Experimental Study of the Tl₄PbTe₃-Tl₉TbTe₆-Tl₉BiTe₆ Section of the Tl-Pb-Bi-Tb-Te System

Abstract

The aim of the present study was to determine the phase relations in the Tl₄PbTe₃-Tl₉TbTe₆-Tl₉BiTe₆ section of the Tl-Pb-Bi-Tb-Te system. Based on a set of the methods of the physicochemical analysis (differential thermal analysis, powder X-ray diffraction method as well as microhardness measurements), the phase diagram of the Tl₄PbTe₃-Tl₉TbTe₆ boundary system, some isopleth sections, liquidus and solidus surfaces projections, as well as isothermal sections at 840 and 860 K, were plotted. Unlimited solid solutions with the Tl₅Te₃ structure (δ-phase) were found in the system, which are of interest as a thermoelectric materials.

Keywords:
thallium-lead telluride; thallium-terbium tellurides; thallium-bismuth tellurides; phase equilibria; liquidus and solidus surfaces; solid solutions

1. Introduction

Presently, great interest has devoted to the chalcogenides of heavy metals as prospective functional materials which found applications in the wide range of devices such as computer memories, chemical sensors, photo-detectors, solar cells, thermoelectric and optical devices, and ionic sensors^1-4. Some of them expected as a good candidates for use in the spintronic devices^5,6. The rare-earth materials, including chalcogenides, have been intensively investigated owing to their promising functional properties^7-11.

Tl₅Te₃ compound crystallizes in tetragonal structure (Sp.gr.I4/mcm, a = 8.930; c = 12.598 Å, z=4)^12,13. The formula Tl₅Te₃ can thus be rewritten as Tl₁₆[TlTe₃]₄. The thallium atoms on the 4c site can be partially or fully replaced by other elements, resulting in a group of ternary compounds: Tl₄A^IVTe₃ and Tl₉B^VTe₆ -type (A^IV-Sn, Pb; B^V-Sb, Bi)^14-17. Above-stated compounds possess a good thermoelectric performance^18-21 whereas Tl₉BiTe₆ found to have excellent thermoelectric properties with extremely low thermal conductivity at room temperature¹⁸. As it was shown by authors of the Ref.²¹, the bulk superconductor Tl₅Te₃ and its tin-doped derivative [Tl₄](Tl_1−xSn_x)Te₃ have Dirac-like surface states. Moreover, Tl₄SnS₃, Tl₄SnSe₃, Tl₄SnTe₃ compounds may be used for fabrication of IR induced electrooptically operated gratings²².

New structural analogs of Tl₅Te₃ with common formula Tl₉LnTe₆ (Ln-Ce, Nd, Sm, Gd, Tb, Tm) were found in the^23,24. Later, the crystal structure, thermoelectric and magnetic properties of a number Tl₉LnTe₆-type compounds were determined by H.Kleinke and co-workers^25-27.

The design and development of novel methods for controlled synthesis and growth of large single crystals require detailed studies of respective phase diagrams^28-30. On the other hand, the improvement of thermoelectric performance can be achieved by introduction of the heavy metals into the crystal lattice³¹. With this aim, we presented the results of the study of phase relations for a number of systems including the Tl₅Te₃ compound and its structural analogs^32-35. The formation of unlimited solid solutions was found for all these systems.

In this paper, we continued to study similar systems and presented the results of the study of the phase relations in the Tl₄PbTe₃-Tl₉TbTe₆-Tl₉BiTe₆ section of the Tl-Pb-Bi-Tb-Te system.

The initial compounds of above-mentioned system were studied in a number of papers. Tl₄PbTe₃ and Tl₉BiTe₆ melt congruently at 893 K¹⁵ and 830 K¹⁷ respectively, while Tl₉TbTe₆ is formed incongruently at 780 K³². The tetragonal lattice constants are following: a=8.841, c=13.056Å, z=4 (Tl₄PbTe₃)³⁶; a=8.871; c=12.973 Å, z=2 Tl₉TbTe₆³³; a = 8.855, c = 13.048 Å, z=2 (Tl₉BiTe₆)³⁷.

According to Ref.³⁸, the boundary system Tl₄PbTe₃-Tl₉BiTe₆ is quasi-binary and characterized by the formation of unlimited solid solutions (δ-phase) with Tl₅Te₃-structure.

Other boundary system Tl₉TbTe₆-Tl₉BiTe₆ was shown to contain a continuous series of solid solutions with a Tl₅Te₃ tetragonal structure, but not quasi-binary due to the incongruent melting of Tl₉TbTe₆ compound³².

2. Experimental

2.1. Materials and syntheses

The ternaries synthesized from the high purity elements (Tl-99.999%, Pb-99.99%, Tb-99.9%, Bi-99.999%, Te-99.999%).

All synthesis were carried out in previously cleaned and dried quartz ampoules. Taking into account the high toxicity of thallium and its compounds, we used protective gloves at all times when working.

Stoichiometric amounts of the starting components were put into silica tubes of about 20 cm in length and diameter of about 1.5 cm and sealed under a pressure of 10^-2 Pa. Tl₄PbTe₃ and Tl₉BiTe₆ were synthesized by direct synthesis of elemental components in a resistance furnace at 920 K followed by cooling in the switched-off furnace.

