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AZERBAIJAN CHEMICAL JOURNAL No 2 2020
ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
UDC 544.344.3:546.28924
PHASE EQUILIBRIA IN THE TlTe-Tl9ErTe6 SYSTEM
I.F.Mekhdiyeva
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
mehdiyeva.ilahe2@gmail.com
Received 01.07.2019 Accepted 02.10.2019
Phase equilibria in the TlTe-Tl9ErTe6 system were experimentally studied using differential thermal analysis, powder X-ray diffraction. The system was shown to be non-quasibinary due to the incongruent nature of the melting of both starting compounds, but stable under the solidus. Limited solid solutions (~5 mol%) based on Tl9ErTe6 are revealed in the system.
Keywords: thallium-erbium tellurides, phase equilibria, polythermal section.
doi.org/10.32737/0005-2531-2020-2-74-77 Introduction
Heavy ^-elements chalcogenides attract a lot of attention of scientists due to their functional properties, such as thermoelectric, photoelectric, optical, magnetic et al. properties [17]. Furthermore, despite the toxicity of thallium, complex thallium chalcogenides are closely monitored as topological insulators [6, 8, 9], Weyl semimetals [10], photo-, X-ray and gamma radiation detectors [11-13], as well as thermoelectric materials [4, 14, 15]. Doping by rare-earth elements can improve their properties and give them additional functionality, such as the magnetic properties [16, 17].
This work is a continuation of the studies on the phase equilibria in the ternary systems based on thallium-rare earth elements tellurides.
Earlier authors of [18-20] showed the formation of compounds of the Tl9LnTe6 type (Ln-Ce, Nd, Sm, Gd, Tm, Tb), and determined the nature and temperature of their melting as well as the crystal lattice parameters. According to [21], unlike the indicated lanthanides, ytterbium does not form a compound of the Tl9LnTe6 type. The authors of [16, 17] confirmed the data of [18-20] and investigated the crystal structures and thermoelectric and magnetic properties of some compounds of the Tl9LnTe6 type.
The results of the phase equilibria investigations in the Tl2Te-Tl5Te3-Tl9LnTe6 (Ln -Gd, Tb, Sm, Er, Tm) [22-26] systems showed
that the studied systems are characterized by the formation of a wide field of S-solid solutions with the Tl5Te3 structure, occupying of the significant part of the concentration triangle.
The aim of the present work is the investigation of the phase relations in the TlTe-Tl9ErTe6 system.
TlTe and Tl9ErTe6 compounds melt in-congruently at 573 [27] and 705 K [28] and have the tetragonal structure (Sp.Gr.I4/mcm) with the following parameters: a = 12.953, c = 6.173 A, z = 16 [29]; to a = 8.8501; c = 12.9524 A, z = 2 [28].
Experimental part
Starting compound TlTe was synthesized by melting of high purity elements in evacuated (~ 10-2 Pa) quartz ampoules at 750 K with following slow cooling. After fusion, given the incongruent nature of the melting of this compound, the intermediate ingot was annealed at 550 K for 500 h in order to achieve a homogeneous state.
Obtaining homogeneous Tl9ErTe6 requires the development of a special synthesis technique, which is described in detail in [28].
The purity of the synthesized compounds was checked by differential thermal analysis (DTA) and powder X-ray diffraction (XRD) techniques.
Alloys of the TlTe-Tl9ErTe6 system were prepared by melting the stoichiometric quantities of the TlTe and Tl9ErTe6 in evacuated silica
tubes at 1000 K in a tube furnace. After the synthesis, alloys were powdered in an agate mortar, pressed into pellets and annealed at 550 K within 500 h. In order to prevent a reaction between the ampoule walls and erbium, the silica tubes were coated with a carbon film via the decomposition of ethanol.
DTA and XRD analyses were employed to analyze the samples.
DTA was performed using a NETZSCH 404 F1 Pegasus differential scanning calorimeter. The crystal structure was analyzed by a powder X-ray diffraction technique at room temperature using a Bruker D8 diffractometer utilizing CuKa -radiation within 20 = 10 to 700.
