Granitsy https://joumals.vsu.ru/kcmf/
Original articles
Original article
https://doi.org/10.17308/kcmf.2021.23/3296
Phase relations in the Tl2Te-TlBiTe2-TlTbTe2 system
S. Z. Imamaliyeva10, G. I. Alakbarzade2, D. M. Babanly13, M. V. Bulanova4, V. A. Gasymov1, M. B. Babanly1
1Institute of Catalysis and Inorganic Chemistry of the Azerbaijan National Academy of Sciences, 113 H. Javid ave., Baku AZ-1143, Azerbaijan
2Azerbaijan National Aerospace Agency, 159 Azadlig ave, AZ-1106, Baku, Azerbaijan
3Azerbaijan State Oil and Industry University, French-Azerbaijani University (UFAZ), 16/21 Azadliq prospekti, Baku AZ-1101, Azerbaijan
4Frantsevich Institute for Problems of Materials Science, NASU, 3 Krzhizhanovsky st., Kiev 03142, Ukraine
Abstract
The phase equilibria in the Tl2Te-TlBiTe2 -TlTbTe2 concentration area of the Tl-Bi-Tb-Te quaternary system were investigated by using the differential thermal analysis and powder X-ray diffraction techniques. The diagram of the solid-phase equilibria of this system at room temperature was constructed. It was established that the Tl9BiTe6-Tl9TbTe6 section divides the Tl2Te-TlBiTe2-TlTbTe2 system into two independent subsystems. It was found that the Tl2Te-Tl9BiTe6-Tl9TbTe6 subsystem is characterized by the formation of a wide field of solid solutions with a Tl5Te3 structure (S-phase) that occupy more than 90% of the area of the concentration triangle. The results of X-ray phase analysis; of alloys of the Tl9BiTe6-Tl9TbTe6-TlTbTe2-TlBiTe2 subsystem showed the formation of wide regions of solid solutions based on TlTbTe2 and TlBiTe2 along the section of TlTbTe2-TlBiTe2 ((Pj- and P2-phases) and made it possible to determine the location of the heterogeneous phase regions in this subsystem. The parameters of crystal lattices of mutually saturated compositions of the Pt-, P2-, and S-phases are calculated from powder diffraction patterns.
The paper also presents some polythermal sections, isothermal sections at 740 and 780 K of the phase diagram, as well as projections of the liquidus and solidus surfaces of the Tl2Te-Tl9BiTe6-Tl9TbTe6 subsystem. The liquidus surface consists of three fields of the primary crystallization of a (Tl2Te)-, S- and Pj-phase. The constructed isothermal sections clearly demonstrate that the directions of the tie lines do not coincide with the T-x planes of the studied internal sections, which is characteristic of non-quasi-binary polythermal sections. The obtained new phases are of interest as potential thermoelectric and magnetic materials.
Keywords: Tl2Te-TlBiTe2-TlTbTe2 system, phase equilibria, solid solutions, powder X-ray diffraction, crystal lattice, topological insulators
Acknowledgements: the work has been carried out within the framework of the international joint research laboratory "Advanced Materials for Spintronics and Quantum Computing" (AMSOC) 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, a grant ElF/ MQM/Elm-Tehsil-1-2016-1(26)-71/01/4-M-33.
For citation: Imamaliyeva S. Z., Alakbarzade G. I., Babanly D. M., Bulanova M. V., Gasymov V. A., Babanly M. B. Phase relations in the Tl2Te-TlBiTe2-TlTbTe2 system. Kondensirovannye sredy i mezhfaznye granitsy = Condensed Matter and Interphases. 2021;23 (1): 32-40. https://doi.org/10.17308/kcmf.2021.23/3296
El Samira Zakir Imamaliyeva, e-mail: samira9597a@gmail.com
© Imamaliyeva S. Z., Alakbarzade G. I., Babanly D. M., Bulanova M. V., Gasymov V. A., Babanly M. B., 2021 The content is available under Creative Commons Attribution 4.0 License.
S. Z. ImamaLiyeva et al.
Original articles
Для цитирования: Имамалиева С. З., Алекберзаде Г. И., Бабанлы Д. М., Буланова М. В., Гасымов В. А., Бабанлы М. Б. Фазовые равновесия в системе Т12Те-ТШГГе2-ТПЪТе2. Конденсированные среды и межфазные границы. 2021;23(1): 32-40. https://doi.org/10.17308/kcmf.2021.23/3296
1. Introduction
Binary and multinary chalcogenides of metals are of great interest as prospective materials with different functional properties such as electronic, optical, thermoelectric, topological insulators et al. [1-9].
