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UDC 541.123.6:546.289'24
PHASE RELATIONS IN Tl9GdTe6-T^SbTe6 AND Tl9TbTe6-T^SbTe6 SYSTEMS
S.Z. Imamaliyeva1, T.M. Hasanly2, V.A. Gasymov 1, D.M. Babanly 1, F.M. Sadygov 2
institute of Catalysis and Inorganic Chemistry, ANAS H.Javid ave., 113, Baku AZ1143, Azerbaijan Republic; e-mail: [email protected] 2Baku State University Z.Xalilov str., 23, Baku AZ 1148, Azerbaijan Republic : e-mail: info@,bsu.az
Phase equilibriums in the Tl9GdTeg-Tlç$bTe6 and Tl9TbTe6-TlçS>bTe6 systems have been examined by means of differential thermal analysis, X-ray diffraction and microhardness measurements over equilibrium alloys. Phase diagram and concentration dependence of the unit cell parameters and microhardness of both systems plotted. It found that the systems are non-quasi-binary due to incongruent melting of Tl9Gd (Tb) Te6 compositions but proved to be stable below the solidus. Systems are characterized by formation of continuous solid solutions with Tl5Te3 structure. Solid solutions obtained may be of interest as thermoelectric and magnetic materials.
Keywords: thallium-terbium telluride, thallium-gadolinium telluride, thallium-antimony telluride, phase equilibriums, solid solutions, crystal structure.
1. INTRODUCTION
A number of works are illustrative of the growing interest in new multinary chalcogenide materials. This is due to their specific functional properties, such as thermal, electrical and optical [1-3]. Furthermore, recent studies have shown that some of them exhibit topological insulator properties [4,5]. Doping by rare-earth elements may improve their properties to provide them with additional functionality [6-8].
Thallium subtelluride, Tl5Te3 is suitable "matrix" for production of novel complex materials. This composition is crystallized in tetragonal structure (Sp.gr. I¥/mcm) [9, 10] and has a number of ternary cation- [11-14] and ani on-substituted [15-18] ternary structural analogs. Cation-substituted compositions of Tl4AIVTe3 [A-IVSn, Pb] and Tl9BVTe6 [BV-Sb, Bi] types form an important class of thermoelectric materials with anomalous low thermal conductivity [19-21]. Particularly, Tl9BiTe6 shows high ZT value comparable to the state-of-the-art thermoelectric materials [21]. On the other hand, according to recent investigations, anion-substituted Tl5Se2I composition is a prospective material for efficient X-ray and y-ray detection [22].
A new substitution variant of Tl5Te3,
thallium lanthanide tellurides, Tl9LnTe6 (Ln-Ce, Nd, Gd, Gd, Tm, Tb) has been obtained first by authors [23-25] to ensure their melting property and crystal lattice parameters. Moreover, according to [25, 26], ytterbium does not form the composition Tl9YbTe6. Later, a number of tellurides, Tlio-xLnxTe6, were synthesized, structurally characterized and their thermoelectric properties identified by authors [27-29].
Earlier, with the purpose of obtaining a solid solution with Tl5Te3 structure the phase relations in the Tl9NdTe6-Tl9BiTe6, Tl9TbTe6-Tl9BTe6 and Tl9GdTe6-Tl9BTe6 systems had been studied in [30-32]. Authors showed the formation of continuous areas of solid solutions with Tl5Te3 structure.
The goal of the present work is to determine phase equilibria in the Tl9GdTe6-Tl9SbTe6 and Tl9TbTe6-Tl9SbTe6 systems and thus obtain phase relationships and provide more accurate experimental data for preparation of pure and high quality materials.
Tl9SbTe6 melts congruently at 798 K [11] and has a low symmetry crystal structure of Tl5Te3 (Sp.gr.I¥/m), a = 8.829 A and c = 13.001 A, Z = 2 [33].
Tl9GdTe6 and Tl9TbTe6 melt with decomposition by peritectic reactions at 800
and 780K with the following lattice parameters: a =8.870, c = 13.027 A, Z = 4 [31]
and a =8.871, c = 12.973 A, Z =4 [32].
