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AZERBAIJAN CHEMICAL JOURNAL № 1 2021
ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
UDC 544.344.3:546.28924
PHASE DIAGRAM OF THE TlTe-Tl9TmTe6 SYSTEM
I.F.Mehdiyeva
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
Received 15.07.2020 Accepted 17.11.2020
Phase equilibria in the TlTe-Tl9TmTe6 system were experimentally studied by methods of differential thermal and powder X-ray diffraction analyses. The system was found to be non-quasibinary due to the incongruent nature of both initial components melting, but it is stable below solidus and is characterized by formation limited solid solutions (~2 mol%) based on Tl9TmTe6 are revealed in the system.
Keywords: thallium-thulium tellurides, phase relations, polythermal section, differential thermal analysis, X-ray diffraction analysis.
doi.org/10.32737/0005-2531-2021-1-18-22
Introduction
Chalcogenides of heavy ^-elements including thallium, due to their properties, such as thermoelectric, photoelectric, optical, magnetic et others properties are prospective functional materials [1-12]. Even though thallium toxicity, complex thallium chalcogenides are thorough studied as Weyl semimetals [6], photo-, X-ray and gamma radiation detectors [7-9], materials with thermoelectric [10-12] and topological insulator properties [1-5].
It is known that the incorporation of heavy atoms into a crystal lattice may significantly reduce the lattice contribution to the total thermal conductivity, which leads to an increase in the thermoelectric performance [13]. Moreover, doping by rare-earth elements can improve their properties and give them additional functionality, such as the magnetic properties [14, 15].
The formation of compounds with general formulae of Tl9LnTe6 and Tl4LnTe3 (Ln-lan-thanides) were found by authors of [16-20]. They showed and determined the nature and temperature of their melting. Moreover, the authors showed that these compounds are ternary structural analogs of the Tl5Te3 and calculated their crystal lattice parameters. According to [21], ytterbium does not form a compound of the Tl9LnTe6 type. The formation of Tl9LnTe6 compounds was confirmed in [12, 14]. The authors of [12, 14, 15] showed that these compounds possess thermoelectric and magnetic properties.
The results of the phase equilibria investigations of the Tl-Ln-Te ternary systems in the Tl2Te-Tl5Te3-Tl9LnTe6 compositions area [2226] showed that the above systems are characterized by the formation of wide S-solid solutions with the Tl5Te3 structure, which occupy 90% of the concentration triangle.
The aim of the present work is the investigation of the phase equilibria in the TlTe-Tl9TmTe6 system.
TlTe and Tl9TmTe6 compounds melt in-congruently at 573 [27] and 755 K [26] corre-spondently and have the tetragonal structure (Sp.Gr.I4/mcm) with the following parameters: a= 12.953, c=6.173, z=16 [28]; a= 8.910(5), c=12.741(10), z=2 [25].
Experimental part
Thallium (granules, 99.999 %), thulium (powder, 99.9%) and tellurium (broken ingots 99.999 %) were used as initial components.
Telluride TlTe was synthesized by melting of 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 to achieve a homogeneous state
Taking into account the incongruent character of melting of the Tl9TmTe6 [25] this compound was synthesized by the ceramic method according to a specially developed technique. We used stoichiometric amounts of
thallium telluride Tl2Te, thulium, and tellurium. This is due to thulium with thallium form ther-modynamically stable compounds that prevent the further synthesis of ternary compounds. After alloying at 1000 K, the non-homogenized samples were ground into a fine powder, mixed, pressed into a cylindrical tablet, and annealed at 700 K for 1000 h.
The purity of the synthesized compounds was checked by differential thermal analysis (DTA) and powder X-ray diffraction (XRD) techniques.
Alloys of the TlTe-Tl9TmTe6 system were prepared by melting the stoichiometric quantities of the TlTe and Tl9TmTe6 in evacuated quarts tubes at 1000 K in a tube furnace. After the synthesis, intermediate ingots were powdered, pressed into pellets, and annealed at 550 K within 500 h. In order to prevent a reaction between the ampoules and thulium, the synthesis of Tl9TmTe6 and alloys of the studied system were carried out in silica tubes coated with a carbon film via the decomposition of ethanol.
DTA and XRD analyses were used 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-Tl9TmTe6 system (Table, Figure 1).
was constructed
DTA results of the alloys of the
TlTe-Tl9TmTe6 system
Composition, mol % Thermal effects,
Tl9TmTe6 K
0(TlTe) 573, 635
10 572-615, 650
20 572-635
40 572-665
50 572-678
60 572-680, 1060
80 572-720, 1090
90 572-730, 1120
100 745, 1123
TlTe-Tl9TmTe6 system is non-quasibi-nary since both compounds melt with decomposition by the peritectic reactions (Figure 1).