Despite the toxicity of thallium, complex thallium chalcogenides are closely monitored as topological insulators [10-15], Weyl semimetals [16, 17], photodetectors [18, 19], X-ray and gamma radiation detectors [20, 21], as well as materials which exhibit abnormally low thermal conductivity [22-25].
Insertion to the crystal structure of chalcoge-nides of d- and f- elements can improve their properties and give them additional functionality, for example, the magnetic properties [26-29].
For the optimization of the functional properties of the above materials, it is necessary to plot phase diagrams of these systems, especially for the systems consisting of structural analogues, since it can be expected that they form wide areas of solid solutions [7, 30-32].
This work is a continuation of our studies on the phase equilibria in systems based on thallium-REE tellurides, in which wide areas of solid solutions with a Tl5Te 3 structure are revealed, which are of practical interest as thermoelectric materials with anomalously low thermal conductivity [32-36]
The aim of the present work is the investigation of the solid-phase relations in the Tl2Te-TlBiTe2-TlTbTe2 system.
The starting compounds and phase equilibria in the boundary systems were studied in a number of works [33, 37-43]
Tl2Te melts congruently at 698 K [37], and has a monoclinic structure (Sp.Gr. C2/C; a = 15.662; b = 8.987; c = 31.196A, p = 100.7(6°, z = 44) [38].
TlBiTe2 melts congruently at 820 K [39], and crystallizes in a hexagonal structure (Sp. Gr.R-3m) with parameters a = 4.526; c = 23.12 A; z = 3 [40].
TlTbTe2 compound is structural analogue of TlBiTe2 and has the following lattice parameters: a = 4.416; c = 24.27 A; z = 3 [41].
Tl2Te-TlBiTe2 system studied by the authors of [38] is characterized by the formation of the
Tl9BiTe6 compound which melts congruently at 830 K. This compound crystallizes in a tetragonal structure with the following lattice parameters: a = 8.855; c = 13.048 A, z = 2 [42]. According to Ref. [39], in the Tl2Te-Tl9BiTe6 system, continuous solid solutions with a morphotropic phase transition near Tl2Te were detected. Considering that Tl2Te and Tl9BiTe6 crystallize in different crystal structures, this statement seems unlikely. Therefore, the authors of [43] re-studied the phase relations in the Tl2Te-Tl9BiTe6 system and showed that the system is a quasi-binary system of the peritectic type and is characterized by the formation of limited solid solutions based on the initial compounds.
Tl2Te-TlTbTe2 system was studied only in the composition interval of > 80 mol% Tl2Te. It is shown that it is characterized by the formation of a tetragonal Tl9TbTe6 compound which melts with decomposition by a peritectic reaction at 780 K and has the following lattice parameters: a = 8.871; c = 12.973A, z = 2 [35]. The Tl2Te-Tl9TbTe6 subsystem is characterized by the formation of solid solutions with Tl5Te 3 type tetragonal structure based on Tl TbTe .
9 6
In the Tl9TbTe6-Tl9BiTe6 system, continuous solid solutions based on the starting compounds were found [33].
In the TlBiTe2-TlTbTe2 system, it was shown that despite the isostructural character of the initial compounds, the system is characterized by the limited mutual solubility of the initial components. The solubility based on TlBiTe2 reaches ~45 mol% and the solubility based on TlTbTe2 is about 22 mol% [44].
2. Experimental
2.1. Materials and synthesis
Initial binary and ternary compounds were synthesized by the direct interaction of the high purity elements, all from Alfa Aesar (Germany): (thallium, CAS No 7440-28-0; tellurium, 1349480-9; bismuth, 7440-69-9; terbium, 7440-27-9).
Tl2Te, Tl9BiTe6, and TlBiTe2, 10 grams each, were prepared by the melting of the elements in evacuated (~10-2 Pa) quartz ampoules in a single-zone electric furnace at 850 K. To
S. Z. ImamaLiyeva et al.
Original articles
achieve an equilibrium state, after synthesis, the intermediate ingot of TlBiTe2 was subjected to heat treatment 700 K for 500 h.
