CC BY
31
366
CHEMICAL PROBLEMS 2019 no. 3 (17) ISSN 2221-8688
UDC 541.123/.123.8/9:546.57'81'86/23
PHASE RELATIONS IN THE PbTe-AgSbTe2 SYSTEM
Shabnam Hamlet Mansimova
Baku State University
23, Z.Khalilov str., AZ-1148 Baku, Azerbaijan; e-mail: [email protected]
Received 10.06.2019
Abstract: Phase equilibria in the PbTe-AgSbTe2 section of the quasiternary Ag2Te-PbTe-Sb2Te3 system were studied by means of differential-thermal and X-ray analyses, as well as microhardness measurements. For investigations, two series of alloys of the explored section were prepared by two different ways. Based on the experimental data, the T-x diagram of the PbTe-AgSbTe2 section was constructed. It found that a wide area (30-100 mol% PbTe) of solid solutions based on lead telluride was formed in the system. A characteristic feature of this system is a large temperature range (up to 150 °) of crystallization (melting) of solid solutions which leads to strong segregation and heterogeneity of solid solutions.
Keywords: PbTe-AgSbTe2 system, phase diagram, solid solutions, silver-lead-antimony tellurides DOI: 10.32737/2221-8688-2019-3-366-372
Introduction
In recent years, thermoelectric (TE) materials have been widely studied to be used as alternative energy sources and in novel energy conversion applications. In this respect, many semiconducting chalcogenides have attracted much attention for the development of TE materials [1-3]. Various complex tellurides such Ag-AIV-BV-Te (AIV- Ge, Sn, Pb; BV-Sb, Bi) alloys have high ZT values and are mentioned among the most promising thermoelectric materials [4-6]. It must be noted that the AgSbTe2 alloy has been used as an important component to construct the TE materials with excellent TE properties, such as (PbTe)m(AgSbTe2) (denoted as LAST) and (GeTe)x(AgSbTe2)100-x (named as TAGS). The ZT value of LAST reaches 2.2 at 800 K when m is 18. The TAGS-x has a ZT value over 1.5 at 800 K when x is 80 or 85 [7-11].
Recent studies showed that complex tellurides of heavy p-metals exhibit topological surface states as well, and can be used in spintronics and quantum computing [12-14].
It has to be kept in mind that optimization of functional properties of these materials can be achieved by changing their composition. This is based, in turn, on the research into phase equilibriums in relevant systems [1517]. In the case of Ag2Te-AIVTe-BV2Te3
systems, it would be very interesting to look for new complex phases because binary and ternary compounds in these systems have already been recognized as promising matrix phases.
The phase equilibriums in the Ag2Te-SnTe-Sb2Te3, Ag2Te-SnTe-Bi2Te3 and Ag2Te-PbTe-Bi2Te3 systems had already been described in communications [18-21]. For the both systems, several new non-stoichiometric phases were found, and their primary crystallization and homogeneity fields determined. It revealed that the homogeneity region of solid solutions formed along AIVTe-Ag BVTe2 sections expanded greatly in both directions.
Herein, we present the phase relationships in the Ag2Te-PbTe-Sb2Te3 system over the PbTe-AgSbTe2 section. In [22], it was shown that the compound of the AgSbTe2 composition previously mentioned in the literature [23, 24] did not exist, and the cubic phase in the Ag2Te-Sb2Te3 system had a slightly different composition (Ag19Sb29Te52). According to [25], Ag19Sb29Te52 decomposed by solid-phase reaction upon cooling does not exist below 250 K. Thus, the results of [22, 25] cast doubt on the existence of continuous solid solutions in the PbTe-AgSbTe2 system.
Experimental part
For the experiments, binary tellurides Ag2Te, PbTe, and Sb2Te3 were first synthesized. These compounds were prepared through melting high-purity elements (99.999 wt. %) in evacuated (~10-3 Pa) silica ampoules at a temperature of~50 K higher than their melting points [26]. The Ag2Te was additionally annealed at 1200 K for 3 hours and then quenched with cold water in order to obtain a homogeneous stoichiometric composition. All starting compounds were identified through the use of differential thermal analysis and powder X-ray diffraction techniques.
More than ten alloys of the PbTe-AgSbTe2 section were prepared from the pre-synthesized binary compounds also by means of vacuum alloying. Two series of alloys were obtained in two ways. One series of samples after fusion was slowly cooled to 750 K and annealed at this temperature for 700 hours. The second series of samples were quenched by injecting ampoules into cold water from 1150 K (alloys with compositions of 70, 80 and 90 mol% PbTe) and from 1000 K (other alloys), and then annealed at 750 K for 700 h.
