Phase Equilibria in the Tl4PbTe3-Tl9SmTe6-Tl9BiTe6 Section of the Tl-Pb-Bi-Sm-Te System

Phase equilibria in the section Tl4PbTe3-Tl9SmTe6-Tl9BiTe6 of the Tl-Pb-Bi-Sm-Te system were determined by combination of differential thermal analysis, powder X-ray diffraction methods as well as microhardness measurements. The phase diagrams of the boundary systems Tl4PbTe3-Tl9SmTe6, Tl9SmTe6-Tl9BiTe6, isothermal section at 820 and 840 K, some isopleth sections and as well as liquidus and solidus surfaces projections, were plotted. Unlimited solid solutions, which crystallize in Tl5Te3 structure type were found in the system at the solidus temperatures and below.

A new thallium lanthanide tellurides of Tl 9 LnTe 6 -type (Ln-Ce, Nd, Sm, Gd, Tb, Tm) were found to be a new structural analog of Tl 5 Te 3 . 18,19 H. Kleinke and co-workers [20][21][22] confirmed the results of the studies, 18,19 and determined the thermoelectric and magnetic properties for a number Tl 9 LnTe 6 -type compounds.
The development of the novel preparative methods for direct synthesis of functional materials requires to provide an accurate study of phase relations and plot the phase diagram.
Early, we presented the results of a study of phase relations for a number of systems including the Tl 5 Te 3 compound or its structural analogs. [23][24][25] The formation of unlimited solid solutions was shown for these systems.
In this paper, we continue to study similar systems and present the experimental results on phase equilibria in the Tl 4 PbTe 3 -Tl 9 SmTe 6 -Tl 9 BiTe 6 section of the Tl-Pb-Bi-Sm-Te system.
We used protective gloves at all times when working with thallium because thallium and its compounds are highly toxic and contact with skin is dangerous.
Stoichiometric amounts of the starting components were weighed with accuracy ±0.0001 g. Then they were put into silica tubes of about 20 cm in length and diameter about 1.5 cm and sealed under a vacuum of 10 -2 Pa. Tl 4 PbTe 3 and Tl 9 BiTe 6 were synthesized by heating in a resistance furnace at 920 K followed by cooling in the switched-off furnace.
In the case of Tl 9 SmTe 6 , the ampoule was graphitized using pyrolysis of toluene in order to prevent the reaction of samarium with quartz. Taking into account the results of the work 26 , the intermediate ingot of Tl 9 SmTe 6 was powdered in an agate mortar, carefully mixed, pressed into a pellet and annealed at 700 K within ∼700 h.
The resulting ingots were homogeneous polycrystals alloys that were established by the differential thermal analysis (DTA) and X-ray diffraction (XRD).
The alloys of the Tl 4 PbTe 3 -Tl 9 SmTe 6 -Tl 9 BiTe 6 system were prepared by melting of previously synthesized ternary compounds. After synthesis the samples containing >60% Tl 9 SmTe 6 were powdered, carefully mixed, pressed into pellets and annealed at 700 K during ~ 800 h in order to complete the homogenization. The total mass of each ingot is about 1 g.

Methods
DTA and XRD analyses, as well as microhardness measurements, were used to analyze the samples of the investigated system.
The phase transformation temperatures 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 about ±2 K. The phase identification was performed using a Bruker D8 diffractometer utilizing CuK α radiation. The powder diagrams of the ground samples were collected at room temperature in the 2θ range of 6-75°. 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). Microhardness measurements were done with a microhardness tester PMT-3, the typical loading being 20 g and accuracy about 20 MPa.