Results and discussion
Based on DTA data the phase diagram of the TlTe-Tl9ErTe6 system was plotted (Table and Figure 1).
DTA results of the alloys of the TlTe-Tl9ErTe6 system
Composition, mol % Tl9ErTe6 Thermal effects, K
0(TlTe) 573, 635
10 570-615, 650
20 570-640
40 570-665
60 570-680, 1050
80 570-690, 1090
90 570-700, 1110
100 705, 1120
STITe 20
Fig. 1. Phase diagram of the TlTe-Tl9ErTe6 system.
TlTe-Tl9ErTe6 system is non-quasibinary since both compounds melt with decomposition by the peritectic reactions (Figure 1). As can be seen, the section is not quasibinary, what is associated with the incongruent nature of the melting of both initial compounds. Liquidus consists of two curves of the primary crystallization of the 5-phase (0-20 mol% Tl9ErTe6) and TlErTe2 (80-100 mol% Tl9ErTe6). Below the liquidus, in the composition interval of 25100 mol % Tl9ErTe6 and temperatures of 650705 K, a curve with a monovariant peritectic equilibrium L+TlErTe2o£ is observed (Figure 1, p1K curve). In the region of richer TlTe, this curve continues under the liquidus curve up to the composition of 3 mol% Tl9ErTe6. This indirectly indicates the transformation of the peritectic reaction L+TlErTe2o5 into the eutectic L^TlErTe2 + 5 (KU curve) at point K. Near TlTe in a narrow range of compositions (0-3 mol% Tl9ErTe6), the peritectic equilibrium L+5oTlTe (curve p2U) is take place. The horizontal (U) at 570 K corresponds to the invariant transition reaction L+TlErTe2oTlTe+5. As a result of this reaction, a two-phase region TlTe+5 is formed, which ensures the stability of this section below the solidus.
The results of X-ray phase analysis confirm the constructed phase diagram. The powder XRD results of annealed alloys along this section showed their two-phase state: TlTe +5 (solid solutions based on Tl9ErTe6). For example, Figure 2 shows a powder X-ray diffraction pattern of an alloy with a composition of 60 mol% Tl9ErTe6. As is seen, it consists of reflection lines of two initial compounds. Reflexes of other phases were not detected.
Conclusion
Phase relations in the TlTe-Tl9ErTe6 section of the Tl-Er-Te ternary system were experimentally studied based on differential thermal analysis and powder X-ray diffraction technique results. It was found that this section is generally non-quasibinary, but stably below solidus and forms limited (~5 mol%) solid solutions based on Tl9ErTe6.
7б
I.F.MEKHDIYEVA
Fig.2. Powder XRD pattern of the alloy with composition б0 mol% TlgErTe^
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) and partially supported by the Science Development Foundation under the President of the Republic of Azerbaijan, a grant EÍF/MQM/ Elm-Tehsil-1 -201б- 1(26)-71/01/4-M-33.
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TlTe-Tl9ErTe6 SISTEMINDO FAZA TARAZLIGI i.F.Mehdiyeva
TlTe-Tl9ErTe6 sistemi differensial-terniki va rentgenfaza analizi üsullan ils ôyranilmiç, onun faza diaqrami qurul -muçdur. Gôstarilmiçdir ki, sistem har iki ilkin birlaçmanin inkonqruent arimasi sababindan qeyri-kvazibinardir, lakin solidusdan açagida stabildir va Tl9ErTe6 asasinda 5 mol%-a qadar bark mahlul amala gatirir.
Açar sözlar: tallium-erbium telluridlari, faza tarazligi, politermik kasiklar.
ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ TlTe-Tl9ErTe6 И.Ф.Мехдиева
Фазовые равновесия в системе Т1Те-Т19ЕгТе6 изучены методами дифференциально-термического и рентгенофа-зового анализов и построена диаграмма состояния. Показано, что система неквазибинарна в силу инконгруэнт-ного характера плавления обоих исходных соединений, но стабильна ниже солидуса и характеризуется образованием ~5мол% твердых растворов на основе Т19ЕгТе6.
Ключевые слова: теллуриды таллия-эрбия, фазовые равновесия, политермическое сечение.
