Phase Equilibria in the Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ Section of the Tl-Bi-Tm-Te Quaternary System

Abstract

Phase relations in the Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ section of the Tl-Bi-Tm-Te quaternary system were studied by differential thermal analysis, powder X-ray diffraction technique and microhardness measurements applied to equilibria alloys. Some isopleth sections and isothermal section at 760 K, as well as projections of the liquidus and solidus surfaces, were constructed. The system is characterized by formation of continuous series of solid solutions at the solidus temperatures and below. Solid solutions are crystallized in the tetragonal Tl₅Te₃ structure type.

Keywords:
thallium-thulium tellurides; thallium-bismuth tellurides; phase relations; projections of the liquidus and solidus; solid solutions; crystal structure

1. Introduction

Due to their important properties, chalcogenides based materials find applications in a range of devices such as optoelectronic and memory devices, ion-selective sensors, modern day solar cells, and thermoelectric energy conversion^1,2. In recent years, a number of studies are devoted to the investigation of interactions of heavy metals chalcogenides with rare-earth elements^3-7.

Thallium subtelluride, Tl₅Te₃, because of features of crystal structure (Sp.gr.I4/mcm, a = 8.930; c = 12.598 Å)⁸ has a number of ternary derivatives such as of Tl₄A^IVTe₃ and Tl₉B^VTe₆ (A^IV-Sn, Pb; B^V-Sb, Bi)^9-11.

Pointed compounds show a good thermoelectric performance, whereas Tl₉BiTe₆ exhibits the highest ZT value^2,12. Furthermore, authors¹³ found the Dirac-like surface states in the [Tl₄]TlTe₃ (Tl₅Te₃) and its non-superconducting tin-doped derivative [Tl₄](Tl_1−xSn_x)Te₃.

Earlier we presented some new thallium lanthanide tellurides of Tl₉LnTe₆-type (Ln-Ce, Nd, Sm, Gd, Tm, Tb), which are also ternary substitution variant of Tl₅Te₃^14-16. As it was shown^16,17, ytterbium does not form the compound of pointed type. Later, the crystal structure, magnetic and thermoelectric properties for a number of Tl₉LnTe₆-type compounds were determined by authors^18-20.

Doping is an effective way for improve the thermoelectric properties, because incorporation of heavy atoms into crystal lattice may significantly reduce the lattice contribution to the total thermal conductivity, which leads to an increase of the thermoelectric performance²¹. At this aim, we have presented the results of phase equilibria investigations of a number of systems including Tl₅Te₃ compound or its structural analogues^22-24. We found that these systems are characterized by the formation of continuous series of solid solutions.

The present paper is aimed to investigate phase equilibria in the Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ section of the Tl-Bi-Tm-Te quaternary system.

Starting compounds Tl₅Te₃ and Tl₉BiTe₆ melt congruently at 723 K²⁵ and 830 K¹⁰ while Tl₉TmTe₆ melts with decomposition by the peritectic reaction at 745 K²⁶. The crystal lattice parameters of Tl₉TmTe₆ and Tl₉BiTe₆ are following: a=8.9095, c=12.7412 Å, z=2; a = 8.855, c = 13.048 Å, z=2^26,27.

2. Experimental

2.1. Materials and syntheses

The following reagents were used as starting components: thallium (granules, 99.999 %), bismuth (granules, 99.999 %), thulium (powder, 99.9%), and tellurium (broken ingots 99.999 %).

The reagents were weighed according to the compositions and put into silica tubes of about 20 cm in length. Then the ampoules were sealed under a vacuum of 10^-2 Pa. The samples, 1 gram each, were prepared by melting of the reagents in evacuated quartz ampoules in one zone electric furnace at the 30-50⁰ above the melting point of the compounds followed by cooling in the switched-off furnace.

In the case of Tl₉TmTe₆, the ampoule was graphitized using pyrolysis of acetone in order to prevent the reaction of thulium with quartz. Taking into account the results of the²⁶, the intermediate ingot of Tl₉TmTe₆ was powdered in an agate mortar, pressed into a pellet and annealed at 700 K within ~700 h.

The purity of the synthesized starting compounds was checked by the differential thermal analysis (DTA) and X-ray diffraction (XRD).

Only one thermal effect was observed for Tl₉BiTe₆ (830 K) and Tl₅Te₃ (723 K), and two peaks for Tl₉TmTe₆ which are relevant to the peritectic reaction at 745 K and its liquidus at 1123 K. These data agree with the literature data^10,25,26.

XRD confirmed that the synthesized Tl₅Te₃, Tl₉BiTe_6, and Tl₉TmTe₆ compounds were phase-pure. Their powder XRD patterns were indexed using Topas V3.0 software. Obtained unit cell parameters were practically equal to those given in^8,26,27 (Table 1).