2. EXPERIMENTAL
2.1. Materials and syntheses
Thallium (granules, 99.999 mass%), antimony (granules, 99.999 mass%), gadolinium (powder, 99.9%), terbium (powder, 99.9%) and tellurium (broken ingots 99.999 mass%) were used as starting materials. The elements were weighed to total about 20 g (Tl9SbTe6) and 10 g (Tl9GdTe6, Tl9TbTe6) as per the molar ratio of the corresponding ternary composition, and placed in silica tubes, 20 cm long, and then sealed under a vacuum of 10-3 Pa. The synthesis was carried out by heating in one zone an electric furnace at 850K (Tl9SbTe6) and 1200K (Tl9GdTe6, Tl9TbTe6), followed by cooling in the switched-off furnace. To prevent a reaction between rare-earth elements and tubes, the silica tubes were coated with a carbon film via the decomposition of ethanol.
In considering that the equilibrium state could not be obtained even after a long-time (1000 h.) annealing [30-32], intermediate ingots of Tl9GdTe6 and Tl9TbTe6 were powdered in agate mortar, pressed into pellets and annealed at 730K within ~700h.
The purity of the synthesized compositions was examined by the differential thermal analysis DTA) and X-ray diffraction analysis (XRD).
Just one endothermic effect was revealed for Tl9SbTe6 (790K), and two effects for Tl9GdTe6 (800 and 1190 K) and 9TbTe6 (780 and 1110 K) showed the completion of the synthesis.
Powder XRD pattern for the Tl9SbTe6, Tl9GdTe6 and Tl9TbTe6 were similar to that of Tl5Te3. The lattice parameters were refined using the Topas V3.0 software (Table 1). They are practically equal to those shown in [34] for Tl9SbTe6, and slightly differ from [28] for Tl9TbTe6.
The samples of the Tl9GdTe6-Tl9SbTe6 and Tl9TbTe6-Tl9SbTe6 systems were prepared by melting from pre-synthesized ternary compositions in evacuated silica ampoules. Total mass of the ingot was 1 g. The synthesis was carried out by heating an electric furnace on a zone. Initially ampoules were heated from room temperature to 1200 K at a rate of 5 K/min and complied with this temperature within 3 h, then slowly cooled to 73 0K and kept at this temperature within 200 h. DTA and XRD analyses showed that alloys containing >60mol% Tl9(Gd)TbTe6 proved to be non-homogeneous after the heating. Therefore, the samples were powdered and pressed into pellets and then reheated in fused silica tubes at 730 K for a 500h in order to complete the homogenization.
2.2. Methods
Differential thermal analysis (DTA), X-ray powder diffraction (XRD), and microhardness measurements were made to analyze the samples. DTA was performed using a NETZSCH 404 F1 Pegasus differential scanning calorimeter. Measurements were carried out at room temperature and ~1400 K. Temperatures of thermal effects were read mainly from the heating curves. But in some samples thermal effects were read from cooling curves in order to establish the onset of crystallization.
X-ray powder diffraction (XRD) data were collected at room temperature in reflection mode using a Bruker D8 ADVANCE powder diffractometer and CuKa radiation within 20 =10 to 70°.
Microhardness measurements were performed with a microhardness meter PMT-3 with typical loading reaching 20 g.
3. RESULTS AND DISCUSSION
The Tl9GdTe6-Tl9SbTe6 and Tl9TbTe6-Tl9SbTe6 systems (Table 1, Fig.1) are non-quasi-binary section of the Tl-Gd(Tb)-Sb-Te quaternary system due to peritectic melting of Tl9Gd(Tb)Te6 compositions. However, they are characterized by the formation of continuous solid solutions (S).
Note that the S-solid solutions are primarily crystallized in 0-63 mol% Tl9GdTe6 composition area. Primary crystallization of X-phase occurs in the range of >63 mol% Tl9GdTe6. The mono-variant peritectic L+X • S reaction takes place below 800K and leads to the formation of three-phase area L+X+S. This area is not experimentally fixed due to narrow temperature interval and shown by dotted line (Fig.1).
