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AZ9RBAYCAN KIMYA JURNALI № 3 2015
93
UDC 541.123.6:546.289'24
PHASE DIAGRAM OF THE Tl2Te-Tl9TbTe6 SYSTEM
S.Z.ImamaUyeva\ T.M.Gasanly2, F.M.Sadygov2, M-B.Babanly1
M.Nagiyev institute of Catalysis and Inorganic Chemistry NAS of Azerbaijan
2Baku State University
Received 08.05.2015
The phase equilibria in the Tl2Te-Tl9TbTe6 system were investigated by using DTA, XRD, SEM-EDS technique and microhardness measurements. The phase diagram of the system as well as concentration dependence of the unit cell parameters and microhardness were constructed. It was established that the system is characterized by formation of the wide fields of solid solutions with Tl5Te3-type structure. However, the system is non-quasi-binary due to the peritectic melting of the Tl9TbTe6 compound.
Keywords: thallium-terbium tellurides, phase equilibria, solid solutions, crystal structure.
Introduction
Rare earth metal chalcogenides have attracted an increasing level of attention because of their interesting functional properties [1-4].
The tellurides, Tl9LnTe6 (Ln - Ce, Nd, Sm, Gd, Tm) were for the first time reported in [5-7]. These compounds are ternary derivatives of Tl5Te3 (Sp.gr. I4/mcm). Their calculated lattice parameters were within expected body centered tetragonal Bravais lattice.
Later, in [8-12] thallium lanthanide tellurides Tl10-xLnxTe6 (Ln=La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er) were synthesized, as well as structurally characterized and their thermoelectric and magnetic properties determined by Kleinke's group. Moreover, authors confirmed the results of [6, 7] that these compounds are substitution variants of Tl5Te3.
Investigation of phase equilibria in the Tl5Te3-Tl9NdTe6-Tl9BiTe6 (I), Tl5Te3-Tl4PbTe3-Tl9NdTe6 (II), Tl5Te3-Tl9NdTe6-Tl9SbTe6 (III) and Tl2Te-Tl9NdTe6-Tl9BiTe6 (IV) systems showed that these systems are characterized by formation of continuous (I-III) or wide (IV) areas of solid solutions with Tl5Te3 structure [13-15].
Here, we report the results of the complete investigation of the phase equilibria and some properties of solid solutions in the Tl2Te-Tl9TbTe6 quasi-binary system.
Although the existence of Tl2Te has been reported by many authors [16-18], the existence of this compound as an individual compound
outside the homogeneity region of the previously mentioned Tl5Te3 [19] for the first time has been reported by authors [20]. Tl2Te melts congruent-ly at 698 K. Despite the fact Tl2Te is confirmed in [21, 22] its existence has even been questioned [23] prior to the publication of [24], which found that it crystallizes in the monoclin-ic system and has its own structure type (space group C2/c; a = 15.662, b = 8.987, c = 31.196Â; p = 100.760, z = 44). Finally, Okamo-to confirmed the Tl2Te compound [25].
Tl9TbTe6 compound melts incongruently at 780 K. This compound is substitution variant of Tl5Te3 crystallizing in the space group I4/mcm [10]. Unit cell lattice parameters of Tl9TbTe6 were determined from a least-squares refinement: a = 8.871 Â and c = 12.973 Â; z = 2.
Experiments
Materials and synthesis. The elements of high purity were used as starting materials: Tl granules, 99.99%; Gd powder, - 40 mesh, 99.9%; Te broken ingots, 99.99%, all purchased from Alfa Aesar. Tl2Te was prepared from the stoichiometric amounts of the corresponding elements by one-step melting in vacuum-sealed silica ampoules (~10 Pa) at 750 K, followed by cooling in the switched-off furnace.
The Tl9LnTe6 compounds are formed by peritectic reactions and it is very difficult to achieve equilibrium solidification with the peritectic composition [13-15]. Therefore to accelerate the interaction on interfaces, the obtained intermediate ingot of Tl9TbTe6 was crushed to
fine powders using agate mortar and pestle, pressed into pellets and again annealed at 750 K. This procedure was repeated several times until the targeted compound is obtained without any side products. The total annealing time of tablet was ~500 h. To prevent a reaction between the quartz ampoules and the terbium, the syntheses were carried out in graphitized ampoules.
The samples of the Tl2Te-Tl9TbTe6 system were prepared by melting of pre-synthe-sized starting compounds in evacuated silica ampoules. The total mass of one ingot was 1 g. The synthesis was realized in a tube furnace. Ampoules were heated to maximal temperature 1000 K, kept at this temperature within 3 h and then were cooled slowly to 650 K, kept at this temperature during 1000 h and then slowly cooled in a switched-off furnace.
Analysis. Differential thermal analysis (DTA), X-ray powder diffraction (XRD), scanning electron microscope with energy dispersive analysis (SEM-EDS), as well as microhardness measurements were used 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. Temperatures of thermal effects were taken mainly from the heating curves, but in some cases the 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.
X-ray powder diffraction (XRD) data were collected at room temperature in the range of 29 from 10 to 700 in reflection mode using a Bruker D8 ADVANCE powder diffractometer equipped with a Cu-target tube and a diffracted beam graphite monochromator.
The microstructure and equilibrium compositions of the phases were determined by FEI Quanta™ 250 scanning electron microscope with Oxford Instruments energy dispersive X-ray spectrometer (SEM-EDS).
Microhardness measurements were carried out on microhardnesmeter PMT-3 with loading 20 grams.
Results and discussion Tl2Te-Tl9TbTe6 system (Fig.1, Table) -is a part of the Tl2Te-Tb2Te3 system.
Fig.1. Phase diagram of the Tl2Te-Tl9TbTe6 system.
