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UDC 541.123.6:546.289'24
TliTe-Tl9BiTe6-TlsGeTe5 SYSTEM T.M. Alakbarova1, Y.I. Jafarov1, A.L. Mustafayeva1, M.B. Babanly2
1Baku State University 23, Z.Khalilov str., Baku, Az-1148, Azerbaijan 2Acad. M. Nagiyev Institute of Catalysis and Inorganic Chemistry National Academy of Sciences of Azerbaijan H.JavidAve., 113, Baku, Az-1143, Azerbaijan; e-mail: babanlymb@gmail.com
Phase equilibria in the quaternary system Tl-Ge-Bi-Te in the composition area Tl2Te-Tl9BiTe6-Tl8GeTe5 were investigated by using differential thermal analysis (DTA) and powder X-ray diffraction (XRD) technique. Some isopleth sections and isothermal sections at 300, 740 and 800 K, as well as projections of liquidus and solidus surfaces were constructed. It found that homogeneity area of solid solutions with Tl5Te3 structure (S-phase) occupied more than 80% of the concentration triangle. A narrow area of solid solutions (a-phase) based on Tl2Te were detected along boundary system Tl2Te-TlBiTe6.
Keywords: thallium-germanium telluride, thallium-bismuth telluride, phase equilibriums, solid solutions, crystal structure.
1. INTRODUCTION
Chalcogenides of heavy metals belong to important functional materials with thermoelectric, photoelectric, optical and other properties [1-3]. Some of them exhibit properties of topological insulators and can be used in spintronic devices [4-6]. Moreover, complex thallium chalcogenides constitute a new class of promising thermoelectric materials with anomalously low thermal conductivity [7-10].
Thallium subtelluride, Tl5Te3, because of crystal structure features (Sp.gr.I¥/mcm, a = 8.930; c = 12.598 A) [11,12], has a number of ternary cation- and anion analogs such as TUA Te3 and Tl9BVTe6 as well as T№Hal (AIV-Sn, Pb; BV-Sb, Bi; X-Se, Te; Hal-Cl, Br, I) [13-17]. The above-mentioned compounds have a good thermoelectric performance, whereas Tl9BiTe6 exhibits the highest ZT value [2, 18-22]. Furthermore, authors [23] found Di-rac-like surface states in the [Tl4]TlTe3 (Tl5Te3) and its non-superconducting tin-doped derivative [Tl4](Tl1-xSnx)Te3.
Direct search for new multicomponent compounds based on known ones requires the study of phase equilibria in the corresponding systems. Early we presented the results of
phase relations study in the systems including Tl5Te3 and its analogs [24-26]. It was shown that the systems are characterized by the formation of continuous areas of solid solutions with Tl5Te3-type structure.
In this work, we continue to study similar systems and present a detailed analysis of phase equilibria in the quaternary system Tl-Ge-Bi-Te on the composition area Tl2Te-Tl9BiTe6-Tl8GeTe5.
Initial compounds of the investigated system are studied in detail. Tl2Te, Tl9BiTe6 and Tl8GeTe5 compounds melt congruently at 698 [27], 830 [15] and 753 K [28] respectively. Tl2Te crystallizes in the monoclinic system (space group C2/c; a = 15.662; b = 8.987; c=31.196Â, P=100.76°, z=44) [29], while Tl9BiTe6 and Tl8GeTe5 have tetragonal structure of Tl5Te3-type (I4/mcm) with following parameters: a = 8.855, c = 13.048 Â, z=2; a=8.918, c=13.055 Â, z=2 [22,30]. Boundary Tl9BiTe6-Tl8GeTe5 and Tl2Te- Tl5Te3-Tl8GeTe5 systems are studied in [31, 32] and characterized by the formation of unlimited fields of solid solutions.
2. EXPERIMENTAL
2.1. Materials and syntheses
The following elementary components were used for the investigations: thallium (granules, 99.999 %), bismuth (granules, 99.999 %), germanium (powder, 99.999%), and tellurium (broken ingots 99.999 %).
The components were weighed according to stoichiometric compositions and put into silica tubes, about 20 cm in length and 1 cm in diameter. Then the ampoules were sealed under a vacuum of 10" Pa. The compounds, 20 gram each, were prepared by melting of the elements in evacuated silica tubes in a single zone electric furnace at a temperature of 30500 above the melting point of the compounds followed by cooling in the switched-off furnace.
The purity of synthesized compounds was checked by differential thermal analysis (DTA) and powder X-ray diffraction (XRD) techniques.
Previously synthesized binary and ternary compounds were used to synthesize the alloys of the Tl2Te-Tl9BiTe6-Tl8GeTe5 system. All the samples were annealed at 650 K after synthesis to achieve complete homogenization.