Results and discussion
Fig. 1 shows thermograms for heating alloys of both series with compositions 60, 80 and 90 mol% PbTe. As can be seen, the melting onset temperatures of the two series of alloys are greatly different. For samples of the 1st series (red curves) obtained by slow cooling, the melting onset temperatures are
significantly (up to 100°) lower than those of the 2nd series (blue curves). Note that rise in the annealing time up to 1000 h did not change the DTA curves of the alloys of the 2nd series, whereas for those of the 1st series some (~ 1020 °) rise in the temperatures of the onset of melting was observed.
Fig.1. Fragments of the DTA curves for the PbTe-"AgSbTe2" system alloys with the compositions 60 (a), 80 (b) and 90 (c) mol% PbTe. DTA curves for samples from the 1st series
are red, and those from the 2nd series are blue.
The results above are indicative that the Series II samples can be considered practically in equilibrium. Therefore, to construct the phase diagram (Fig. 2) of the PbTe-"AgSbTe2" system, data from DTA alloys of series II were used (see Table). According to Fig. 2, the PbTe-"AgSbTe2" system is characterized by the formation of about 70 mol% of solid solutions based on PbTe (P-phase); however, as a whole, the system is
The powder X-ray analysis results confirmed the formation of a wide area (30100 mol% PbTe) of solid solution with a cubic structure in the explored system. PbTe-poor alloys are three-phase (P+Ag2Te+ Sb2Te3).
A characteristic feature of the PbTe-"AgSbTe2" system is a very large temperature range of crystallization (melting) of the P-phase (up to 150 °). For this reason, slow cooling of melts leads to strong segregation and heterogeneity of solid solutions which makes it difficult to achieve an equilibrium state of the samples. The heterogeneity of solid solutions in the 1st series alloys is clearly apparent by a powder X-ray patterns of an alloy with a composition of 70 mol% PbTe (Fig. 3). As can be seen, X-ray patterns of samples of this alloy obtained through the use
generally non-quasi-binary. This is due to the fact that one of the starting components, i.e. "AgSbTe2", is not an individual compound but a two-phase alloy Ag2Te+Ag19Sb29Te52 [11,14]. This goes to show that the solid phase (Ag2Te+Ag19Sb29Te52) not located on its T-x plane of the PbTe-«AgSbTe2» section is involved in the phase equilibria of this section along the of <30 mol% PbTe composition area.
of two different ways, differ strongly. The alloy from the 1st series has very diffuse reflection peaks while the alloy from the 2nd series has a very high quality X-ray pattern.
The results of microhardness measurements (Table, Fig.2) are in accordance with the phase diagram. The microhardness values of the ß-phase are due to continuous function of the composition and expressed by a curve with a gentle maximum. In alloys with compositions of 10 and 20 mol% PbTe in addition to the ß-phase, there are two more phases with microhardness values of ~ 380 and 590 MPa. Following the results of X-ray diffraction analysis of these alloys, these microhardness values refer to Ag2Te and Sb2Te3 compounds.
Table. DTA and microhardness measurement data
for the PbTe-"AgSbTe2" system
Composition, Thermal effect,
mol % PbTe K MPa
0 500; 635; 813; 813-840 580;750
5 500; 620; 805; 805-827
10 500; 603; 805; 805-820 580;790
20 808-825 580;820
30 815-853
40 827-885 840
50 848-935
60 872-1007 730
70 913-1058
80 965-1105 620
90 1045-1150
PbTe 80 60 40 20 «AgSbTe,'!
mol % PbTs;
Fig.2. Phase diagram and concentration dependence of microhardness for the PbTe-"AgSbTe2" system
Fig.3. Powder X-ray patterns of alloy with composition of 70 mol% PbTe: a) a sample from the 1st series; b) a sample from the 2nd series
Conclusion
Based on the data of DTA, XRD and microhardness measurements, the nature of phase equilibriua in the PbTe-AgSbTe2 section of the Ag2Te-PbTe-Sb2Te3 quasi-ternary system was uncovered. In particular, it found that this section is partially quasi-binary and characterized by the formation of a wide (up to 70 mol %) PbTe (p-phase)-based solid solutions region. Within the 0-30 mol% PbTe composition range the alloys of the system
consist of a three-phase mixture Ag2Te+Sb2Te3+p. A characteristic feature of this system is a large temperature range (up to 150°) of crystallization (melting) of the P-phase that involves a strong segregation and heterogeneity of solid solutions in composition. The obtained solid solutions are of practical interest as medium-temperature thermoelectric materials.
References
1. Gayner C., Kar K.K. Recent advances in thermoelectric materials. Progress in Materials Science, 2016, vol. 83, pp. 330382.
2. Twaha S., Zhu J., Yan Y., Li B. A comprehensive review of thermoelectric technology: Materials, applications, modelling and performance improvement. Renewable and Sustainable Energy Reviews, 2016, vol. 65, pp. 698-726.
