Phase Equilibria and some Properties of Solid Solutions in The Tl5Te3-Tl9SbTe6-Tl9GdTe6 System

Phase equilibria in the Tl5Te3-Tl9SbTe6-Tl9GdTe6 system were experimentally studied by thermal analysis, X-ray diffraction and microhardness measurements applied to equilibrated alloys. Some isopleth sections, isothermal section at 760 K, and also projections of the liquidus and solidus surfaces, were constructed. A continuous series of solid solutions was found in this system. Solid solutions crystallize in the tetragonal Tl5Te3 structure type.


1. Materials and Syntheses
For the synthesis, we used the high purity thallium, antimony, gadolinium, and tellurium (the purity of the ingredient was better than 99.99 mass. %).
The surface of thallium was coated by a thin oxide film, which was removed before use.
It should be noted that, thallium and its compounds are extremely toxic, and should be handled with great care. Thallium is readily absorbed through the skin and care should be taken to avoid this route of exposure. Therefore, we used protective gloves at all times when working with thallium. However, no respiratory tract covers are required since thallium is not volatile.
The elements were weighed to be about 10 g in total according to the molar ratio of the corresponding binary and ternary compound, were placed in silica tubes of about 20 cm in length and then were sealed under a vacuum of 10 -2 Pa.
Taking into account the congruent melting of Tl 5 Te 3 and Tl 9 SbTe 6 , their synthesis was carried out by heating of elements in one zone electric furnace at the 750 and 830 K, respectively followed by cooling in the switchedoff furnace.
The obtained intermediate ingot of Tl 9 GdTe 6 was carefully ground in an agate mortar, pressed into the circular pellet of about 10 mm diameter and annealed at 770 K within ∼1000 h as it was done in previous work. 24 The weight losses during the pellet preparation were less than 0.5 mass. %. In order to prevent a reaction between the gadolinium and the quartz during high temperature reactions, quartz tubes coated internally with a thin layer of carbon were used.
The purity of the synthesized compounds was checked by the X-ray diffraction (XRD) and differential thermal analysis (DTA).
Only one thermal effect was observed for Tl 5 Te 3 (723 K) and Tl 9 SbTe 6 (790 K); whereas two peaks for Tl 9 GdTe 6 which were relevant the peritectic reaction at 800 K and its liquidus at 1190 K. These data are in good agreement with the literature references. 7,23,24 XRD confirmed that synthesized compounds were phase-pure. Powder XRD pattern of the Tl 9 SbTe 6 and Tl 9 GdTe 6 were similar to that of Tl 5 Te 3 . The unit cell parameters were practically equal to literature data (Table 1). 24,25 Synthesized binary and ternary compounds were used for the fabrication of the alloys of the Tl 5 Te 3 -Tl 9 Sb-Te 6 -Tl 9 GdTe 6 system. The alloys weighing 1 g were synthesized in quartz tube evacuated to 10 -2 Pa. Taking into account the fact that an equilibrium state could not be obtained even after the long-time (1000 h) annealing, after synthesis the samples containing more than 60 mol% Tl 9 GdTe 6 were powdered, mixed, pressed into circular pellets of about 10 mm diameter and annealed at 700 K for 1 month.

Methods
X-ray powder diffraction (XRD), differential thermal analysis (DTA) and also microhardness measurements were employed to check the purity of the synthesized starting compounds and analyze the samples in order to plot the phase diagrams.
DTA was performed using a NETZSCH 404 F1 Pegasus differential scanning calorimeter within room temperature and ∼1400 K depending on the composition of the alloys at a heating rate of 10 K min -1 and accuracy about ±3°. Temperatures of thermal effects were taken mainly from the heating curves.
The XRD measurements of the powdered specimen were recorded using a Bruker D8 diffractometer utilizing CuK α radiation within 2θ = 10 ÷ 70°. The unit cell parameters 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 microhardnesmeter PMT-3, the typical loading being 20 g and accuracy about 20 MPa.