Due to incongruent melting of Tl9TbTe6 compound (780 K) this system is non-quasi-binary and refers to the peritectic type. Above 780 K from melt primarily phase X (supposedly TlTbTe2 compound) crystallizes and on the diagram in the concentration area 70-100% Tl9TbTe6 the L+X area is formed.
Two horizontals at 703 and 780 K correspond to the peritectic reactions L+5o-a (a-and 5- are solid solutions based on Tl2Te and Tl5Te3) and L+X^5.
Maximum homogeneity areas of a- and 5-phases at the peritectic temperature are 15 and 67 mol%, respectively. These fields continuously narrow with decreasing of temperature and are 12 and 65 mol%, respectively, at room temperature.
The formation of the wide solid solution areas in investigated system is confirmed by XRD and SEM analysis. Some XRD patterns and SEM images of the annealed alloys are presented in Fig.2 and 3.
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Some properties of initial compounds and alloys of the Tl2Te-Tl9TbTe6 system
Phase Thermal effects, K Space group, lattice parameters, Â Microhardness, Hu, MPa
Tl2Te 698 monoclinic, C2/c; a = 15.662, b = 8.987, c = 31.196Â; p = 100.760, z = 44 1400
Tl9.95Tb0.05Te5.05 703 1420
Tl9.9Tb0.lTe5.! 703-715 1460
Tl9.85Tb0.15Te5.15 703-724 -
Tl9.8Tb0.2Te5.2 703-735 1240;1480
Tl9.7Tb0.3Te5.3 705-750 1240;1480
Tl9.6Tb0.4Te5.4 710-760 tetragonal, I4/mcm, a=8.903, c=12.695, z=2 1220
Tl9.5Tb0.5Te5.5 718-768 -
Tl9.4Tb0.6Te5.6 732-776 a=8.892, c=12.758, z=2 1200
Tl9.3Tb0.7Te5.7 739-780 -
Tl9.2Tb0.8Te5.8 750-780 a=8.890, c=12.832, z=2 1150
Tl9.1Tb0.9Te5.9 765-780; 1085 -
Tl9.05Tb0.95Te5.95 773-780;1097 -
Tl9TbTe6 780;1110 a=8.871, c=12.973, z=2 1000
o o
2-Theta - Scale
Fig. 2. XRD patterns for different compositions of the Tl2Te-Tl9TbTe6 system: 1 - Tl2Te; 2 - 30% Tl9TbTe6; 3 -40% Tl9TbTe6; 4 - Tl9TbTe6.
a b
Fig.3. SEM images of some alloys of the Tl2Te-Tl9TbTe6 system: a - 30% Tl9TbTe6, b -40% Tl9TbTe6.
XRD patterns of alloys containing >40 mol% Tl9TbTe6 show diffraction lines corresponding to Tl9TbTe6, whereas alloys from a+5 two-phase area consisted of a set of patterns of both phases. For example, XRD pattern of alloy from this area is presented (30 mol% Tl9TbTe6).
Within the limits of the a- and 5-phase areas the microhardness is continuous function of composition. It remains constant in the two-phase region a+5. This is related by the well-known iso-compositions of coexisting phases in such two-phase areas.
Conclusion
The phase diagram of the Tl2Te-Tl9TbTe6 system has been constructed using various experimental methods. A wide range of the substi-tutional solid solutions (5-phase) which crystallize in Tl5Te3 crystal type is obtained in the system. Pointed system is non-quasi-binary due to the peritectic melting of the Tl9TbTe6 compound. Below 780 K system is quasi-binary and characterized by peritectic equilibrium L+5o-a. At room temperature homogeneity areas of a-and 5-phases are 12 and 65 mol% respectively.
Acknowledgement
The work was supported by the Science Foundation of the State Oil Company of the Republic of Azerbaijan (Grant for the project "Preparation and investigation of new functional materials based on complex metal chal-cogenides for alternative energy sources and electronic engineering", 2014).
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Tl2Te-Tl9TbTe6 SISTEMININ FAZA DIAQRAMI
S.Z. imamaliyeva, T.M.Hasanli, F.M.Sadiqov, M.B.Babanli
DTA, RFA, SEM-EDS va mikrobarkliyin ôlçûlmasi üsullari ils Tl2Te-Tl9TbTe6 sisteminda faza tarazliqlan öyranil-miçdir. Sistemin faza diaqrami, hamçinin kristal qafas parametrlarinin va mikrobarkliyin tarkibdan asililiq qrafiklari qurulmuçdur. Müayyan edilmiçdur ki, sistemda Tl5Te3 tipli kristal quruluça malik olan geniç bark mahlul sahalari amala galir. Lakin sistem Tl9TbTe6 birlaçmasinin peritektik arimasi sababindan qeyri-kvazibinardir
Açar sözlar: tallium-terbium telluridlari, faza tarazliqlari, bark mahlullar, kristal qurulu§.
ФАЗОВАЯ ДИАГРАММА СИСТЕМЫ Tl2Te-Tl9TbTe6
С.З.Имамалиева, Т.М.Гасанлы, Ф.М.Садыгов, М.Б.Бабанлы
Методами ДТА, РФА, СЭМ-ЭДС, а также измерением микротвердости исследованы фазовые равновесия в системе Т12Те-Т19ТЬТе6. Построена фазовая диаграмма, а также концентрационные зависимости параметров кристаллической решетки и микротвердости. Установлено, что система характеризуется образованием широких областей твердых растворов со структурой Т15Те3. Однако система не квазибинарна в силу перитектического характера плавления соединения Т19ТЬТе6.
Ключевые слова: теллуриды таллия-тербия, фазовые равновесия, твердые растворы, кристаллическая ^руктура.