3. Tesfaye F., Moroz M. An Overview of Advanced Chalcogenide Thermoelectric Materials and Their Applications. Journal of Electronic Research and Application, 2018, vol. 2, no. 2, pp. 28-41.
4. Tan G., Shi F., Sun H., Zhao L.-D., Uher C., Dravid V. P., Kanatzidis M. G. SnTe-AgBiTe2 as an efficient thermoelectric material with low thermal conductivity. J. Mater. Chem. A, 2014, vol. 2, pp. 2084920854.
5. Wu J., Yang J., Zhang H., Zhang J., Feng
S., Liu M., Peng J., Zhu W., Zou T. Fabrication of Ag-Sn-Sb-Te based thermoelectric materials by MA-PAS and their properties. Journal of Alloys and Compounds, 2010, vol. 507, pp. 167-171.
6. Wu H.-J., Wei P.-C., Cheng H.-Y., Deng J.-R., Chen Y.-Y. (2017). Ultralow thermal conductivity in n -type Ge-doped
AgBiSe2 thermoelectric materials. Acta Materialia, 2017, vol. 141, pp. 217-229.
7. Perlt S., Hoche T., Dadda J., Muller E., Rauschenbach B. Compositional sensitivity of microstructures and thermoelectric properties of Ag1-xPb18Sb1+yTe20 compounds. Journal of Electronic Materials, 2013, vol. 42, issue 7, pp.1422-1428.
8. Horichok I., Ahiska R., Freik D., Nykyruy L., Mudry S., Matkivskiy O., Semko T. Phase Content and Thermoelectric Properties of Optimized Thermoelectric Structures Based on the Ag-Pb-Sb-Te System. Journal of Electronic Materials, 2016, vol. 45, no. 3, pp.1576-1583.
9. Chen Y., He B., Zhu T.J., Zhao X.B. Thermoelectric properties of non-stoichiometric AgSbTe2 based alloys with a small amount of GeTe addition. Journal of Physics D: Applied Physics, 2012, vol. 45, no. 11, p.115302.
10. Kusz B., Miruszewski T., Bochentyn B., et al. Structure and Thermoelectric Properties of Te-Ag-Ge-Sb (TAGS) Materials Obtained by Reduction of Melted Oxide Substrates. J. Electron. Mater., 2016, vol. 45, no. 2, pp. 10851093.
11. Zhang H., Luo J., Zhu H.-T., Liu Q.-L., Liang J.-K., Li J.-B., Liu, G.-Y. Synthesis and thermoelectric properties of Me-doped AgSbTe2 compounds. Chin. Phys. B, 2012, vol. 21, no. 10, p. 106101.
12. Shvets I.A., Klimovskikh I.I., Aliev Z.S., Babanly M.B., Sánchez-Barriga J., Krivenkov M., Shikin A.M. and Chulkov E.V. Impact of stoichiometry and disorder on the electronic structure of the PbBi2Te4-xSex topological insulator. Phys. Rev. B, 2017, vol. 96, pp. 235124 -235127.
13. Pacile D., Eremeev S. V., Caputo M., Pisarra M., De Luca O., Grimaldi I., Fujii J., Aliev Z. S., Babanly M. B., Vobornik I., Agostino R. G., A. Goldoni, E. V. Chulkov, and M. Papagno. Deep insight into the electronic structure of ternary topological insulators: A comparative study of PbBi4Te7 and PbBi6Te10. Physica
status solidi (RRL) - Rapid Research Letters, 2018, vol. 12, no. 12, pp. 1800341-1800348.
14. Aliev Z. S., Amiraslanov I. R., Nasonova D. I., Shevelkov A. V., Abdullayev N. A., Jahangirli Z. A., Orujlu E. N., Otrokov M. M., Mamedov N. T., Babanly M. B., Chulkov E. V. Novel ternary layered manganese bismuth tellurides of the MnTe- Bi2Te3 system: Synthesis and crystal structure. J. All. Comp., 2019, vol. 789, pp. 443-450.
15. Babanly M.B., Chulkov E.V., Aliev Z.S., Shevel'kov A.V., Amiraslanov I.R. Phase diagrams in materials science of topological insulators based on metal chalkogenides. Russ. J. Inorg. Chem., 2017, vol. 62, no. 13, pp. 1703-1729.
16. Zlomanov V.P., Khoviv A.M., Zavrazhnov A.Yu. Physicochemical Analysis and Synthesis of Nonstoichiometric Solids. In: InTech. Materials Science - Advanced Topics, 2013, pp. 103-128.
