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496 CHEMICAL PROBLEMS 2018 no. 4 (16) ISSN 2221-8688
UDC 541.123.6:546.289'24
THE Tl4PbTe3-Tl9GdTe6-Tl9BiTe6 ISOPLETH SECTION OF THE Tl-Pb-Bi-Gd-Te SYSTEM
1S.Z. Imamaliyeva, 2G.I. Alakbarzade, 2Z.E. Salimov, 3S.B. Izzatli, 3Ya.I. Jafarov,1M.B. Babanly
1 Acad. M. Nagiyev Institute of Catalysis and Inorganic Chemistry National Academy of Sciences of Azerbaijan, 113, H.JavidAve., AZ-1143, Baku, Azerbaijan, e-mail: samira9597a@gmail.com 2Azerbaijan State Oil and Industry University 16/21, Azadlig Ave., AZ-1010, Baku, Azerbaijan
3Baku State University Z.Khalilov str., 23, AZ-1148, Baku, Azerbaijan
Received 17.10.2018
The purpose of this study was to investigate the phase relations in the Tl-Pb-Gd-Te quaternary system on the TlgGdTe6-TlgBiTe6-TUPbTe3 composition area. Based on the results of the differential thermal analysis, the powder X-ray diffraction technique and the microhardness measurements, it revealed that the system is characterized by unlimited solid solutions crystallized in Tl5Te3 structure type.
Keywords: thallium-lead telluride; thallium-gadolinium tellurides; thallium-bismuth tellurides; phase equilibria; liquidus and solidus surfaces; solid solutions. DOI: https://doi.org/10.32737/2221-8688-2018-4-496-504
1. INTRODUCTION
Heavy metal chalcogenides, multicomponent phases and composites based thereon are valuable functional materials [1-3]. In recent years, interest in complex thallium chalcogenides has increased due to topological insulators [4-6], Weyl semimetals [6] and thermoelectric materials with anomalously low thermal conductivity [8-10]. Some of these compounds have photoconductivity and are promising for use as detectors of y and X-ray radiation [11, 12].
Among thallium chalcogenides, Tl5Te3 is one of the most suitable matrix compounds for the preparation of new ternary compounds -structural analogues and multicomponent phases. According to the phase diagram of the Tl-Te system [13], this compound melts congruently at 725 K and has a wide homogeneity region (34.5-38 at.% Te). Due to the crystal structure features [14], TbTe3 has a number of the ternary cation-substituted structural analogues [15-18]. This compound and its ternary analogues attract the attention
of researchers, primarily as potential thermoelectric materials [19-22]. TbBiTe6 compound shows the best thermoelectric figure among these compounds (ZT = 0.65 at 300 K and ZT = 1.2 at 500 K) [22].
The development of physicochemical bases for goal-oriented synthesis of new multicomponent inorganic phases and materials is based on fundamental studies on phase equilibria and thermodynamic properties of the corresponding systems [23-25]. In this respect, the most interesting are systems where it is possible to expect the formation of structural analogues of known binary and ternary compounds or solid solutions based on them.
In recent years, thallium- REEs tellurides with the common formula of Tl9LnTe6 have attracted the attention of researchers as promising materials that combine thermoelectric and magnetic properties [26-30]. It is assumed that these compounds can have good thermoelectric
properties, such as Tl9BiTe6 since the introduction of lanthanide atoms lighter than thallium and bismuth into the TbTe3 crystal lattice should lead to significant mass fluctuations and as a result to a decrease in thermal conductivity and an improvement in thermoelectric properties.
Earlier we presented the results of the research of some TbLnTe6 compounds as well as the phase relations for a number of systems, including the TbTe3or TbLnTe6 compounds [31-35]. The formation of unlimited solid solutions was found for these systems.
In this study, we continue to investigations of the similar systems and present the experimental results on phase relations in the Tl4PbTe3-Tl9GdTe6-Tl9BiTe6 section of the Tl-Pb-Bi-Gd-Te system.
The initial compounds of the above-mentioned system were studied in a number of papers. TUPbTe3 and TbBiTe6 melt congruently at 893 K [18], and 830 K [17], respectively, while Tl9GdTe6 is formed by the peritectic reaction at 800 K [32].These compounds have the following lattice parameters: a=8.841, c=13.056A, z=4 (Tl4PbTe3)[36]; a = 8.855, c = 13.048 A, z=2(Tl9BiTe6) [37];a=8.870; c=13.027 A, z=2 (Tl9GdTe6) [32].