Table 1
Dependence of the properties of the alloys annealed at the 700 K (800 h) on the composition for the Tl₅Te₃- Tl₉TmTe₆ and Tl₉BiTe₆-Tl₉TmTe₆ sections of the Tl-Bi-Tm-Te quaternary system

Solid phase compositions	Thermal effects, K	Microhardness, MPa	Parameters of tetragonal lattice, Å
			a	c
Tl5Te3	723	1130	8.9292(3)	12.5995(6)
Tl9.9Tm0.1Te6	725-728	-	-	-
Tl9.8Tm0.2Te6	726-730	1200	8.9259(4)	12.6266(11)
Tl9.6Tm0.4Te6	730-735	1240	8.9218(5)	12.6553(12)
Tl9.5Tm0.5Te6	732-737	-	-	-
Tl9.4Tm0.6Te6	734-739	1260	8.9177(6)	12.6839(11)
Tl9.2Tm0.8Te6	738-743; 1040	1240	8.9135(5)	12.7126(11)
Tl9.1Tm0.9Te6	741-744; 1095	-	-	-
Tl9TmTe6	745; 1123	1210	8.9095(4)	12.7412(8)
Tl9Bi0.1Tm0.9Te6	750-770; 1100	-	-	-
Tl9Bi0.2Tm0.8Te6	755-788; 1050	1290	8.8981(4)	12.8022(11)
Tl9Bi0.4Tm0.6Te6	770-800	1260	8.8872(5)	12.8641(12)
Tl9Bi0.5Tm0.5Te6	775-805	-	-	-
Tl9Bi0.6Tm0.4Te6	777-810	1200	8.8761(5)	12.9263(10)
Tl9Bi0.8Tm0.2Te6	800-820	1120	8.8650(6)	12.9873(11)
Tl9BiTe6	830	980	8.8545(4)	13.0476(7)

Previously synthesized binary and ternary compounds were used to synthesize the alloys of the Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ system. Taking into account the results of previous studies that an equilibrium state could not be obtained even after the long-time (1000 h.) annealing^22,26, after synthesis the samples containing >60% Tl₉TmTe₆ were powdered, mixed, pressed into pellets and annealed at 700 K during ~ 800 h in order to complete the homogenization.

2.2. Methods

All alloys were studied by using differential thermal analysis, X-ray diffraction method and microhardness measurements.

DTA was performed using a NETZSCH 404 F1 Pegasus differential scanning calorimeter within room temperature and ~1400 K at a heating rate of 10 K·min^-1 and accuracy about ±2 K. X-ray examination of powdered specimen was carried using a Bruker D8 diffractometer utilizing CuK_α radiation within 2θ = 10÷70°. The unit cell parameters of intermediate alloys were calculated by indexing of powder patterns using Topas V3.0 software. An accuracy of the crystal lattice parameters is shown in parentheses (Table 1).

Microhardness measurements were done with a microhardness tester PMT-3, the typical loading being 20 g and accuracy about 20 MPa.

3. Results and Discussion

The Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ section of the Tl-Bi-Tm-Te system was constructed based on obtained experimental results and literature data on boundary systems^10,26 (Figures 1-5).

Figure 1
Phase diagram (a), concentration relations of microhardness (b), and lattice parameters (c) for the 2Tl₅Te₃-Tl₉TmTe₆ section of the Tl-Bi-Tm-Te system.

Figure 2
Phase diagram (a), concentration relations of microhardness (b), and lattice parameters (c) for the Tl₉TmTe₆-Tl₉BiTe₆ section of the Tl-Bi-Tm-Te system.

Figure 3
XRD patterns for some alloys of the Tl₅Te₃-Tl₉TmTe₆ and Tl₉TmTe₆-Tl₉BiTe₆ systems.

Figure 4
Polythermal sections 2Tl₅Te₃-[A], Tl₉TmTe₆-[B] and Tl₉BiTe₆-[C] of the phase diagram of the Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ section of the Tl-Bi-Tm-Te system. A, B and C are equimolar compositions of the boundary systems as shown in Fig.5a.

Figure 5
The liquidus and solidus surfaces projections and isothermal section at 760 K in the Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ composition area of the Tl-Bi-Tm-Te quaternary system. Dash-dot lines show the investigated sections. A, B and C are equimolar compositions of the boundary systems.

Tl₉TmTe₆ melts with decomposition and loses the properties of the component of the system above the temperature of the peritectic reaction. Therefore, this compound inverted commas on the axes of the phase diagrams (Figures).

The results of DTA and microhardness measurements for alloys of boundary systems, as well as the parameters of the crystal lattices for some intermediate alloys, are given in the Table 1. Based on these data, T-x diagrams and the composition dependencies of corresponding properties are constructed.