2.2. Methods
DTA and XRD analyses were used to analyze the samples of the Tl2Te-Tl9BiTe6-Tl8GeTe5 system.
The phase transformation temperatures have been determined using a NETZSCH 404 F1 Pegasus differential scanning calorimeter between a room temperature and ~900 K. The phase identification was performed using a Bruker D8 diffractometer utilizing CuKa radiation in the 20 range of 6-70°. The unit cell parameters of starting compounds and intermediate alloys were calculated by indexing powder patterns using Topas V3.0 software.
3. RESULTS AND DISCUSSION
The combined analysis of experimental and literature data on boundary systems Tl2Te-Tl8GeTe5 [28] and Tl9BiTe6-T^GeTe5 [32]
enabled us to construct the self-consistent diagram of phase equilibriums in the Tl2Te-Tl9BiTe6-Tl8GeTe5 system (Table, Fig.1-4).
Table. Some properties of phases in the Tl2Te-Tl9BiTe6-Tl8GeTe5 system
Alloy compositions, Fig.1,3 Thermal effects, K Lattice parameters, Â
mol % Tl9BiTe6
100 830 Tetragomal, a=8.8545(4), c=13.0476(9)
CP (U 80 800-824 a=8.866(4), c=13.013(10)
50 760-800 a=8.888(4), c=12.961(10)
H S 20 720-770 a=8.891(4), c=12.910(9)
H 10 700-756 -
5 698-747 -
[B] 695-730 a=8.915(5); c=12.876(12)
mol % Tl2Te
100 698 monoclinic, C2/c; a = 15.658 (8); b = 8.989 (4); c = 31.192Â (12), ß = 100.76, z = 44
< 90 698 -
<à H 80 696-740 -
IN H 60 715-765 a=8.919(5); c=12.783(12)
40 740-782 a=8.913(5); c=12.875(12)
20 760-798 a=8.908(5); c=12.968(12)
[A] 782-806 a=8.903(5); c=13.060(13)
Isopleth sections of the T12Te-T19BiTe6-T18GeTe5 system (Fig.1).
Figs.1a,b present the isopleth sections Tl2Te-[A] and Tl9BiTe6-[B] of the ThTe-Tl9BiTe6-Tl8GeTe5 system, where A and B are equimolar alloys from the respective boundary system as shown in Fig.3. The liquidus of Tl2Te-[A] section consists of two curves of primary crystallization of a- and 5-
phases. The intersection point of these curves corresponds to the monovariant peritectic reaction L+ôoa (700-703 K). The L+ô+a area is indicated by a dashed line because it was not fixed experimentally due to a narrow temperatures range. Below the solidus this section passes through a, a+ô and ô phase areas.
jTl2Te so 60 40 20 [A] [B] ^ 20 40 60 SO T]TBiTc, mol%[A] mol % TLBiTe,,
Fig.1. Polythermal sections Tl2Te-[A] and Tl9BiTe6-[B] of the phase diagram of the Tl2Te-Tl9BiTe6-Tl8GeTe5 system where A and B are equimolar alloys from the respective boundary system as shown in Fig.3.
Along the Tl9BiTe6-[B] section, the 5-phase primary crystallizes over the entire compositions range. Then a monovariant peri-tectic process L+5o-a takes place (Fig.3a). The L+5+a phase is shown by a dashed line because not fixed experimentally due to a narrow interval of temperatures. A narrow (10 mol%) two-phase region a+5 is found below 700 K.
X-ray analysis of the selected powdered samples testifies to the phase diagrams of the
above-mentioned systems. For example, the XRD patterns for different compositions of the Tl2Te-[A] section are presented (Fig.2). The alloys with compositions <60 mol% Tl2Te are monophasic with Tl5Te3-type diffraction patterns (Fig.2, diffraction patterns 1-3), while alloy with 80mol% Tl2Te composition is bi-phasic and besides the 5-phase reflections contains weak reflections of a-phase based on Tl2Te (Fig.2, diffraction pattern 4).
Fig.2. XRD patterns for different compositions in the Tl2Te-[A] section. [A] are equimolar alloy from the boundary system Tl9BiTe6-Tl8GeTe5 as shown on Fig.3.
The liquidus surface projection (Fig. 3).
The liquidus of the Tl2Te-Tl9BiTe6-Tl8GeTe5 system consists of two fields of the primary crystallization of a- and 5-phases. These fields are separated by pe line which corresponds to the monovariant peritectic equilibrium L+5o-a. Near the eutectic point (e) the peritectic equilibrium L+5o-a must be
transformed into Lo-a+5 eutectic equilibrium. However, coordinates of this transformation are not experimentally fixed due to a narrow temperature range. Solidus surface consists of two areas corresponding to the completion of crystallization a- and 5-phases.