The Tl4PbTe3-Tl9BiTe6 boundary system is quasi-binary and characterized by the formation of unlimited solid solutions (5) with Tl5Te3-structure [38]. The TbGdTe6-TbBiTe6 boundary system is not quasi-binary due to the peritectic character of TbGdTe6 melting [39].
2. EXPERIMENTAL
2.1. Materials and syntheses
For the experiments we used the following chemical reagents: thallium (granules, 99.999 %), lead (ingot, 99.99 %), gadolinium (powder, 99.9%), bismuth (granules, 99.999 %), and tellurium (broken ingots 99.999 %). Taking into account that thallium and its compounds are highly toxic, the protective gloves were constantly used when working with thallium.
Stoichiometric amounts of the starting components were put into silica tubes of about 20 cm in length and a diameter of about 1.5 cm, sealed under a vacuum of 10-2 Pa. Tl4PbTe3 and Tl9BiTe6 were synthesized by heating in a resistance furnace at 920 K followed by cooling in the switched-off furnace.
In order to avoid the reaction of gadolinium with quartz, the ampoule was graphitized using pyrolysis of toluene. After synthesis, the intermediate ingot of Tl9GdTe6 was powdered in an agate mortar, carefully mixed, pressed into a pellet and annealed at 750 K within-700 h.
The purity of resulting compounds was established by the differential thermal analysis (DTA) and X-ray diffraction (XRD).
3. RESULTS AND DISCUSSION
The combined analysis of experimental TUPbTe3-TbBiTe6 [38] results and literature data on boundary system TbBiTe6 [39] allowed us
The alloys of the Tl4PbTe3-Tl9GdTe6-Tl9BiTe6 system were prepared by melting of the previously synthesized and identified ternary compounds. After synthesis, the samples containing >60% TbGdTe6 were powdered, mixed, pressed into pellets and annealed at 750 K during ~ 800 h in order to complete the homogenization.
2.2. Methods
DTA and XRD analyses, as well as micro-hardness measurements, were used to analyze the samples of the investigated system.
DTA heating curves were obtained in the range within room temperature ~1400 K at a heating rate of 10 K- min-1 using a Netzsch 404 F1 Pegasus differential scanning calorimeter system. The crystal structure of the initial compounds and intermediate alloys was studied by XRD method on a Bruker D8 powder diffractometer (CuKa radiation) in the 20 range of 10-75°. The tetragonal lattice parameters were indexed using the Topas V3.0 software. Micro-hardness measurements were performed on a PMT-3 micro-hardness tester and an indenter load of 0.2 N.
and Tl9GdTe6-to establish the
scheme of phase relations in theTUPbTe3-Tl9GdTe6-Tl9BiTe6 (Fig.1-5).
The Table presents the results of DTA, micro-hardness measurements, and parameters of the tetragonal lattice for some intermediate alloys of the TUPbTe3-TbGdTe6 system.
Tl4PbTe3-Tl9GdTe6 section (Fig. 1) is non-quasi-ternary due to the peritectic character of melting of Tl9GdTe6 but stable below the solidus and characterized by the formation of a continuous series of solid solutions (5-phase with Tl5Te3-structure).
Within the composition range 0-60 mol % TbGdTe6, the 5-solid solutions primarily crystallize, and within a range richer in TbGdTe6, the TlGdTe2 does. According to the Gibbs phase rule, L+TlGdTe2+5 three-phase area should form within the range 60-100 mol % TbGdTe6 below 800 K in the phase diagram because of the mono variant peritectic reaction L+TlGdTe2^5. However, this area was not experimentally fixed because of the narrow temperature range and was shown by a dashed line.