The synthesis of the incongruently melting compounds, Tl9TbTe6 and TlTbTe2, was carried out by the ceramic method at 1000 K for 100 h. We used a graphitized ampoule in order to prevent the reaction of terbium with quartz. Then the ingots were slowly cooled down to room temperature, crushed in an agate mortar, pressed into pellets and the heating procedure was repeated at 900 K for 500 h.
The purity of the synthesized compounds was controlled by the differential thermal analysis (DTA) and powder X-ray diffraction (PXRD) method.
Samples of the Tl2Te-TlSbTe2-TlTbTe2 system, 1 g each, were prepared by fusing pre-synthesized and identified binary and ternary compounds in evacuated quartz ampoules in a single-zone electric furnace at a temperature 30500 higher than the melting temperature of the compounds, followed by cooling in a switched off furnace.
2.2. Methods
The PXRD (Bruker D8 diffractometer,CuKa radiation) was used to control the purity of the synthesized compounds and intermediate
samples. The analysis was carried out at room temperature between 10° < 20 < 70°. The lattice constants were calculated by indexing of powder patterns using Topas V3.0 software.
DTA was performed using a NETZSCH 404 F1 Pegasus differential scanning calorimeter within room temperature and ~1400 K depending on the composition of the alloys at a heating rate of 10 Kxmin_1. The temperatures of thermal effects were taken mainly from the heating curves.
3. Results and discussion
3.1. Solid-phase equilibria diagram of the Tl2Te-TlBiTe2-TlTbTe2 system
Fig. 1 presents the solid-phase equilibria diagram of the Tl2Te-TlBiTe2-TlTbTe2 system.
As can be seen, the stable section Tl9BiTe6-Tl9TbTe6 characterized by the formation of a continuous series of solid solutions [36] divides this system into two independent subsystems.
Tl2Te-Tl9BiTe6-Tl9TbTe6 subsystem is characterized by the formation of a wide field of solid solutions with a Tl5Te 3 structure (8-phase) that occupy more than 90% of the area of the concentration triangle. Solid solutions based on Tl2Te (a-phase) form within a narrow region. The regions of the a- and 8-phases are separated by a two-phase region a + 8. It should be noted
Fig. 1. The solid-phase equilibria diagram of the Tl2Te-TlBiTe2-TlTbTe2 system
Condensed Matter and Interphases / Конденсированные среды и межфазные границы S. Z. Imamaliyeva et al.
2021;23(1): 32-40 Original articles
that a similar scheme of phase equilibria was found when studying the Tl2Te-Tl9BiTe6-Tl9ErTe6 system [43].
While studying the Tl9BiTe6-Tl9TbTe6-TlTbTe2-TlBiTe2 subsystem, a number of alloys from this concentration region were investigated. Also, we used the results from our previous papers [36, 44].
The interaction of the S-phase with solid solutions based on TlTbTe2 (P1) and TlBiTe2 (P2)
2500 -
leads to the formation of wide two-phase (P1+S and P2+S) fields separated by a P1+P2+S three-phase area. The location and extent of the phase regions are confirmed by XRD data. As an example, Fig. 2 shows PXRD patterns from the P1+S two-phase (# 1) and P1+P2+S three-phase (# 2) regions.
Based on the index of the PXRD patterns of the samples # 1 and # 2, we obtained the following crystal lattice parameters:
2000 -
В с
-ß2- phase - 6- phase
îs 1500 -
1000
500 -
30 40
Diffraction Angle [°2G]
В с
2500
2000
1500
1000
500
0
•
#2 ф - pr phase
A - 02- phase
# — 5- phase •
i к
10
20
50
60
70
30 40
Diffraction Angle [°26]
Fig. 2. The PXRD patterns of samples #1 and #2 from the two- and three-phase areas of the Tl9BiTe6-Tl9TbTe6 TlTbTe2-TlBiTe2 subsystem
S. Z. ImamaLiyeva et al.
Original articles
Sample #1: a=4.4883, c=23.580 A (Prphase); a=8.8626, c=13.008 A (8-phase)
Sample #2: a=4.4793, c=23.481A (Prphase); a=4.4472, c=24.007 A (p2-phase); a=8.8630, c=13.005 A (8-phase).
A comparison of these data with the results of [36, 44] shows that sample #1 consists of a two-phase mixture of a Prphase with a composition of 40 mol% TlTbTe2 along the section TlBiTe2-TlTbTe2 and a 8-phase with a composition of 50 mol% Tl9TbTe6 along the Tl9BiTe6-Tl9TbTe6 section. Sample #2 consist of a three-phase mixture of P1+P2+8 with the following phase compositions: P1 and P2 - respectively, 45 and 77 mol% TlTbTe2 along the TlBiTe2-TlTbTe2 section, and 8 - 50 mol% Tl9TbTe6. These coincide with the data in Fig. 1.
