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AZ9RBAYCAN KÍMYA JURNALI № 2 2018 ISSN 2522-1841 (Online)
ISSN 0005-2531 (Print)
УДК 544.344.3:546.28924 THE Tl9ErTe6-Tl9BiTe6 SYSTEM AND SOME PROPERTIES OF SOLID SOLUTIONS
I.F.Mekhdiyeva, K.N.Babanly, M.A.Mahmudova*, S.Z.Imamaliyeva
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan * Institute of Physics, NAS of Azerbaijan
Received 05.02.2018
The phase relations in the Tl-Bi-Er-Te quaternary system on the Tl9ErTe6-Tl9BiTe6 composition area were investigated by using differential thermal analysis, X-ray diffraction, and microhardness measurements. Phase diagram, as well as concentration dependencies of the lattice parameters and microhard-ness, were constructed. It is established that the system is characterized by the formation of unlimited series of solid solutions with tetragonal Tl5Te3 structure type (5). Obtained experimental data can be used for choosing the composition the growth of high-quality crystals of 5-phase which are of interest like thermoelectric and magnetic materials.
Keywords: thallium-erbium tellurides, thallium-bismuth tellurides, phase equilibria, solid solutions, crystal structure.
Introduction
Complex thallium chalcogenides attracted increased interest as topological insulators [1, 2], Weyl semimetals [3] and thermoelectric materials with anomalously low thermal conductivity [4, 5]. Some of these compounds have photoconductivity and are promising for use as detectors of y- and X-ray radiation [6].
Subtelluride Tl5Te3 one of the most suitable matrix compounds for the synthesis of new ternary compounds - structural analogs and mul-ticomponent phases. According to the phase diagram of the Tl-Te system [7, 8], this compound melts congruently at 725 K and has a wide (34.5-38 at.% Te) homogeneity area. Due to the peculiarities of the crystal structure [9], Tl5Te3 has a number of ternary structural analogs [1013] which form an important class of thermoelectric materials with anomaly low thermal conductivity [14-16]. Particularly, Tl9BiTe6 shows high ZT value comparable to the state-of-the-art thermoelectric materials [14].
A new thallium lanthanide tellurides with composition Tl9LnTe6 (Ln - Ce, Nd, Sm, Gd, Tb, Tm) founded by authors of [17-19] are also ternary analogs of Tl5Te3. Authors of [1921] determined the thermoelectric properties for a number Tl9LnTe6-type compounds and showed that Tl9LaTe6 possess the highest ZT value (~0.21) at 581 K [20]. The Tl9CeTe6, Tl9PrTe6 and Tl9TbTe6 compounds also were found to be paramagnetic due to the presence of unpaired
electrons [22].
It is well known that the development of physicochemical bases for directed synthesis of new multicomponent inorganic phases is associated with fundamental research in the phase equilibria area and thermodynamic properties of the corresponding systems [23]. In this case, systems in which it is possible to expect the formation of structural analogs of known binary and ternary compounds or solid solutions based on them are the most interest.
The results of phase equilibria investigations for a number of systems including the Tl5Te3 compound or its structural analogs showed the formation of continuous series of solid solutions [24-28].
In this paper, we present the experimental results on phase equilibria in the Tl-Bi-Er-Te quaternary system on the Tl9ErTe6-Tl9BiTe6 composition area.
Experimental part
Materials and syntheses
High purity elements (Tl granules, 99.99%; Er powder, 99.9%; Bi foil, 99.999%, Te broken ingots, 99.99%), all purchased from Alfa Aesar were used for the synthesis of the Tl9ErTe6 and Tl9BiTe6 compounds. The elements were weighed to be about 10 g in total according to the molar ratio of the corresponding ternary compound, placed in silica tubes of about 20 cm in length and then sealed under a
vacuum of 10" Pa. The surface of thallium was coated by a thin oxide film, which was removed before use. To prevent a reaction between the quartz and the erbium, the syntheses of the Tl9ErTe6 was carried out in the graphitized ampoule.
The synthesis of the Tl9BiTe6 was carried out by heating of elements in one zone electric furnace at the 850 K, followed by cooling in the switched-off furnace.
