Thermodynamic investigation of the Bi2Se3–Bi2Te3 system by the EMF method

Abstract

Binary and complex chalcogenides with a tetradimite-like layered structure are of great practical interest as topological insulators, thermoelectric, and optoelectronic materials. Their fundamental thermodynamic functions in combination with phase diagrams are important for the development and optimization of methods for the synthesis and growth of crystals. The work presents the results of a thermodynamic study of the starting compounds and solid solutions of the Bi2Se3-Bi2Te3 system using the method of electromotive forces (EMF). Various modifications of this method are widely used to study binary and complex metal chalcogenides. Studies were carried out by EMF measurements of the concentration chains of the following type: (–) Bi (solid) | ionic liquid + Bi3+ | Bi in the alloy (solid) (+) in the temperature range 300–450 K. The pre-synthesized equilibrium Bi2Se3–хTex alloys (x = 0; 0.6; 1.2; 1.8; 2.0; 2.4; 3.0) with a 0.5 at% excess tellurium were used as right electrodes. Ionic liquid (morpholine formate) with the addition of BiCl3 was used as the electrolyte. The acquired experimental data were processed by the Microsoft Office Excel 2003 computer program using the least-squares method and linear equations of the type E = a + bT were obtained. The obtained equations of the EMF temperature dependences were used to calculate the relative partial molar functions of bismuth in the alloys. The diagram of solid-phase equilibria of the Bi–Se–Te system was used to determine the equations of potential-forming reactions and the latter were used to calculate the standard thermodynamic functions of the formation and standard entropies of Bi2Se3, Bi2Te3 compounds and Bi2Se3–xTex solid solutions of the above compositions. The thermodynamic functions of the formation of Bi2Se3–xTex solid solutions from the initial binary compounds were also calculated. The results correlate well with the structural data that suggests some ordering in the arrangement of selenium and tellurium atoms in the b-phase crystal lattice of the Bi2SeTe2 composition: selenium atoms predominantly occupy the central layer of the five-layer, and tellurium atoms are located in the two outer layers.

Keywords

• bismuth selenides
• bismuth telluride
• solid solutions
• emf method
• thermodynamic properties.