PHASE EQUILIBRIA IN THE Cu2Se-SnSe-Sb2Se3 SYSTEM

Phase equilibria in the Cu2Se-SnSe-Sb2Se3 system were studied along the SnSe-Cu3SbSe3 section by means of differential-thermal and X-ray phase analysis and its phase diagram was constructed. It was found that the SnSe-Cu3SbSe3 section is stable below the solidus and is nonquasibinary thanks to the incongruent melting of the Cu3SbSe3 compound.


INTRODUCTION
Ternary and complex copper chalcogenides have attracted much attention thanks to their outstanding photoelectric, thermoelectric, non-linear optic and etc. properties [1][2][3].In particular, the Cu-Sb-Sn-X (X = S, Se) systems are of great interest for the development of new ecologically safe thermoelectric materials [4][5][6].In recent years, these compounds are very interesting due to the possibility of increasing their thermoelectric gure of merit.One of the ways to increase the thermoelectric gure of merit of these materials is to obtain solid solutions based on them.For this purpose, it is expedient to study phase equilibria in the corresponding systems [7,8].
Earlier we carried out a multitude of comprehensive studies [9-11] of phase equilibria and thermodynamic properties of complex systems based on copper chalcogenides.
The purpose of this work is to clarify the phase equilibria in the Cu 2 Se-SnSe-Sb 2 Se 3 quasiternary system along the SnSe-Cu 3 SbSe 3 polythermal section.

EXPERIMENTAL 2.1. Materials and syntheses
The initial compounds SnSe and Cu 3 SbSe 3 were synthesized by melting of elementary components of high purity (99.999 %) in vacuumed (~ 10 -2 Pa) quartz ampoules.The synthesis was carried out at temperatures 50 °C higher than the melting points of the synthesized compounds.Further ampoule with tin selenide was slowly cooled to room temperature.The ampoule with Cu 3 SbSe 3 according to the recommendations of [20] was rapidly cooled from the melt and then annealed at 600-673 K. Synthesized compounds SnSe and Cu 3 SbSe 3 were identi ed by differential-thermal analysis (DTA) and powder X-ray diffraction (XRD) method.
In order to achieve the equilibrium state in alloys, cast non-homogenized samples obtained by slow cooling of melts were ground into a powder, thoroughly mixed and pressed into tablets with a mass of 0.8-1g, and then annealed at 700 K for 500 hours.

Methods
Studies carried out by DTA and XRD methods.The differential-thermal analysis was carried out in the temperature range from room temperature to 1400 K with a heating rate of 10 K• min -1 on a differential scanning calorimeter (NETZSCH 404 F1 Pegasus system).The measurement results were processed using the NETZSCH Proteus Software.The accuracy of the temperature measurement was within ±2 K.
X-ray phase analysis was carried out at room temperature on a Bruker D8 ADVANCE diffractometer with CuKα 1 radiation.The X-ray images were indexed using Topas V3.0 software Bruker.

RESULTS AND DISCUSSION
The results of XRD of annealed alloys showed that they are two-phase mixtures of the starting compounds.This indicates the stability of this section below the solidus.For example, Fig. 1 shows X-ray image of the alloy with composition 40 mol % SnSe -60 mol % Cu 3 SbSe 3 and con rmed its biphasic composition.As can be seen, the XRD pattern of this alloy is entirely composed of diffraction peaks of the SnSe (circles) and Cu 3 SbSe 3 (triangles).
Based on the DTA data (Table ), a phase diagram of the SnSe-Cu 3 SbSe 3 section was plotted (Fig. 2).As can be seen, this section is a quasistable cross-section of the Cu 2 Se-SnSe-Sb 2 Se 3 system but is nonquasibinary due to the incongruent melting of the Cu 3 SbSe 3 compound.Solubility on the basis of SnSe (β-phase) with an extension of ~3 mol % is observed.Liquidus of the SnSe-Cu 3 SbSe 3 system consists of two branches, which characterize primary crystallization of the β-phase and solid solutions based on a high-temperature modi cation of Cu 2 Se compound (α-phase) formed along the Cu 2 Se-Sb 2 Se 3 section [2].Below the liquidus in the 0-30 mol % SnSe composition range, thermal effects related to the monovariant peritectic reaction L+α↔ = 4.46, b = 4.19, c = 11.57Å; Z = 4 [13].
Cu 3 SbSe 3 are observed.During this reaction, a threephase region L+α+Cu 3 SbSe 3 is formed.In the 30-95 mol % SnSe composition range the joint crystallization of the α-and β-phases takes place.The horizontal at 725 K corresponds to an invariant transition reaction L+α↔Cu 3 SbSe 3 +β.Crystallization is completed by the formation of a two-phase mixture Cu 3 SbSe 3 +β.КОНДЕНСИРОВАННЫЕ СРЕДЫ И МЕЖФАЗНЫЕ ГРАНИЦЫ, ТОМ 20, № 2, 2018 4. CONCLUSION The SnSe-Cu 3 SbSe 3 section is studied by means of the DTA and XRD methods and its phase diagram is constructed.It was established that the SnSe-Cu 3 SbSe 3 is a nonquasibinary and stable below the solidus crosssection of the phase diagram of the Cu 2 Se-SnSe-Sb 2 Se 3 system.