The synthesis of Tl₉TbTe₆ was carried out at 1000 K in the graphitized ampoule in order to avoid the interaction between the terbium and quartz. Then the intermediate ingot of Tl₉TbTe₆ was powdered in an agate mortar, thoroughly mixed, pressed into a pellet and annealed at 750 K during ~700 h.

The ampoules were shaken during all the heating process in order to help the complete mixing of all the elements.

We used the differential thermal analysis (DTA) and X-ray diffraction (XRD) in order to control the purity of synthesized compounds. Only one thermal effect was observed for Tl₉BiTe₆ (830 K) and Tl₄PbTe₃ (893 K), while two peaks for Tl₉TbTe₆ compound which are relevant to the peritectic reaction at 780 K and its liquidus at 1110 K. These data agree with the literature data^13,17,33. The X-ray patterns showed that the desired compounds Tl₄PbTe₃, Tl₉TbTe₆ and Tl₉BiTe₆ formed as pure phases.

The multicomponent alloys of the Tl₄PbTe₃-Tl₉TbTe₆-Tl₉BiTe₆ section were prepared by melting of previously synthesized ternary compounds. After thermal treatment at 1000 K for 24-36 h, the samples were slowly cooled (20-30 K per hour) down to 750 K and annealed within 1000 h in order to complete the homogenization.

2.2. Methods

We used the DTA and XRD methods, as well as microhardness measurements to analyze the samples of the Tl₄PbTe₃-Tl₉TbTe₆-Tl₉BiTe₆ section.

The temperatures of the thermal effects were determined 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 of ±2 K. The phase composition of the powdered samples was identified by powder X-ray diffraction Bruker D8 diffractometer with CuK_α radiation within 10°≤ 2θ ≤ 70° at room temperature. 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.

Table 1
Dependence of the properties of the alloys annealed at the 750 K (1000 h) on the composition for the Tl₄PbTe₃-Tl₉TbTe₆ section of the Tl-Pb-Bi-Tb-Te system

Solid phase compositions	Thermal effects, K	Microhardness, MPa	Tetragonal lattice parameters, Å
			a	c
Tl4PbTe3	893	1120	8.8409(9)	13.0556(12)
Tl8.2Pb1.6Tb0.2Te6	860-885	1145	8.8470(8)	13.0390(11)
Tl8.4Pb1.2Tb0.4Te6	835-865	1155	8.8530(7)	13.0226(10)
Tl8.5Pb1.0Tb0.5Te6	825-853	-	-	-
Tl8.6Pb0.8Tb0.6Te6	815-845	1145	8.8590(8)	13.0060(10)
Tl8.8Pb0.4Tb0.8Te6	800-815; 1030	1125	8.8650(9)	12.9895(12)
Tl8.9Pb0.2Tb0.9Te6	790-800; 1080	-	-	-
Tl9TbTe6	780; 1110	1100	8.871(10)	12.9730(14)

3. Results and Discussion

The character of the phase relations along the Tl₄PbTe₃-Tl₉TbTe₆-Tl₉BiTe₆ section is established based on combined analysis of experimental results and literature data on boundary systems Tl₄PbTe₃-Tl₉BiTe₆³⁸ and Tl₉TbTe₆-Tl₉BiTe₆³³ (Fig.1-6).

Figure 1
Phase diagram (a), concentration dependencies of microhardness (b), and lattice parameters (c) for the alloys of the Tl₉TbTe₆-2Tl₄PbTe₃ section.

Figure 2
XRD powder patterns for Tl₅Te₃ (a), as well as Tl₄PbTe₃, Tl₉TbTe₆ and some alloys of the Tl₄PbTe₃-Tl₉TbTe₆ section (b). 1- Tl₄PbTe₃; 2-20 mol% Tl₉TbTe₆;3-50 mol% Tl₉TbTe₆; 4-80 mol% Tl₉TbTe₆; 5-Tl₉TbTe₆.

Figure 3
Polythermal sections Tl₉TbTe₆-[A], Tl₉BiTe₆-[B] and Tl₄PbTe₃-[C] of the Tl₄PbTe₃-Tl₉TbTe₆-Tl₉BiTe₆ concentration area of the phase diagram of the Tl-Pb-Bi-Tb-Te system. A, B, and C are equimolar alloys from the respective boundary system as shown in Fig.4.

Figure 4
The liquidus and solidus surfaces projections Tl₄PbTe₃-Tl₉TbTe₆-Tl₉BiTe₆ section of the Tl-Pb-Bi-Tb-Te system. The investigated polythermal sections are shown by dash-dot lines. A, B and C are equimolar compositions of the boundary systems. Primary crystallization phases: 1-δ; 2-TlTbTe₂.

Figure 5
Isothermal sections at 860 and 840 K in the Tl₄PbTe₃-Tl₉Tb₆-Tl₉BiTe₆ section of the Tl-Pb-Bi-Tb-Te system.