The Tl9LnTe6 compounds form by peri-tectic reactions and it is difficult to achieve equilibrium [17-19]. Therefore, the obtained intermediate ingot of Tl9ErTe6 was carefully ground in an agate mortar, pressed into the circular pellet of about 8 mm in diameter and annealed at 680 K within -1000 h. The weight losses during the pellet preparation were less than 0.5 wt %.
Differential thermal analysis (DTA) and X-ray diffraction (XRD) indicated the single-phase synthesis of both compounds. DTA showed that Tl9BiTe6 melts congruently at 830 K. There are two peaks in the heating curve of Tl9ErTe6, which are relevant to the peritectic reaction at 705 K and its liquidus at 1120 K (Figure 1).
o
c,A 13.05 13.00 12.95
[I I 100
M Pa
1 one
900
T. K 1100'
1000
900
800
700
0 - C • - a
©
L+TlErTe3\ \ L ©
\ L+5—Tl ErTe, \ S
*---
TLErTe«
20
40 60
mol%TL,BiTe,l
80
TLBiTe,
Fig.1. Phase diagram of the Tl9ErTe6-Tl9BiTe6 system.
Powder XRD pattern of the Tl9BiTe6 and Tl9TbTe6 compounds were identical to that of Tl5Te3. Unit cell lattice parameters were deter-
mined from a least-squares refinement (Table).
Composition dependence of the properties for the alloys of the Tl9ErTe6-Tl9BiTe6 section of the Tl-Bi-Er-Te system
Phase Temperature of melting, K Microhardness, MPa Parameters of the tetragonal lattice, Â
a c
Tl9ErTe6 705;1120 1070 8.850 12.952
Tl9Bi0.2Er0.8Te6 720-760; 1030 1110 8.851 12.972
Tl9Bi0.4Er0.6Te6 730-790 1120 8.852 12.991
Tl9Bi0.6Er0.4Te6 750-810 1080 8.853 13.011
Tl9Bi0.8Er0.2Te6 780-820 1040 8.854 13.030
Tl9BiTe6 830 980 8.855 13.048
These are practically equal to the literature data [29] for Tl9BiTe6 and slightly differ from literature data [21] for Tl9ErTe6.
The alloys of the Tl9ErTe6-Tl9BiTe6 system were prepared by melting from pre-syn-thesized starting compounds in evacuated silica ampoules. The total mass of the ingot was 1 g. Ampoules were heated to maximal temperature 1200 K, kept at this temperature during 4 h and then cooled slowly to 680 K and kept at this temperature during 300 h. DTA and XRD analyses showed that mixtures containing >70 mol% Tl9ErTe6 were non-homogeneous after the first heating. Therefore, the samples were ground and pressed into pellets under argon and reheated in fused silica tubes at 680 K for a 500 h for the homogenization.
Methods
DTA and XRD analysis, as well as mic-rohardness measurements, were employed to analyze the samples. DTA was performed using a NETZSCH 404 F1 Pegasus system with two chromel-alumel thermocouples. The measurement was performed between room temperature and -1400 K with a heating and cooling rate of 10 K min-1. Temperatures of thermal effects were taken mainly from the heating curves. But, in some samples thermal effects were taken from cooling curves in order to determine the onset of crystallization. The overall uncertainty of the determined phase transformation temperatures was estimated to be ±1 K.
The crystal structure was studied by XRD
at room temperature on a Bruker D8 powder dif-fractometer (CuKoi-radiation) in the range 29=(10-70)°. Microhardness measurements were performed on a PMT-3 microhardness tester.
Results and discussion
The results of DTA and microhardness measurements, as well as the parameters of the crystal lattices for some intermediate alloys of the Tl9ErTe6-Tl9BiTe6 section, are given in the Table. Based on these data, phase diagram and the composition dependencies of properties are plotted (Figure 1). This section is characterized by the formation of continuous solid solutions (5-phase) with Tl5Te3-structure. However, this system is a non-quasi-binary section of the Tl-Bi-Er-Te quaternary system due to the peritectic melting of Tl9ErTe6. This leads to crystallization TlErTe2 compound in a wide
composition interval and formation L+TlErTe2 two-phase and L+TlErTe2+5 three-phase areas. The L+TlErTe2+5 area is shown by a dotted line because not fixed experimentally due to narrow temperature interval.
