The quasi-binary thallium(I) selenide – bismuth(III) selenide system is one of possible (reasonable) polythermal cross sections of the ternary system Tl - Bi - Se. It is quite interesting because of chemical compounds formed by its components. The compounds can exist in both the glassy and the crystalline states, and have advantageous thermoelectric properties.
The phase equilibria in the system Tl2Se – Bi2Se3 were studied three times [1 - 3], the main experimental method being DTA. The data published in the papers are generally in agreement with one another only in that in the system two chemical compounds are formed: TlBiSe2 and Tl9BiSe6. The many inconsistencies relate to a problem whether or not the compounds are phases of variable composition and what are their homogeneity ranges and melting temperatures.
The system Tl2Se – Bi2Se3 is analogous to the Tl2Te – Bi2Te3. The phase diagram of the latter system has been precisely determined earlier by us [4] employing common thermal analysis (TA). From the diagram it followed that four compounds were formed in the system: TlBiTe2 and Tl9BiTe6 as in the former one, but also Tl0.83Bi1.06Te2 and TlBi7Te11. Accordingly, it seemed to be quite interesting to re-examine the Tl2Se – Bi2Se3 system using TA method which is different from DTA applied in [1 – 3]. The aim of the re-examination was to test how the analogy between the two systems under discussion, reflected on their phase diagrams.
Investigation of the system Tl2Se – Bi2Se3 by the TA method appeared difficult because of easy penetration of selenium through the alumina well of the thermopile Pt/Pt,Rh which resulted in its damage. Another difficulty was that the molten Tl2Se + Bi2Se3 mixtures frequently solidified as glasses which made the phase transition temperature measurements impossible. Notwithstanding, the phase diagram for the system Tl2Se – Bi2Se3 was successfully established. It appeared that the components of the system formed three compounds: TlBiSe2, Tl9BiSe6 and Tl0.78Bi1.07Se2. Only the first of the compounds was incongruently melting.
In such a way it has been shown that the phase diagram of the title system widely resembles that of Tl2Te – Bi2Te3 which was to be expected because of the analogy between them.
References
[1] Zbigli K.R., Raevskii S.D.: Izv. AN SSSR, Neorg. Mater., 20, 1984, 211.
[2] Barchii M.B., Lazarev V.B., Peresh Y.Y., Voroshitov Y.V., Tkachenko V.Y.: Izv. AN SSSR, Neorg. Mater., 24, 1988, 11.
[3] Babanly M.B., Zamani I.S., Akhmadyar A., Kuliev A.A.: Zhur. Neorg. Khim., 35, 1990, 1285.
[4] Gaweł W., Zaleska E., Terpiłowski J.: J. Thermal Anal., 35, 1989, 59.