3.2. The liquidus surface
of the Tl2Te-Tl9BiTe6-Tl9TbTe6subsystem
The liquidus surface of the Tl2Te-Tl9BiTe6-Tl9TbTe6 system consists of three fields of the primary crystallization of the a- and 8-phases and the P2-phase based on the TlTbTe2 compound (Fig. 3). These fields are separated by p1p1' and p2p2' lines, which correspond to the L+P2 ^ P and L+8 ^ a monovariant peritectic process. The solidus surface consists of two areas of
the completion of crystallization of the a- and 8-phases.
3.3. Some polythermal and isothermal sections of the phase diagram of the Tl2Te-Tl;BiTe6-Tl9TbTe6 subsystem
In order to confirm the correct construction of the liquidus surface of the Tl2Te-Tl9BiTe6-Tl9TbTe6 subsystem and to refine the boundaries of areas of primary crystallization of the 8-phase and TlTbTe2, the isopleth sections Tl2Te-[A] and Tl9TbTe6Te-[B] (A and B - are alloys with compositions 1:1 from the boundary sections Tl9BiTe6-Tl9TbTe6 and Tl2Te-Tl9BiTe6) of the
9 6 9 6 2 9 67
phase diagram were constructed.
The liquidus curve along the Tl2Te- [A] section consists of two curves corresponding to the primary crystallization of the a- and 8- phases. Their intersection point corresponds with the onset of the monovariant peritectic reaction L+8 ^ a.
In the Tl9TbTe6- [B] section, in the composition range up to ~65 mol% Tl9TbTe6, the 8-phase crystallizes from the melt, while in the TlTbTe2 -rich alloys the P1-phase based on TlTbTe2 first crystallizes, then the monovariant peritectic equilibrium L+P1 ^ 8 takes place. In the latter reaction, the P1-phase is completely consumed and the excess of melt crystallizes into the 8-phase.
Tl,Te
Pz A
Lii о A
80 / -74OA
\ \
60 /-,720
—** \ \
У 40 "
40/ © ^^ ^780 \
Pi/
/ __ —* 800 \
20/"
Х^Глйо i 820\
¿¿Z—-*г « > ^CTSOO" " i - -820 \
TIJbTe, 20 Pi 40 [A] 60
mol. % TLBiTe,
80
TLBiTe,
Fig. 3. Projections of the liquidus (solid lines) and solidus (dashed lines) surfaces of the TLTe-Tl9BiTe-Tl9TbTe6
subsystem. Primary crystallization fields of phases: 1 - a; 2 and Tl9TbTe6Te-[B] polythermal sections of the subsystem
5; 3 - ßj. Red lines show the studied Tl2Te-[A]
S. Z. Imamaliyeva et al.
Original articles
The presence of monovariant peritectic reactions L+P1 ^ S and L+S ^ S (Fig. 3, p2p2' and p2p2 curves) in the Tl2Te-Tl9BiTe6-Tl9TbTe6 system should lead to the formation of L+a+S and L+P1+S three-phase regions on the polythermal sections of Tl2Te-[A] and Tl9TbTe6-[B], accordingly (Fig. 4). The very narrow temperature ranges of these reactions do not allow us to determine these areas by the DTA method. Taking into account the well-known principles [45] of the construction of polythermal sections, the regions L+P1+S and L+a+S in the relevant section were delimited by dashed lines.
The isothermal sections of the phase diagram are important for choosing the composition of
solution-melts when growing single crystals by directional crystallization.
As can be seen, from the isothermal sections at 740 and 780 K, the first consists of conjugated liquidus and solidus curves, delimiting singlephase regions L and S. These curves are connected by tie lines and delimit the L + S two-phase area. The isothermal section at 780 K in addition to these phase regions, also reflects the heterogeneous regions L+P1, P1+S, and L+P1+S, which are delimited taking into account data on the Tl2Te-Tl9TbTe6 and Tl2Te-Tl9TBiTe6 boundary systems [35, 43].