Microhardness measurements confirmed the phase diagram (Figure1 b). The curve of mi-crohardness has a flat maximum, which is typical for substitutional solid solutions.
The XRD powder patterns for some alloys of the Tl9ErTe6-Tl9BiTe6 section are presented in Figure 2. All intermediate samples have the same diffraction pattern typical to Tl5Te3 and differ from each other by some displacement of the diffraction patterns. The dependence of the lattice parameters of the composition obeys the Vegard's rule within the error limits.
2SDDD 21QQD 23DDD 22DDÛ 21DDD 2DDDQ 19ППП -1SDDÛ -T7DDD -16DDQ -
15DDD -
CO. ■ • HDDÛ -
О 1ЭППП -
О
^12ППЦ
С
11DDD — 1DDDÛ -ЭППП — F.aaa -TDDD -6DDD ~ SQDD -lODD — 3DDD -2П00 -1GDD —
......... hm,A*J
I
LL
TbErTw
20 mol %T№iTee
40 mol %TbBiTe«
60 mol %Т№Тк
80 mol %TkBiTe6
TbBiTM
16 20
2-Theta - Scale
Fig.2. XRD patterns for different compositions in the Tl9ErTeg-Tl9BiTe6 system.
70
Conclusion
The phase diagram of the Tl9ErTe6-Tl9BiTe6 section of the Tl-Bi-Er-Te quaternary system is constructed. This section is characterized by an unlimited solubility of components in the solid state. Obtained experimental data can be used for choosing the composition of solution-melt and to evaluate technologically parameters necessary for the growth of crystals of 5-phase of high quality which are of interest like thermoelectric materials.
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 Institute of Catalysis and Inorganic Chemistry of ANAS (Azerbaijan) and Donostia International Physics Center (Basque Country, Spain).
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Tl9ErTe6-Tl9BiTe6 SiSTEMi УЭ BORK MOHLULLARIN BOZi XASSOLORi
i.F.Mehdiyeva, K.N.Babanli, M.A.Mahmudova, S.Z. imamaliyeva
DTA, RFA va mikrobarkliyin ôlçûlmasi üsullari ils Tl-Bi-Er-Te dördlü sisteminda Tl9ErTe6-Tl9BiTe6 qatiliq sahasinda faza tarazliqlari ôyranilmiçdir. Sistemin faza diaqrami, hamçinin kristal qafas parametrlarinin va mikrobarkliyin tarkibdan asililiq qrafiklari qurulmuçdur. Müayyan edilmiçdur ki, sistemda Tl5Te3 tipli kristal quruluça malik olan qeyri-mahdud bark mahlul sirasi (5) amala galir. Alinmiç tacrübi naticalar termoelektrik va maqnit xassali materiallar kimi böyük maraq kasb edan 5-fazanin monokristallannin alinmasi ûçûn istifada oluna bilar.
Açar sözlar: tallium-erbium telluridlari, tallium-bismut telluridlari faza tarazliqlari, bark mahlullar, kristal qurulu§.
СИСТЕМА Tl9ErTe6-Tl9BiTe6 И НЕКОТОРЫЕ СВОЙСТВА ТВЕРДЫХ РАСТВОРОВ
И.Ф.Мехдиева, К.Н.Бабанлы, М.А.Махмудова, С.З.Имамалиева
Методами ДТА и РФА, а также измерением микротвердости изучены фазовые равновесия в четверной системе Tl-Bi-Er-Te в концентрационной области Tl9ErTe6-Tl9BiTe6. Построены фазовая диаграмма, а также графики концентрационных зависимостей параметров кристаллических решеток и микротвердостей. Установлено, что система характеризуется образованием непрерывного ряда твердых растворов замещения с тетрагональной структурой типа Tl5Te3 (5). Полученные экспериментальные данные могут быть использованы для выбора составов расплавов для выращивания монокристаллов 5-фазы, представляющих интерес как термоэлектрические и магнитные материалы.
Ключевые слова: теллуриды таллий-эрбий, таллий-висмут теллуриды, фазовые равновесия, твердые растворы, кристаллическая структура.
