Elsevier

Thermochimica Acta

Volume 686, April 2020, 178567
Thermochimica Acta

Phase equilibria in the quasi-binary thallium(I) selenide– silver selenide system

https://doi.org/10.1016/j.tca.2020.178567Get rights and content

Highlights

  • Phase diagram for the Tl2Se-Ag2Se system was constructed from DTA, XRD and emf methods.

  • Two new compounds: 9Tl2Se•8Ag2Se and 3Tl2Se•17Ag2Se were found in the system.

  • On the matrix of the former compound, a phase (γ) of variable composition was formed.

  • The results were compared with both earlier data and those on Tl2Te-Ag2Te system.

Abstract

The phase diagram for the quasi-binary Tl2Se-Ag2Se system has been established in its final form from the results of phase studies by differential thermal analysis (DTA) and X-ray diffraction (XRD), involving those of our earlier electromotive force measurements (emf) of concentration cells. The diagram for the title system presented in this paper has been compared with those published earlier by other authors. The study resulted in substantial correction of the former phase diagram

Introduction

The quasi-binary thallium(I) selenide - silver(I) selenide system was formerly studied three times [[1], [2], [3]]. In the papers [1 and 2] there were published two almost identical phase diagrams for this system, accordingly, they might be presented in one figure (Fig. 1). The lines in the diagrams have an identical shape, accordingly, any differences in temperatures or compositions have been shown in form of double values in the figure.

According to both of them, three ternary compounds, i.e. AgTlSe (or Tl2Se•Ag2Se, at 50.0 mol% Ag2Se) melting congruently, Ag3TlSe2 (Tl2Se•3Ag2Se, at 75.0 mol% Ag2Se) and Ag7TlSe4 (Tl2Se•7Ag2Se, at 87.5 mol% Ag2Se) melting incongruently, were formed in this system. We decided to verify these ca. forty year old results by the electromotive force (emf) measurement method of concentration cells and showed [3] that both the phase diagrams in Fig. 1 were to be corrected. The new data obtained by the electrochemical method differed significantly from the former (Fig. 2). It appeared that the components of the system under consideration formed three compounds: 9Tl2Se•8Ag2Se (9:8) at 47 mol% Ag2Se, Tl2Se•3Ag2Se 1:3) at 75 mol% Ag2Se and 3Tl2Se•17Ag2Se (3:17) at 85 mol% Ag2Se. The compound 9:8 formed a phase γ of variable composition within the range 45 mol% Ag2Se – 51 mol% Ag2Se, undergoing a polymorphic transition αβ at variable temperature.

The compound Tl2Se•3Ag2Se (75 mol% Ag2Se) was the only detail of the former data [1,2] that has been confirmed in [3].

The emf measurement method of concentration cells is a very effective one for precise determining number, composition and nature of the solid phases formed by the components of binary and quasi-binary metal systems [4]. In other words, it is suitable for determining phase concentration limits in the systems. It is, however, less useful in obtaining information on liquidus temperatures because of technical limitations. This is the reason why the phase diagram for the title system, presented in Fig. 2, covers temperature ranges up to 493 K only. The dotted lines in Fig. 2 show a probable, supposed general shape of phase limits to be determined in the next studies.

The present study has been performed mainly by using differential thermal analysis to determine temperatures of phase transformations in the higher region, including those of the liquidus line, to enable the phase diagram to be completed.

The powder X-ray diffraction method was also employed to obtain complementary information.

Section snippets

Materials

The components of the system examined, i.e. thallium selenide and silver selenide, were prepared from pure elements: thallium 99.99 mass%, silver 99.99 mass% and selenium 99.99 mass%. The source and the purity of the starting materials are summarized in Table 1. The metal chalcogenides were synthesized by simply fusing stoichiometric quantities of the elements, weighed with an accuracy of ±0.0001 g, in quartz tubes, in a purified argon atmosphere (5 N pure, BOC Gazy, Poznan) and then mixed for

Results and discussion

The DTA measurements of phase transition temperatures in the system thallium(I) selenide - silver selenide were effected within the whole concentration range. Results of these measurements have been listed in the Table 2 to enable other authors to employ them in future computational treatment of the system.

The phase transition temperatures of pure components (Tl2Se and Ag2Se) of the title system, prepared for needs of the present study, have been determined and compared with available

Conclusions

The phase diagram for the thallium(I) selenide - silver selenide system has been constructed in its final form based on data obtained by three experimental methods: electrochemical [3], thermal and X-ray diffraction (this work).

The results of the present study differ considerably from those of [1,2] in a large majority of details: phase transition temperatures, compositions corresponding to characteristic points of the system and formulae of the chemical compounds, including their nature.

It

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was financed with national funds for scientific research by the Ministry of Science and Higher Education, Poland (Grant No. ST. D050.18.003).

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