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

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.


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][4][5][6][7] .
Earlier we presented some new thallium lanthanide tellurides of Tl 9 LnTe 6 -type (Ln-Ce, Nd, Sm, Gd, Tm, Tb), which are also ternary substitution variant of Tl 5 Te 3 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 Tl 9 LnTe 6 -type compounds were determined by authors [18][19][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 performance 21 .At this aim, we have presented the results of phase equilibria investigations of a number of systems including Tl 5 Te 3 compound or its structural analogues [22][23][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 Tl 5 Te 3 -Tl 9 BiTe 6 -Tl 9 TmTe 6 section of the Tl-Bi-Tm-Te quaternary system.
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 30-50 0 above the melting point of the compounds followed by cooling in the switched-off furnace.
In the case of Tl 9 TmTe 6 , the ampoule was graphitized using pyrolysis of acetone in order to prevent the reaction of thulium with quartz.Taking into account the results of the 26 , the intermediate ingot of Tl 9 TmTe 6 was powdered in an agate mortar, pressed into a pellet and annealed at 700 K within ~700 h.
a Institute of Catalysis and Inorganic Chemistry named after acad.M.Nagiyev, ANAS H.Javid ave., 113, Az-1143, Baku, Azerbaijan 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 Tl 9 BiTe 6 (830 K) and Tl 5 Te 3 (723 K), and two peaks for Tl 9 TmTe 6 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 .
Previously synthesized binary and ternary compounds were used to synthesize the alloys of the Tl 5 Te 3 -Tl 9 BiTe 6 -Tl 9 TmTe 6 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% Tl 9 TmTe 6 were powdered, mixed, pressed into pellets and annealed at 700 K during ~ 800 h in order to complete the homogenization.

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.

Results and discussion
The Tl 5 Te 3 -Tl 9 BiTe 6 -Tl 9 TmTe 6 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).
Tl 9 TmTe 6 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.
2Tl 5 Te 3 -Tl 9 TmTe 6 and Tl 9 BiTe 6 -Tl 9 TmTe 6 systems.As can be seen (Figures 1, 2), these systems are characterized by the formation of continuous solid solutions (δ) with Tl 5 Te 3structure.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 Tl 9 TmTe 6 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 Table 1.Dependence of the properties of the alloys annealed at the 700 K (800 h) on the composition for the Tl 5 Te 3 -Tl 9 TmTe 6 and Tl 9 BiTe 6 -Tl 9 TmTe 6 sections of the Tl-Bi-Tm-    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 TlTmTe 2 .This assumption is confirmed by the presence of the most intense reflection peaks of TlTmTe 2 28 on diffractograms of the as-cast alloys from region more than 63 mol% Tl 9 TmTe 6 .
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 Tl 5 Te 3 -Tl 9 TmTe 6 and Tl 9 TmTe 6 -Tl 9 BiTe 6 systems.As can be seen, powder diffraction patterns of starting compounds and intermediate alloys are single-phase and have the similar with Tl 5 Te 3 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).
The liquidus and solidus surfaces projections and isothermal section at 760 K in the Tl 5 Te 3 -Tl 9 BiTe 6 -Tl 9 TmTe 6 composition area of the Tl-Bi-Tm-Te quaternary system (Figure 5).
Liquidus of Tl 5 Te 3 -Tl 9 BiTe 6 -Tl 9 TmTe 6 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% Tl 9 TmTe 6 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 (Tl 9 TmTe 6 -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.

Conclusion
The phase diagram of the Tl-Bi-Tm-Te system in the Tl 5 Te 3 -Tl 9 BiTe 6 -Tl 9 TmTe 6 composition area is constructed, including the T-x diagrams of boundary systems Tl 5 Te 3 -Tl 9 TmTe 6 and Tl 9 BiTe 6 -Tl 9 TmTe 6 , 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.

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).
Figures 4a-c show the isopleth sections Tl 9 BiTe 6 -[A], Tl 9 TmTe 6 -[B] and Tl 5 Te 3 -[C] of the Tl 5 Te 3 -Tl 9 BiTe 6 -Tl 9 TmTe 6 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 Tl 9 BiTe 6 -[A] и Tl 5 Te 3 -[C] systems (Figures 4 a,c) only δ-phase crystallizes from the melt.

Figure 4 .
Figure 4. Polythermal sections 2Tl 5 Te 3 -[A], Tl 9 TmTe 6 -[B] and Tl 9 BiTe 6 -[C] of the phase diagram of the Tl 5 Te 3 -Tl 9 BiTe 6 -Tl 9 TmTe 6 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 Tl 5 Te 3 -Tl 9 BiTe 6 -Tl 9 TmTe 6 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.

Figure 5 .
Figure 4. Polythermal sections 2Tl 5 Te 3 -[A], Tl 9 TmTe 6 -[B] and Tl 9 BiTe 6 -[C] of the phase diagram of the Tl 5 Te 3 -Tl 9 BiTe 6 -Tl 9 TmTe 6 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 Tl 5 Te 3 -Tl 9 BiTe 6 -Tl 9 TmTe 6 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.