We have assumed that the X phase has a composition of TlGdTe2. This assumption is confirmed by the presence of the most intense reflection peaks of TlGdTe2 [34] on diffractograms of cast alloys from an area exceeding 63 mol% Tl9GdTe6.
Note that the nature of phase equilibriums in the Tl9TbTe6-Tl9SbTe6 system is qualitatively identical.
It should be noted that irrespective of the very close melting temperature of Tl9SbTe6 (790K) and peritectic decomposition of Tl9GdTe6 (800 K) and T№Te6 (780 K) compositions, the liquids and solidus curves of both studied systems have no extremum points.
Table 1. Properties of initial compositions and alloys of the Tl9SbTe6-Tl9GdTe6
and Tl9SbTe6-Tl9TbTe6 systems
Phase Temperature of melting, K Microhardn ess, MPa Parameters of tetragonal lattice, Â
a c
Tl9TbTe6 78o;1ioo 11oo 8.8713 12.9737
Tl9Sbo,iTbo,9Te6 781; 1o8o - - -
Tl9Sbo,2Tbo,8Te6 782;1o3o 116o 8.8626 12.9786
Tl9Sbo,4Tbo,6Te6 784 114o 8.8542 12.9842
Tl9Sbo,6Tbo,4Te6 786;1o3o 113o 8.8458 12.9898
Tl9Sbo,8Tbo,2Te6 788 1o8o 8.8374 12.9954
Tl9SbTe6 79o 1ooo 8.8312 13.o132
Tl9Sbo,8Gdo,2Te6 791 1o5o 8.8412 13.o152
Tl9Sbo,6Gdo,4Te6 793 112o 8.8482 13.o181
Tl9Sbo,4Gdo,6Te6 794 114o 8.8563 13.o211
Tl9Sbo,2Gdo,8Te6 796; 11oo 116o 8.8631 13.o242
Tl9Sbo,iGdo,9Te6 798;116o - - -
Tl9GdTe6 8oo; 119o 11oo 8.87o3 13.o276
8.88
S.82
H„, I2Ü0 MPa
1100
1000
T,K 1100
1000
810
800
790
780
o - C • - a © .
- ®
®
L+X \ L
L+Ö+X \ ; L+5 l \ ■ i *—~~ S
13.03
13.01
12.99-
12.97
TLTbTe,,
20
40
60
80
TLSbTe,
mol%Tl,SbTe,
Fig.1. Phase diagrams of the Tl9SbTe6-Tl9GdTe6 and Tl9SbTe6-Tl9TbTe6 systems.
Both examined systems go to show that the temperature interval of the crystallization of the 5-phase is less than 3 K. The fact makes it possible to characterize the 5-solid solutions as quasi-ideal solution.
Results of microhardness measurements are in line with the plotted phase diagrams (Figs.1b). Curves have a flat maximum which is typical for systems with continuous solid solutions.
Phase diagrams of the above-mentioned systems are confirmed by powder X-ray
9uu I e6 <
analysis (Fig.2). Powder diffraction patterns of starting compositions and intermediate alloys are qualitatively identical with slight displacement of reflections from one composition to another. For example, we provide the powder diffraction pattern of alloy with compositions 50mol%Tl9SbTe6+50 mol%Tl9Gd(Tb)Te6. Note that the lattice parameters of the solid solutions depend linearly on composition, i.e. subject to the Vegard's law.
i o
ThTbTeé
1 n»Sbo.s f bojTet J, . ..... ...
TbSbTei "i, H......... . . „ ■ 1
ft_ 1. . ..
1 TUGdTe* IK - ..,.., j j . . i. . .A , .
» 20
Fig.2. XRD patterns for different compositions in the Tl9GdTe6-Tl9SbTe6 and Tl9TbTe6-
Tl9SbTe6 systems
Plotted T-x diagrams afford ample monocrystals of 5-solid solution with given opportunity to select compositions for growing composition from the melt.