3.1. Tl₄PbTe₃-Tl₉TbTe₆ boundary section

The results of DTA, XRD and microhardness measurements for starting compounds and some intermediate alloys of the Tl₄PbTe₃-Tl₉TbTe₆ section are presented in Table 1. This section (Fig.1) is characterized by the formation of unlimited solid solutions (δ-phase) with Tl₅Te₃-structure. However, it is a non-quasi-binary section of the Tl-Pb-Tb-Te quaternary system due to the peritectic character of melting of the Tl₉TbTe₆ compound. This leads to the crystallization of TlTbTe₂ compound in a wide composition interval and to the formation of L+TlTbTe₂ two-phase and L+TlTbTe₂+δ three-phase areas. Due to a narrow interval of temperatures, the area L+TlTbTe₂+δ is not fixed experimentally and shown by a dashed line.

The dependences of microhardness on composition have a flat maximum which is typical for systems with unlimited solid solutions (Fig.1b)³⁹.

The XRD powder patterns for some alloys of the Tl₄PbTe₃-Tl₉TbTe₆ section as well as Tl₅Te₃ are presented in Fig.2. Powder diffraction patterns of Tl₄PbTe₃, Tl₉TbTe₆ and also intermediate alloys are single-phase and have the diffraction patterns qualitatively similar to Tl₅Te₃ with slight reflections displacement from one compound to another. For example, we present the powder diffraction patterns of alloys with composition 20, 50 and 80 mol% Tl₉TbTe₆. Parameters of the tetragonal lattice of solid solutions obey the Vegard's law (Table 1, Fig.1c)⁴⁰.

3.2. Isopleth sections of the phase diagram

We plotted some isopleth sections, in order to construct a complete T-x-y diagram. Figs.3a-c present the isopleth sections Tl₉TbTe₆-[A], Tl₉BiTe₆-[B] and Tl₄PbTe₃-[C] of the Tl₄PbTe₃-Tl₉TbTe₆-Tl₉BiTe₆ concentrations area, where A, B, and C are equimolar alloys from the respective boundary system as shown in Fig.4.

According to Fig.3a, b, the Tl₄PbTe₃-[C] and Tl₉BiTe₆-[B] sections are characterized by primary crystallization of the δ-phase from the melt over the entire concentration interval.

In contrast to the above-mentioned sections, along the Tl₉TbTe₆-[A] section, the direct crystallization of the δ-phase from the melt occurs only in the interval <60 mol% Tl₉TbTe₆. In the Tl₉TbTe₆-rich concentration area, the more refractory phase of TlTbTe₂ first crystallizes from the melt. Then a monovariant peritectic process L+TlTbTe↔δ occurs (Fig.3c), as a result of which a three-phase region L+TlTbTe₂+δ should form on the phase diagram. However, according to DTA data, we were unable to fix this region, which is apparently associated with the narrowness of the temperature interval of the above-stated peritectic reaction. Therefore, this region is indicated by the dotted line (Fig. 3b). The crystallization of all alloys is completed by the formation of δ-phase. The TlTbTe₂ phase is completely consumed in the peritectic reaction L+TlTbTe₂↔δ, and the remaining excess of the melt crystallizes into the δ-phase.

The XRD powder patterns for selective alloys on polythermal sections confirmed the formation of continuous solid solutions with the Tl₅Te₃-structure.

3.3. The liquidus and solidus surfaces projections

Projection of liquidus of the Tl₄PbTe₃-Tl₉TbTe₆-Tl₉BiTe₆ section consists of two fields of the primary crystallization of TlTbTe₂ and δ- solid solutions. These fields are separated by a monovariant peritectic curve L+TlTbTe₂↔δ (ab curve). The solidus projection (dashed lines) consist of one surface corresponding to the completion of the crystallization of the δ-phase.

3.4. Isothermal sections at 860 and 840 K

Both sections are consist of areas of L-, TlTbTe₂ and δ-phases (Fig.5). In alloys with composition <60 mol% Tl₉TbTe₆ in the two-phase L+δ region the directions of the tie-lines on the studied composition plane. It should be noted that comparison of the isopleth sections (Fig.3) and isothermal sections (Fig.5) shows that the directions of the connodes in the two-phase region L+δ deviate from the T-x plane and constantly vary with temperature. Isothermal sections at 860 and 840 K clearly confirm this.

4. Conclusion

At the first time, a self-consistent scheme of the phase relations in the Tl₄PbTe₃-Tl₉TbTe₆-Tl₉BiTe₆ section of the Tl-Pb-Bi-Tb-Te system is obtained. The T-x diagrams of boundary system Tl₄PbTe₃-Tl₉TbTe₆, some isopleth sections, an isothermal section at 860 and 840 K, as well as liquidus and solidus surface projections, are plotted. It was shown, that studied system is characterized by the formation of the continuous field of δ-solid solutions with the Tl₅Te₃ structure. Obtained experimental results can be used for choosing the composition of solution-melt for the growth of the high-quality crystals of δ-phase which are of interest as thermoelectric materials.

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
  2018

History

• Received
  13 Mar 2018
• Reviewed
  02 Apr 2018
• Accepted
  20 Apr 2018