Tl,BiTc,
Fig.3. Projection of the liquidus and solidus (dashed lines) surface of the Tl2Te-Tl9BiTe6-Tl8GeTe5 system. Primary crystallization fields of phases: 1-5; 2-a. Investigated sections are shown by dashed lines. [A] and [B] are equimolar alloys from the respec tive boundary systems.
The isothermal sections of the T^Te- and 5-phases are separated by a+5 two-phase Tl9BiTe6-TlsGeTe5 system at 300, 740 and region. It should be noted that comparison of 800 K (Fig.4) consist of three phase areas. At the isopleth sections (Fig. 1) and isothermal 300 K, over 80% of the concentration triangle sections at 740 and 800 K(Fig.4) shows that is occupied by 5-solid solutions with Tl5Te3 the directions of the tie-lines in the two-phase structure. Tl2Te-based a-phase has a narrow area L+5 deviate from the T-x planes of the homogeneity area in the corresponding angle above-mentioned sections and vary with tem-of the triangle. Homogeneity areas of the a- perature.
Fig.4. Isothermal section of the phase diagram of the Tl2Te-Tl9BiTe6-Tl8GeTe5 system at
300, 740 and 800 K.
A complete T-x-y diagram of the Tl2Te-Tl9BiTe6-Tl8GeTe5 system has been constructed, including some isopleth and isothermal sections at 300, 740 and 800 K as well as projections of liquidus and solidus surfaces. The analyzed system is characterized by the formation of the wide field of 5-solid solutions
with the Tl5Te3 structure, occupying more than 80% of the concentration triangle. Experimental data obtained can be used for choosing the composition of solution-melt and for determining of temperature conditions for growing crystals of 5- phase with a given composition.
ACKNOWLEDGMENT
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 the NASA (Azerbaijan) and the Donostia International Physics Center (Basque Country, Spain).
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СИСТЕМА Tl2Te-Tl9BiTe6-Tl8GeTe5
11 12 Т.М. Алекберова , Я.И. Джафаров , А.Л. Мустафаева , М.Б. Бабанлы
1 Бакинский Государственный Университет Az-1148, Баку, ул.З.Халилова, 23 2Институт Катализа и Неорганической Химии Национальной АН Азербайджана Az-1143, Баку, пр.Г.Джавида, 113, е-mail: babanlymb@,gmail.com
Методами ДТА и РФА изучены фазовые равновесия в четверной системе Tl-Ge-Bi-Te в области концентраций Tl2Te-Tl9BiTe6-TlsGeTe5. Построены некоторые политермические сечения фазовой диаграммы, изотермические сечения при 300, 740 и 800 К, а также проекции поверхностей ливидуса и солидуса. Установлено образование широкой области твердых растворов со структурой Tl5Te3 (ё-фаза), занимающей более 80% площади концентрационного треугольника. На основе Tl2Te обнаружена узкая область твердых растворов (а-фаза) вдоль боковой системы Tl2Te-TlgBiTe6.
Ключевые слова: теллуриды таллия-германия, теллуриды таллия-висмута, фазовые равновесия, твердые растворы, кристаллическая структура.
Tl2Te-Tl9BiTe6-TkGeTe5 SISTEMI
T.M. dlakbarova1, Y.l Cafarov1, A.L. Mustafayeva1, M.B. Babanli2
1Baki Dövldt Universiteti Az-1143, Baki, Z.Xdlilov, 23 AMEA Kataliz vd Qeyri-üzvi kimya institutu Az-1143, Baki, H.Cavidpr.,113; e-mail: babanlymb@gmail.com
DTA vd RFA üsullari ils Tl-Ge-Bi-Te sisteminin Tl2Te-Tl9BiTe6-TlgGeTe5 tdrkib sahdsindd faza tarazliqlari öyrdnilmi^äir. Faza diaqraminin bdzi politermik kdsikldri, 300, 740 vd 800 K-dd izotermik kdsikldri, hdmçinin likvidus vd solidus sdthldrinin proyeksiyalari qurulmuçdur. Sistemdd Tl5Te3 tipli tetraqonal quruluçda hristallaçan vd qatiliq ûçbucaginin sahdsinin 80 %-ddn artigini dhatd eddn ô bdrk mdhlul sahdsi a§kar edilmiçdir. Tl2Te dsasinda a-fazanin homogenlik sahdsinin Tl2Te-Tl9BiTe6 yan sistemi boyunca nazik zolaq §dklindd olmasi göstdrilmi§dir.
Açar sözlar: tallium-germnium telluridldri tallium-bismut telluridldri, faza tarazliqlari, bdrk mdhlullar, kristal quruluç.
Received 21.09.2017.