4. CONCLUSION
The phase diagrams of the Tl9GdTe6-Tl9SbTe6 and Tl9TbTe6-Tl9SbTe6 systems have been plotted using various experimental methods. A continuous series of the substitutional solid solutions which are crystallized in Tl5Te3 crystal type were found
ACKNOWLEDGEMENTS
in both systems. Proceeding from respective characteristics of the starting compositions one can assume that the Tl9Sb1-xGd(Tb)xTe6 (o<x<1) phases possibly have thermoelectric and magnetic properties.
The work is a result of international joint research laboratory activities with the participation of the Institute of Catalysis and Inorganic Chemistry of NASA (Azerbaijan) and Donostia International Physics Center (Basque Country, Spain).
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Tl9SbTe(rTl9GdTe6 УЭ Tl9SbTe(rTl9TbTe6 SiSTEMLdRINDd FAZA TARAZLIQLARI
S.Z. imamaliyeva1, T.M. Hasanli2, У.Э. Qasimov1, D.M. Babanli1, F.M. Sadiqov2
AMEA Kataliz vd Qeyri-üzvi Kimya Institutu, AZ1143, Baki, H.Cavidpr., 113; e-mail: [email protected]
Baki Dövldt Universiteti AZ 1148 Baki, Z.Xdlilov küg., 23; e-mail: [email protected]
DTA, RFA vd mikrobdrkliyin ölgülmdsi üsullari ild Tl9SbTe6-Tl9GdTe6 vd Tl9SbTe6-Tl9TbTe6 sistemldrindd faza tarazliqlari öyrdnilmionlarin faza diaqramlari, hdmginin kristal qdfds parametrldrinin vd mikrobdrkliyinin tdrkibddn asililiq qrafikldri qurulmu§dur. Müdyydn edilmi§dur ki, hdr iki sistem Tl9Gd(Tb)Te6 birld§mdldrinin inkongruent drimdsi sdbdbinddn qeyri-kvazibinardir, lakin solidusdan a§agida onlar stabildirldr vd Tl5Te3 tipli kristal qurulu§a malik arasikdsilmdz mdhlullar dmdld gdlmdsild xarakterizd olunurlar. Alinmi§ bdrk mdhlullar termoelektrik vd maqnit materiallari kimi maraq kdsb edirldr. Agar sözlar: tallium-qadolinium telluridldri, tallium-terbium telluridldri, tallium-stibium telluridldri, faza tarazliqlari, bdrk mdhlullar, kristal qurulu§.
ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМАХ Tl9SbTe6-Tl9GdTe6 И Tl9SbTe6-Tl9TbTe6
С.З. Имамалиева1, Т.М. Гасанлы2, В.А. Гасымов1, Д.М. Бабанлы1, Ф.М. Садыгов2
1 Институт Катализа и Неорганической Химии Национальной АН Азербайджана AZ1143 Баку, пр.Г.Джавида, 113; e-mail: 2Бакинский Государственный Университет AZ1148 Баку, ул. З.Халилова, 23; e-mail: [email protected]
Методами ДТА и РФА, а также измерением микротвердости изучены фазовые равновесия в системах Tl9SbTe6-Tl9GdTe6 и Tl$bTe6-Tl9TbTe6. Построены их фазовые диаграммы, а также концентрационные зависимости микротвердости и параметров кристалллической решетки. Показано, что обе системы неквазибинарны в силу инконгруэнтного плавления соединений Tl9Gd(Tb)Te6, однако ниже солидуса стабильны и характеризуются образованием непрерывных рядов твердых растворов со структурой Tl5Te3. Полученные твердые растворы представляют интерес как термоэлектрические и магнитные материалы.
Ключевые слова: теллуриды таллия-гадолиния, теллуриды таллия-тербия, теллуриды таллия-сурьмы, фазовые равновесия, твердые растворы, кристаллическая структура.
Received 12.06.2017.