A comparison of the isothermal (Fig. 5) and polythermal (Fig. 4) sections of the phase diagram
T, к
800
750
700
* Щ L
- L+5
L+a +a+5 5
a a+5 \
ншк
т, к 1100
1000
780
730
L+p 4 \ L
L+p.+й Ö
80
60
40
20
mol. % TI.Te
[A]
TI„TbTc<
80
60
40
20
IBJ
mol% TljTbïb..
Fig. 4. Tl2Te-[A] and Tl9TbTe6-[B] polythermal sections of the phase diagram of the Tl2Te-Tl9BiTe6-Tl9TbTe( subsystem of the Tl-Bi-Tb-Te quaternary system. A and B are equimolar compositions of the Tl9BiTe6-Tl9TbTe( and Tl2Te-Tl9BiTe6 boundary systems on Fig. 3
Fig. 5. Isothermal sections at 740 and 780 K of the TLTe-TLBiTe-TLTbTe, subsystem
S. Z. ImamaLiyeva et al. Original articles
of the Tl2Te-Tl9BiTe6-Tl9TbTe6 system clearly demonstrates that the directions of the tie lines do not coincide with the T-x planes of the studied internal sections, which is characteristic of non-quasi-binary polythermal sections.
4. Conclusion
The character of the solid-phase equilibria in the Tl2Te-TlBiTe2-TlTbTe2 system is established by using the DTA and powder XRD. A diagram of solid-phase equilibria at room temperature of this system is constructed, as well as a number of polythermal and isothermal sections and projections of the surfaces of liquidus and solidus in the Tl2Te-Tl9BiTe6-Tl9TbTe6 composition range. The Tl9BiTe6-Tl9TbTe6 section, characterized by the formation of a continuous series of solid solutions (S-phase), divides the Tl2Te-TlBiTe2-TlTbTe2 system into two independent subsystems.
The TLBiTe -TlBiTe -TlTbTe -Tl TbTe,
9 6 2 2 9 6
subsystem is characterized by the formation of the wide areas of the solid solutions based on TlTbTe2 (P1-phase) and TlBiTe2 (P2-phase). The homogeneity region of the S-phase covers a large (> 90% of the Tl2Te-Tl9BiTe6-Tl9TbTe6 subsystem area). The obtained solid solutions P1, P2, and S are of great interest as potential magnetic topological insulators and thermoelectric materials.
Conflict of interests
The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.
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Information about the authors
Samira Z. Imamaliyeva, PhD in Chemistry, Assistance Professor, Institute of Catalysis and Inorganic Chemistry, Azerbaijan National Academy of Sciences, Baku, Azerbaijan; e-mail: samira9597a@ gmail.com. ORCID iD: https://orcid.org/0000-0001-8193-2122.
Ganira I. Alakbarzade, PhD student, Azerbaijan National Aerospace Agency, Baku, Azerbaijan; e-mail: alakbarzadegi@gmail.com ORCID iD: https://orcid. org/0000-0001-8500-0007
Dunya M. Babanly, DSc in Chemistry, Assistance Professor, Institute of Catalysis and Inorganic Chemistry, Azerbaijan National Academy of Sciences, Azerbaijan State Oil and Industry University, Baku, Azerbaijan; e-mail: dunya.babanly@ufaz.az. ORCID iD: https://orcid.org/0000-0002-8330-7854.
Marina V. Bulanova, DSc in Chemistry, Leading Researcher, I. M. Frantsevich Institute for Problems of Materials Science, NAS of Ukraine, Kiev, Ukraine; e-mail: mvbulanova2@gmail.com. ORCID iD: https:// orcid.org/0000-0002-8691-0982.
Vagif A. Gasymov, PhD in Chemistry, Assistance Professor, Institute of Catalysis and Inorganic Chemistry, Azerbaijan National Academy of Sciences, Baku, Azerbaijan; e-mail: v-gasymov@rambler.ru. ORCID iD: https://orcid.org/0000-0001-6233-5840.
MahammadB. Babanly, DSc in Chemistry, Professor, Associate Member of the Azerbaijan National Academy of Sciences, Deputy-director of the Institute of Catalysis and Inorganic Chemistry, Azerbaijan National Academy of Sciences, Baku State University, Baku, Azerbaijan; e-mail: babanlymb@gmail.com. ORCID iD: https://orcid.org/0000-0001-5962-3710.
All authors have read and approved the final manuscript.
Received 8 January 2021; Approved after reviewing 9 February 2021; Accepted 15 March 2021; Published online 25 March 2021.
Translated by Samira Imamaliyeva
Edited and proofread by Simon Cox