Table. Experimental data of the DTA, micro-hardness measurements and parameters of tetragonal lattice for the alloys of the TUPbTe3-TbGdTe6 section of the Tl-Pb-Bi-Gd-Te system
Solid phase compositions Thermal effects, K Micro-hardness, MPa Tetragonal lattice parameters, A
a c
Tl4PbTe3 893 1120 8.8409 13.0556
Tl8.2Pb1.6Gd0.2Te6 865-882 1170 8.8467 13.0498
Tl8.4Pb1.2Gd0.4Te6 842-865 1200 8.8525 13.0441
Tl8.5Pb1.0Gd0.5Te6 837-856 - - -
Tl8.6Pb0.8Gd0.6Te6 824-845 1190 8.8583 13.0384
Tl8.8Pb0.4Gd0.8Te6 809-824;1110 1150 8.8641 13.0327
Tl8.9Pb0.2Gd0.9Te6 804-812;1150 - - -
TbGdTe6 800; 1190 1100 8.8701 13.0270
Tl,GdT^ 80 60 40 20 2TI4PbTe:
mol % Tl,GdTe„
Fig.1. Phase diagram (a), concentration dependencies of micro-hardness (b), and lattice parameters (c) of the alloys of the TUPbTe3-TbGdTe6 section of the Tl-Pb-Bi-Gd-Te system.
In order to determine the phase constituents, polished surfaces of the intermediate samples were visually observed under the microscope of micro-hardness meter.
The micro-hardness curves have a flat maximum which is characteristic for systems with a continuous series of substitutional solid solutions (Fig.1b) [40].
14000
2-Theta - Scale
Fig. 2. XRD powder patterns for starting compounds and some alloys of the TUPbTe3-TbGdTe6
section.
The XRD data also confirm the phase diagram of theTl4PbTe3-Tl9GdTe6 systems and indicate the formation of a continuous series of solid solutions. As seen in Fig. 2, the XRD patterns of the initial tellurides and intermediate alloys in this system are similar to that of Tl5Te3, with a slight displacement of their reflections.The composition dependences of the lattice parameters of the solid solutions are subject to the Vegard's rule within the experimental uncertainty [41].
The liquidus and solidus surfaces projections (Fig.3).
The projection of the T-x-y phase diagram onto the Gibbs composition triangle (Fig. 3) showed that the liquidus surface consists of two-phase areas of the primary crystallization of the S-solid solutions (area 1)
and TlGdTe2 (area 2). These surfaces are separated by curve ab which represents the L+TlGdTe2^S peritectic equilibrium. The solidus projection is formed by a single surface (dashed isotherms), corresponding to the onset of the melting of the S-phase.
Isopleth sections of the Tl4PbTe3-Tl9GdTe6-Tl9BiTe6 system (Fig.4).
In order to construct a complete T-x-y diagram and refine the boundaries of primary crystallization areas of 5-phase and TlGdTe2, we constructed some isopleth sections. Fig.3 shows the vertical sections of TbGdTe6-[A], Tl9BiTe6-[B] and TUPbTe3-[C] of the TUPbTe3-Tl9GdTe6-Tl9BiTe6 system
(where A, B, and C represent the 1:1 alloys in the boundary systems as shown in Fig.3).
T!,GdTe,
mol % Tl,BiTe,
Fig.3. Li qui dus and solidus surfaces projections of the Tl4PbTe3-Tl9GdTe6-Tl9BiTe6 section of the Tl-Pb-Bi-Gd-Te system. The investigated sections are shown by dash-dot lines. A,B and C are equimolar compositions of the boundary systems. Primary crystallization phases: 1-S; 2-TlGdTe2
It is seen in Figs. 4a and 4c that only the S-phase crystallizes from the melt over the entire composition range on the TUPbTe3-[C] and Tl9BiTe6-[B] systems.
On the TbGdTe6-[A] section, the 5-phase crystallizes from the melt in the composition area <60 mol% TbGdTe6. At higher Tl9GdTe6 content, the TlGdTe2 first crystallizes. This is followed by the
monovariant peritectic reaction
Z+TlGdTe2~5. As a result, the TlGdTe2 disappears and the excess melt crystallizes to give the S-phase(Fig.4a).
The XRD powder patterns for selective alloys on polythermal sections reaffirmed continuous solid solutions with the TbTe3-structure.
1200 T,K
1100 850 800
®
L+TlGdTe, \ \ \ \ i L
i \ L+5+TlGdTe, J*-—^ L+Ô
i i i i
Tl9GdTe(J 80
60 40 mol % Tl9GdTe6
a)
20
[A]
b)
c)
Fig.4. Polythermal sections Tl9GdTe6-[A], Tl9BiTe6-[B] and Tl4PbTe3-[C] of the phase diagram of the TUPbTe3-TbGdTe6-Tl9BiTe6 section of the Tl-Pb-Bi-Gd-Te system. A, B, and C represent the 1 : 1 alloys in the boundary systems as shown in Fig.3.
