Character of Chemıcal Interactıon in Bi₂Se₃-Tb₂Se₃ Systems

Modern scientific and technical progress is inextricably linked with the development of semiconductor technology [1-3]. The rapid development of the latter was the main stimulus for the search for complex semiconductor materials [4-6]. However, the growing need for semiconductor technology in materials is not yet fully satisfied due to the lack of materials with different combinations of optical, magnetic and electrophysical properties.

Terbium chalcogenides of the composition Tb₂X₃(X-Se, Te) and solid solutions based on them are used as thermoelectric materials in the manufacture of n-legs of thermoelectric devices. Chalcogenides of rare earth elements and alloys based on them are promising compounds for the development of thermoelectric materials [7,8]. Therefore, obtaining new materials based on them is an urgent task [9-11].

For the synthesis of samples, terbium metal ingots TbM-1 purity 99.9%, bismuth grade (B-4) and selenium (B-4) were used. The synthesis mode was selected based on the physicochemical properties of the elementary components, binary compounds. The alloys were obtained by direct fusion of the components in evacuated quartz ampoules at 1600K, followed by slow cooling with the furnace turned off.

Samples with a content of 60mol% Tb₂Se₃ and higher were obtained as a sinter. They were re-crushed and converted into tablets. Alloys with a content below 60mol% are compact, dark gray in color with a metallic luster.

To achieve homogeneity of the alloy after synthesis, it was additionally annealed at temperatures 50-100K below the solidus for 250g. The obtained samples were subjected to detailed physicochemical study. The heating and cooling curves of the alloys were recorded on a Termoksan, X-ray diffraction was performed on an X-ray diffractometer.

Differential thermal analysis alloys of the system were carried out on a TERMOSKAN-2 device with an accuracy of 3-5°C, a chromelalumel thermocouple, and calcined Al₂O₃ served as the standard. Heating rate of 9 degrees/min. High-temperature differential thermal analysis was carried out on a device (HTDA) - 8m₂ in an inert atmosphere using a B-B/Re thermal suspension, degreasing speed of 5rev/min.

X-ray phase analysis was performed on an X-ray instrument of the D2 PHASER model through the use of CuKα radiation with a Ni filter. The micro-structural analysis of alloys was carried out using an MIM-8 microscope. In the study of alloy microstructure, an etchant of composition 1 N HNO₃ + HF = 2:1 was used, the etching time was 20s. The microhardness of the phases was measured on a PMT-3 instrument with an accuracy of 5%, and the density of the samples was determined by the pycnometric method.

The amount of selenium was taken in excess to maintain the stoichiometric composition. The thermograms of the alloys show that the observed heating effects are endothermic and reversible. The results of differential thermal analysis are given in Table 1.

Table 1:Results of differential thermal analysis.

Based on the results obtained by the above methods, a phase diagram of the Bi₂Se₃-Tb₂Se₃ system was constructed (Figure 1).

Figure 1:Phase diagram of the Bi₂Se₃-Tb₂Se₃ system.

As can be seen from the figures, the systems are quasi-binary and eutectic. From the phase diagram of the Bi₂Se₃-Tb₂Se₃ system, it is clear that at a ratio of 1:1, peritectic formation of a compound of the TbBiSe₃ composition occurs at a temperature of 1150K.

L + Tb₂Se₃ ↔ TbBiSe₃

A solubility region of up to 5mol.% at 300K was found. TbBiSe₃ forms a eutectic with Bi₂Se₃ at 85mol.%. The compound TbBiSe₃ with an α-solid solution based on Bi₂Se₃ forms a eutectic containing 15mol% Tb₂Se₃, which melts at 1150K. The solubility of Tb₂Se₃ in Bi₂Se₃ at 300K is 5mol%.

By indexing the X-ray diffraction patterns of TbBiSe₃ powders, it was established that the compound crystallizes in the orthorhombic syngony with the Sb₂S₃ type structure. The unit cell parameters are a=12.25, b=12.66, c=4.76Å.

1. Based on the results of physicochemical analysis, phase diagrams of the Bi₂Se₃-Tb₂Se₃ systems were constructed.
2. It was established that a ternary compound of the composition TbBiSe₃ is formed in the Bi₂Se₃-Tb₂Se₃ system. The TbBiSe₃ compound crystallizes in the orthorhombic syngony of the stibnite type: a=12.25, b=12.66, c=4.76Å.

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