Results and Discussion
The Tl 4 PbTe 3 -Tl 9 SmTe 6 -Tl 9 BiTe 6 section was plotted based on combined analysis of experimental results and literature data on boundary system Tl 4 PbTe 3 -Tl 9 BiTe 6 26 ( Fig. 1-6). Table 1. Experimental data of the DTA, microhardness measurements and parameters of tetragonal lattice for the alloys of the Tl 4 PbTe 3 -Tl 9 SmTe 6 and Tl 9 BiTe 6 -Tl 9 SmTe 6 sections of the Tl-Pb-Bi-Sm-Te system  (7) The Table presents the results of DTA, microhardness measurements, and parameters of the tetragonal lattice for starting compounds and some intermediate alloys.
Phase diagrams and the composition dependences of properties are plotted based on these data. Tl 4 PbTe 3 -Tl 9 SmTe 6 and Tl 9 BiTe 6 -Tl 9 SmTe 6 sections ( Fig. 1) are characterized by the formation of unlimited solid solutions (δ) with Tl 5 Te 3 -structure. But, they are non-quasi-binary sections of the Tl-Pb-Sm-Te and Tl-Bi-Sm-Te quaternary systems due to the peritectic character of melting of Tl 9 SmTe 6 . As the result, the crystallization of TlSmTe 2 compound occurs in a wide composition interval which leads to the formation of two-phase L+TlSmTe 2 and three-phase L+TlSmTe 2 +δ areas. The L+TlSmTe 2 +δ area is shown by a dotted line because not fixed experimentally due to a narrow interval of temperatures.
In order to determine the phase constituents, polished surfaces of the intermediate samples were visually observed under the microscope of microhardness meter. The microhardness curves have a flat maximum which is typical for systems with unlimited solid solutions (Fig. 1b). 29 The XRD powder patterns for some alloys of the Tl 4 PbTe 3 -Tl 9 SmTe 6 and Tl 9 BiTe 6 -Tl 9 SmTe 6 sections are presented in Fig. 2. Powder diffraction patterns of Tl 4 PbTe 3 , Tl 9 SmTe 6 , and Tl 9 BiTe 6 as well as intermediate alloys are single-phase and have the diffraction patterns qualitatively similar to Tl 5 Te 3 with slight reflections displacement from one compound to another. For example, we present the powder diffraction patterns of alloy with composition 20, 50 and 80 mol% Tl 9 SmTe 6 for both systems. Parameters of the tetragonal lattice of solid solutions obey the Vegard's law (Table, Fig . 1c).
In order to construct a complete T-x-y diagram and to refine the boundaries of areas of primary crystallization of δ-phase and TlSmTe 2 , we constructed some isopleth sections. Figs.3a-c present the isopleth sections Tl 9 SmTe 6 -[A], Tl 9 BiTe 6 -[B] and Tl 4 PbTe 3 -[C] of the Tl 4 PbTe 3 -Tl 9 Sm-Te 6 -Tl 9 BiTe 6 system, where A, B, and C are equimolar alloys from the respective boundary system as shown in Fig. 4.
The XRD powder patterns for selective alloys on polythermal sections confirmed continuous solid solutions with the Tl 5 Te 3 -structure.
The liquidus and solidus surfaces projections (Fig. 4) Projection of liquidus of Tl 4 PbTe 3 -Tl 9 SmTe 6 -Tl 9 BiTe 6 section consists of two fields of the primary crystallization of TlSmTe 2 and δ-solid solutions. These fields are separated by a monovariant peritectic curve L+TlSmTe 2 ↔ δ (ab curve). The solidus projection (dashed lines) con-  sist of one surface corresponding to the completion of the crystallization of the δ-phase.
Isothermal sections at 820 and 840 K of the Tl 4 PbTe 3 -Tl 9 SmTe 6 -Tl 9 BiTe 6 section (Fig. 5) are consists of areas of L-, TlSmTe 2 and δ-phases. In alloys <60 mol% Tl 9 SmTe 6 in the two-phase L+δ region the directions of the connodes are 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 820 and 840 K clearly confirm this.

Conclusion
A complete phase diagram of the Tl-Pb-Bi-Sm-Te system in the Tl 4 PbTe 3 -Tl 9 SmTe 6 -Tl 9 BiTe 6 composition interval is plotted. Unlimited solubility of components in the solid state is found in the studied section. 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 is of interest as thermoelectric material.

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).