Results and Discussion
The combined analysis of obtained experimental and literature data [7,24,25] allowed us to construct the diagram of the phase equilibria in the Tl 5 Te 3 -Tl 9 SbTe 6 -Tl 9 GdTe 6 system (Table, Fig.1-6).
The 2Tl 5 Te 3 -Tl 9 SbTe 6 system is quasi-binary and characterized by the formation of unlimited solid solutions (δ) with Tl 5 Te 3 -structure. 7 The 2Tl 5 Te 3 -Tl 9 GdTe 6 and Tl 9 SbTe 6 -Tl 9 GdTe 6 systems (Table 1, Figs. 1a, 2a) are characterized by the formation of continuous solid solutions (δ) with Tl 5 Te 3 -structure. However, they are non-quasi-binary sections of the Tl-Gd-Te ternary and Tl-Sb-Gd-Te quaternary systems due to the peritectic melting of the Tl 9 GdTe 6 compound. This leads to crystallization infusible X phase in a wide composition interval and formation two-phase L + X and three-phase L + X + δ areas. These areas are not experimentally fixed due to narrow temperature interval and shown by dotted line.
We have assumed that the X phase has a composition TlGdTe 2 . This assumption is confirmed by the presence of the most intense reflection peaks of TlGdTe 2 on diffractograms of the as-cast alloys from the region more than 63 mol% Tl 9 GdTe 6 . 26 It should be noted that regardless a very close melting temperature of Tl 9 SbTe 6 (790K) and peritectic decomposition of Tl 9 GdTe 6 (800 K) compounds, the liquidus and solidus curves have not extremum points and temperature interval of the crystallization of the δ-phase is less than 3 K. Such phenomenon is realized when the enthalpy of mixing during the formation of solid and liquid solutions from starting compounds is practically equal to zero. In other words, in the studied system the Sb → Gd replacement in the solid and liquid states are not accompanied by a significant thermal effect. This fact allows us to characterize the δ-solid solutions as quasi-ideal solution.
The curves of microhardness dependencies have a flat maximum, which is typical for systems with continuous solid solutions (Fig. 1b and 2b).
The XRD patterns obtained are presented in Fig. 3. Powder diffraction patterns of Tl 5 Te 3 , Tl 9 SbTe 6 and Tl 9 GdTe 6 , and intermediate alloys were very similar to that of Tl 5 Te 3 with slight reflections displacement from one compound to another. The lattice parameters of solid solutions depend linearly on the composition, i.e. obey the Vegard's rule.
Liquidus of the Tl 5 Te 3 -Tl 9 SbTe 6 -Tl 9 GdTe 6 system consists of two fields of the primary crystallization of X-phase and δsolid solutions, limited by the ab curve corresponds to the monovariant peritectic L + X ↔ δ equilibrium (Fig. 4). Table 1. Some properties of phases in the Tl 5 Te 3 -Tl 9 SbTe 6 -Tl 9 GdTe 6 system.  According to the phase diagram of the Tl 9 GdTe 6 -[B] cut, the primary crystallization of the δ-phase occurs from the liquid phase in the composition area < 60 mol% Tl 9 GdTe 6 . In the Tl 9 GdTe 6 -rich alloys the X-phase first crystallizes, then a monovariant peritectic equilibrium L + X ↔ δ takes place.
As can be seen, over the entire compositions area of the Tl 9 SbTe 6 -[A] and Tl 5 Te 3 -[C] cuts only δ-phase crystallizes from the melt.
Comparison between isopleth sections (Fig. 5) with the isothermal section (Fig. 6) shows, that tie-lines positions in two-phase area L + δ do not correspond to the cross section planes and continuously change with temperature. The tie-lines positions at 760 K are shown in Fig. 6.
Figs. 5a-c show the isopleth sections 2Tl 5 Te 3 -[C], Tl 9 SbTe 6 -[A] and Tl 9 GdTe 6 -[B] of the Tl 5 Te 3 -Tl 9 SbTe 6 -Components of the system display unlimited solubility in the solid state. Obtained experimental data can be used for choice the composition of solution-melt and for determining of temperature conditions for growing crystals of δphase with a given composition.

Acknowledgment
This work was done in the international joint research laboratory between Institute of Catalysis and Inorganic Chemistry of ANAS (Azerbaijan) and Donostia International Physics Center (Basque Country, Spain).