T]„GdTe6
mol % Tl9BiTe6
Fig.5.Isothermal sections at 860 K in the Tl4PbTe3-Tl9GdTe6-Tl9BiTe6 section of the Tl-Pb-Bi-Gd-Te system.
Isothermal section at the 860 K of the It should be noted that in the TUPbTe3-
Tl4PbTe3-Tl9GdTe6-Tl9BiT6 section (Fig.5) TbGdTes-TbBiTes section the directions of
connodes in the L+S two-phase regions do not the connodes at 860 K are clearly indicated the coincide with the T-x planes of inner sections corresponding isothermal section of the phase and vary with temperature. The directions of diagram (Fig. 5).
4. CONCLUSION
T-x-y phase diagram of the Tl-Pb-Bi-Gd-Te system in the Tl4PbTe3-Tl9GdTe6-Tl9BiTe6 composition range is constructed. A continuous series of solid solutions is established in the studied section. Received
experimental data can be used for choosing the composition of solution-melt for the growth of the high-quality crystals of the 5- phase which is of interest as a thermoelectric material.
5. 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 Institute of Catalysis and Inorganic Chemistry of ANAS (Azerbaijan) and Donostia International Physics Center (Basque Country, Spain).
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ПОЛИТЕРМИЧЕСКИЙ РАЗРЕЗ TUPb Te3-TbGdTee-TbBiTee СИСТЕМЫ Tl-Pb-Bi-Gd-Te
1С.З. Имамалиева, 2Г.И. Алекберзаде, 2З.Э. Салимое, 3С.Б. Иззатли, 3Я.И. Джафаров, 1М.Б. Бабанлы
'Институт Катализа и Неорганической Химии Национальной АН Азербайджана AZ 1143, Баку, пр.Г.Джавида, 113; e-mail:samira9597a@gmail.com 2Азербайджанский Государственный Университет нефти и промышленности AZ-1010, Баку, пр.Азадлыг,16/21 3Бакинский Государственный Университет AZ 1148, Баку, ул.З.Халилова, 23
В работе приведены новые результаты по фазовым равновесиям в системе Tl-Pb-Bi-Gd-Te в концентрационной области Tl9GdTe6-Tl9BiTe6-Tl4PbTe3. На основании данных, полученных методами ДТА, РФА и измерений микротвердости, установлено, что система характеризуется образованием непрерывных рядов твердых растворов со структурой Tl5Te3.
Ключевые слова: теллуриды таллия-свинца, теллуриды таллия-гадолиния, теллуридыталлия-висмута, фазовые равновесия, твердые растворы, кристаллическая структура.
Tl-Pb-Bi-Gd-Te SiSTEMiNiN ThPbTes-TbGdTee-TbBiTee POLÏTERMÏKKdSÏYÏ
1S.Z. imamaliyeva, 2Q.i. dfakbsrzads, 2Z.E. Sallimov, 3S.B. izzatli, 3Y± Csfsrov, 1M.B. Babanli
'AMEA-nin akad. M.Nagiyev adina Kataliz va Qeyri-uzvi Kimya institutu AZ 1143, Baki, H.Cavidpr., 113; e-mail: samira9597a@gmail.com 2Azarbaycan Dôvlat Neft va Sanaye Universiteti AZ 1010, Baki, Azadliq pr., 16/21
3Baki Dôvlat Universiteti AZ 1148 Baki, Z.Xalilov kuç., 23
i§da Tl-Pb-Bi-Gd-Te sisteminin Tl9GdTe6-Tl9BiTe6-Tl4PbTe3 politermik kasiyi uzra faza tarazliqlarina aid yeni naticalar taqdim olunur. DTA, RFA va mikrobarkliyin ôlçulmasila muayyan edilib ki, sistemda ThTe3tipli tetraqonal quruluça malik fasilasiz bark mahlullar amala galir.
Açar sozlar: tallium-qurguçun telluridlari, tallium-qadolinium telluridlari,tallium-bismut telluridlari, faza tarazliqlari, bark mahlullar, kristal quruluç.