T, K 1200 1100 1000 900 800
700
600 p2
L ..................
✓ / / / L+TlTmTe2 /
- i i i i i i Pi
i i
Al, , L+TlTmTe2+ô f . 572 . . . 8
L+TITe+ô TITe+S -1-1_1 1
123
745
8TITe 20
40 60 mol % TLTmTe.
80 Tl,TmTe„ Fig. 1. Phase diagram of the TlTe-Tl9TmTe6 system.
Liquidus consists of two curves of the primary crystallization of the 5-phase (0-25 mol% Tl9TmTe6) and TlTmTe2 (75-100 mol% Tl9TmTe6). Here 5 - is solid solutions based on Tl9TmTe6. Below the liquidus, in the composition interval of 25-100 mol% Tl9TmTe6 and temperatures of 645-745 K, a curve with a monovariant peritectic equilibrium L + TlTmTe2 o- 5 was observed (Figure 1, p1K curve). In the TlTe richer region, this curve continues under the liquidus curve up to the compositions of 2 mol% Tl9TmTe6. This indirectly indicates the transformation of the peritectic reaction L+ TlTmTe2o5 into the eutectic L^ TlTmTe2+5 (KU curve) at point K. Near TlTe in a narrow interval of compositions (0-2 mol% Tl9TmTe6), the peritectic equilibrium L+5o- TlTe (curve p2U) is take place. The horizontal (U) at 572 K corresponds to the invariant transition reaction L+TlTmTe2 o-TlTe+5. This lead to the formation of a two-phase region TlTe+5, which ensures the stability of this section below the solidus.
Fig. 2. Powder XRD pattern of the alloy with composition 60 mol% Tl9TmTe6.
The results of the powder X-ray diffraction analysis confirmed the plotted T-x diagram. The results obtained showed a two-phase state of intermediate alloys: TlTe + 5. For example, Figure 2 shows a powder X-ray diffraction pattern of an alloy with a composition of 60 mol% Tl9TmTe6. As is seen, it consists of reflection lines of two starting components. Reflexes of other phases were not found.
Conclusion
Phase equilibria in the TlTe-Tl9TmTe6 section of the Tl-Tm-Te ternary system were experimentally studied based on data of differential thermal analysis and powder X-ray diffraction method. It was established that the above section is generally non-quasibinary, but stable below solidus and is characterized formation limited (~2 mol%) solid solutions based on Tl9TmTe6.
Acknowledgment
The work has been carried out within the framework of the international joint research laboratory "Advanced Materials for Spintronics and Quantum Computing" (AMSQC) estab-
lished between the 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 EiF/MQM/Elm-Tehsil-1 -2016-1(26)-71/01/4-M-33.
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TlTe-Tl9TmTe6 SISTEMININ FAZA DIAQRAMI i.F.Mehdiyeva
Т1Те-Т19ТтТе6 differensial-terniki vэ rentgenfaza analizi usu11an ilэ бугэш1т1§, опип faza diaqraml
quru1mu§dur. Gбstэri1mi§dir ki, sistem hэr iki bir1э§mэnin inkonqruent эrimэsi sэbэbindэn qeyri-kvazibinardlr, 1akin so1idusdan a§aglda stabi1dir vэ Т19ТтТе6 эsaslnda 2 mo1%-э qэdэr bэrk mэЫu1 эmэ1э gэtirir.
Адаг sбzlэr: tallium-tulium tellurid, faza tarazhgl, diferensial termiki analiz, rentgen faza analizi, poiltermik kэsiklэr.
ФАЗОВАЯ ДИАГРАММА СИСТЕМЫ TlTe-Tl9TmTe6 И.Ф.Мехдиева
Методами дифференцияально-термического и порошкового рентгенфазового анализов изучены фазовые равновесия в системе Т1Те-Т19ТшТе6 и построена ее фазовая диаграмма. Показано, что система неквазибинарна в силу инконгруэнтного характера плавления обоих исходных соединений, но стабильна ниже солидуса и характеризуется образованием ~2 мол% твердых растворов на основе Т19ТшТе6.
Ключевые слова: теллуриды таллия-тулия, фазовые равновесия, дифференциальный термический анализ, рентгенфазовый анализ, политермический разрез.