2Tl₅Te₃-Tl₉TmTe₆ and Tl₉BiTe₆-Tl₉TmTe₆ systems. As can be seen (Figures 1, 2), these systems are characterized by the formation of continuous solid solutions (δ) with Tl₅Te₃-structure. However, they are non-quasi-binary sections of the Tl-Tm-Te ternary and Tl-Bi-Tm-Te quaternary systems due to peritectic melting of Tl₉TmTe₆ compound. This leads to crystallization infusible X phase in a wide composition range and formation L+Х two-phase and L+Х+δ three-phase areas. The L+Х+δ area is not fixed experimentally due to narrow temperature interval and shown by dotted line.

We have assumed that the X phase has a composition TlTmTe₂. This assumption is confirmed by the presence of the most intense reflection peaks of TlTmTe₂²⁸ on diffractograms of the as-cast alloys from region more than 63 mol% Tl₉TmTe₆.

Microhardness measurements (Figures 1b, 2b) are in good agreement with T-x phase diagram: curves have a flat maximum, which is typical for systems with continuous solid solutions.

Figure 3 presents the XRD patterns for some alloys of the Tl₅Te₃-Tl₉TmTe₆ and Tl₉TmTe₆-Tl₉BiTe₆ systems. As can be seen, powder diffraction patterns of starting compounds and intermediate alloys are single-phase and have the similar with Tl₅Te₃ diffraction pattern with slight reflections displacement from one composition to another. The lattice parameters of solid solutions obey the Vegard's law, i.e. depend linearly on composition (Table 1, Figures 1c, 2c).

Isopleth sections of the Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ system (Figure 4).

Figures 4a-c show the isopleth sections Tl₉BiTe₆-[A], Tl₉TmTe₆-[B] and Tl₅Te₃-[C] of the Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ system, where A, B and C are equimolar compositions of the boundary systems as shown in Figure 5a.

Over the entire compositions range of the Tl₉BiTe₆-[A] и Tl₅Te₃-[C] systems (Figures 4 a,c) only δ-phase crystallizes from the melt.

According to Figure 4a, along the Tl₉TmTe₆-[В] section in the composition area below 70 mol% Tl₉TmTe₆, the primary crystallization of the δ-phase occurs. In the Tl₉TmTe₆- rich interval the X-phase crystallizes first, followed by a monovariant peritectic process L+X↔δ.

The liquidus and solidus surfaces projections and isothermal section at 760 K in the Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ composition area of the Tl-Bi-Tm-Te quaternary system (Figure 5).

Liquidus of Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ section (Figure 5a) consists of two fields of the primary crystallization of Х-phase and δ- solid solutions. These fields are separated by curve corresponding to the monovariant peritectic equilibrium L+Х↔δ (ab curve). The solidus (dashed lines) consist of one surface of the completion of the crystallization of the δ-phase.

The isothermal section at 760 K is shown in Figure 5b. This section consists of five fields. In alloys containing < 65 mol% Tl₉TmTe₆ in the two-phase L+δ region the directions of the tie-lines are on the studied composition plane. Therefore, this part of the section can be considered stable. In the two-phase area L+Х and also in the part of L+δ area (Tl₉TmTe₆-rich composition) the directions of the tie-lines beyond the scope of this composition planes. Narrow three-phase L+Х+δ region that must be between the above-pointed two-phase regions is not fixed and shown in Figure 5b by dotted line.

From Figure 5a it can be shown that the isothermal section at 780 K is qualitatively similar to one at 760 K (Figure 5b). Isothermal sections at 740, 800 and 820 K only consist of three fields of L-, X- and δ- phases.

It is worth noting that, comparison of the isothermal section (Figure 5b) and isopleth sections (Figure 4) shows that the directions of the tie-lines in the L+δ two-phase region deviate from the T-x plane and constantly vary with temperature.

4. Conclusion

The phase diagram of the Tl-Bi-Tm-Te system in the Tl₅Te₃-Tl₉BiTe₆-Tl₉TmTe₆ composition area is constructed, including the T-x diagrams of boundary systems Tl₅Te₃-Tl₉TmTe₆ and Tl₉BiTe₆-Tl₉TmTe₆, some isopleth sections, isothermal section at 760 K as well as the liquidus and solidus surfaces projections. The studied 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 determining the temperature conditions for growing crystals of δ- phase with a given composition.

5. Acknowledgment

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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Publication Dates

• Publication in this collection
  08 June 2017
• Date of issue
  Jul-Aug 2017

History

• Received
  30 Nov 2016
• Reviewed
  14 Apr 2017
• Accepted
  21 May 2017