17. Imamaliyeva S.Z., Babanly D.M., Tagiev D.B., Babanly M.B. Physicochemical Aspects of Development of Multicomponent Chalcogenide Phases Having the Tl5Te3 Structure: A Review. Russ.J.Inorg.Chem., 2018 , vol. 63, no.13, pp. 1704-1730.
18. Mashadiyeva L.F., Kevser J.O., Aliev I.I., Yusibov Y.A., Taghiyev D.B., Aliev Z.S., Babanli M.B. The Ag2Te-SnTe-Bi2Te3 system and thermodynamic properties of the (2SnTe)1-x(AgBiTe2)x solid solutions series. J.Alloys .Compd., 2017, vol. 724, pp. 641-648.
19. Mashadiyeva L.F., Kevser J.O., Aliev I.I., Yusibov Y.A., Taghiyev D.B., Aliev Z.S., Babanli M.B. Phase Equilibria in the Ag2Te-SnTe-Sb2Te3 System and Thermodynamic Properties of the (2SnTe)12x(AgSbTe2)x Solid Solution. Phase equilibria and diffusion, 2017, vol. 38, no. 5, pp. 603-614.
20. Babanly D.M., Aliev I.I., Babanly K.N., Yusibov Yu.A. Phase equilibria in the Ag2Te-PbTe-Bi2Te3 system. Russ. J. Inorg. Chem., 2011, vol. 56, no. 9, pp. 1472-1477.
21. Mashadieva L. F., Yusibov Yu. A., Kevser Dzh., Babanly M. B. Thermodynamic Study of Solid Solutions in the SnTe-AgSbTe2 System by Means of EMF with Solid Electrolyte Ag4Rbl5. Russ. J. Phys. Chem. A, 2017, vol. 91, no. 9, pp. 1642-1646.
22. Marin R.N., Brun G.J., Tedenac J.-C. Phase equilibria in the Sb2Te3-Ag2Te system. J.Mater.Sci., 1985, vol. 20, no. 2, pp. 730-735.
23. Berger L.I., Prochukhan V.D. Triple
diamond-like semiconductors. Moscow: Metallurgy Publ., 1968, 150 p.
24. Lazarev V.B., Berul S.I., Salov A.V. Triple semiconductor compounds in A1-BV-CVI systems. Moscow: Nauka Publ., 1982, 150 p.
25. Wu H.-J., Chen S.W. Phase equilibria of Ag-Sb-Te thermoelectric materials. Acta Mater., 2011, vol. 59, pp. 6463-6472.
26. Massalski T.B., Ed., Binary Alloy Phase Diagrams, 2nd ed., ASM International, Materials Park, 1990.
Pb Te-AgSb Te2 SiSTEMiNDd FAZA TARAZLIQLARI
§эЬпэт Hamlet qizi Mznsimova
Baki Dôvldt Universiteti Z. Xdlilov 23, Az-1148 Baki, e-mail: [email protected]
Îçdd DTA, RFA vd mikrobdrkliyin ôlculmdsi ild Ag2Te-PbTe-Sb2Te3 kvaziuçlu sistemin kdsiyi uzrd faza tarazliqlari ôyrdnilmiç vd T-x diaqrami qurulmuçdur. Gôstdrilmiçdir ki, sistemdd qurguçun tellurid dsasinda geniç bdrk mdhlul sahdsi (30-100 mol% PbTe) dmdld gdlir. Bu sisitemin dsas xususiyydti bdrk mdhlullarin kristallçma (drimd) temperatur intervalinin bôyuk olmasidir. Bu isd bdrk mdhlullarin guclu likvasiyasi vd qeyri-bircinsliyi ild ndticdldnir.
Açar sôzfor: Ag2Te-PbTe-Sb2Te3 sistemi, faza diaqrami, bdrk mdhlullar, gumu$-qurgu$un-stibium telluridldri
ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ PbTe-AgSbTe2
Ш.Г. Мансимова
Бакинский Государственный Университет AZ1148 Баку, ул. З.Халилова, 23; e-mail: [email protected]
Методами дифференциально-термического и рентгенфазового анализов, а также измерением микротвердости изучены фазовые равновесия по разрезу PbTe-AgSbTe2 квазитройной системы Ag2Te-PbTe-Sb2Te3. Для проведения исследований были приготовлены две серии сплавов исследуемого разреза двумя различными способами. На основании экспериментальных данных построена Т-х диаграмма разреза PbTe-AgSbTe2. Показано, что в системе образуется широкая область (30-100 мол% PbTe) твердых растворов на основе теллурида свинца. Характерной особенностью данной системы является большой температурный интервал (до 150°) кристаллизации (плавления) твердых растворов, что приводит к сильной ликвации и неоднородности твердых растворов.
Ключевые слова: система PbTe-AgSbTe2, фазовая диаграмма, твердые растворы, теллуриды серебра-свинца-сурьмы.
