Synthesis and investigation of physico-chemical properties alloys of the As2S3-Tl2Te3 system

Abstract

Introduction

One of the most important problems facing chemists and physicists today is the organization of the synthesis and study of functional semiconductor materials that can meet the requirements of the constantly developing microelectronic and optical industries. Such materials include arsenic chalcogenides with optical properties. As2S3 and As2Se3 compounds and phases based on them are photosensitive [1–8], acousto-optical, and luminescent [9–11] materials and are used in the microelectronics industry. Chalcogenide fibers based on As2S3 and As2Se3 are used to transmit light in the mid-IR region, and have also found application as a compact nonlinear medium, allowing Raman amplification [[ref:12}] and optical regeneration [13,14], wavelength conversion [15]. With the participation of thallium chalcogenides, many semiconductor materials have been obtained, both with electrical and thermoelectric properties [16–18].
In this work, the interaction between As2S3 and Tl2Te3 was studied, which is undoubtedly important  for elucidating the chemistry in the ternary mutual system As,Tl//S,Te, as well as the search for new phases with semiconductor properties.
 

Experiments

The initial components As2S3 and Tl2Te3 were synthesized from elemental arsenic grade B5, high purity sulfur, thallium Tl-000, and tellurium grade A-1; the latter was subjected to sevenfold zone purification. Alloys of the system were synthesized from As2S3 and Tl2Te3 master alloys in quartz ampoules evacuated to 0.133 Pa in a single-temperature furnace.
The study was carried out by DTA, XRD, MSA analysis, as well as by measuring microhardness by determining density.
The DTA of the alloys of the system was carried out on an NTR-73 instrument at a rate of 10 deg/min. XRF was performed on an X-ray device model D2 PHFSER with СuКα-radiation with a Ni-filter.
MSA of the alloys of the system was carried out on a MIM-8 microscope on pre-etched sections polished with GOI paste. The microhardness of the system alloys was measured on a PMT-3 microhardness tester. The density of the alloys of the system was determined by the pycnometric method; toluene was used as a filler.
 

Results and discussions

In order to elucidate the nature of the interaction between arsenic sulfide and thallium telluride, alloys were synthesized over a wide range of concentrations. The obtained samples are compact alloys with a high content of As2S3, brittle, dark brown; further increase in the content of Tl2Te3 alloys become black. The resulting alloys of the As2S3 and Tl2Te3 system are resistant to water and air, dissolve in mineral acids HNO3, H2SO4 and alkalis NaOH, KOH. The synthesis mode was chosen on the basis of a preliminary recording of thermograms of alloy synthesis. The alloys were homogenized at temperatures of 190 and 200°C for 980 h.
DTA analysis of alloys of the As2S3-Tl2Te3 system showed that two and three endothermic heating effects are obtained on the thermograms of the samples. A large number of thermal effects in the system indicates that a complex interaction has taken place.
In the As2S3-Tl2Te3 system, extensive regions of glass formation are obtained. To determine the boundary of the glass formation region, DTA, XRD, MCA, density determination, and microhardness measurements were performed.
DTA of the alloys before annealing showed that the thermograms of the studied samples have two values of softening temperatures, which on the thermograms correspond to Тg As2S3 and Тg of the new Tl2As2S3Te3 phase and are equal to 170 and 135оС. With an increase in the content of Tl2Te3, the softening temperature decreases from 170 to 135°C. Тg =135оС corresponds to the softening temperature of Tl2As2S3Te3  (Table 1). In order to crystallize glassy alloys, rich in As2S3 were annealed at 190°C, and for Tl2As2S3Te3  at 150°C for 800 hours. It was found that glassy alloys could not be crystallized in this mode.
An attempt was made to achieve crystallization from the concentration range of 0-80 mol. % Tl2Te3, having subjected them to annealing in the form of a powder for 1250 h. A microstructural study of alloys of the As2S3-Tl2Te3 system showed that they are glassy, and alloys within 80-100 mol. % Tl2Te3 are phases with crystalline inclusions. Therefore, the alloys in this region consist of glass crystals. On each phase found in the As2S3-Tl2Te3 system, the microhardness was measured before and after annealing.
As can be seen from Table 1 and 2, the microhardness of glasses based on As2S3 before annealing is (1350-1380) MPa, and after annealing - (660-690) MPa. For the new Tl2As2S3Te3 phase, the microhardness is (1130-1160) MPa, and after annealing (900-960) MPa. For the Tl2Te3 compound, the microhardness varies within (680-700) MPa.

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Table 1. Composition, DTA results, microhardness measurements and density determination of alloys of the As2S3-Tl2Te3 system before annealing (glassy)

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Tabel 2. Composition, DTA results, microhardness measurements and density determination of alloys of the As2S3-Tl2Te3 system before annealing (crystalline)

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Figure 1. State diagram of the As2S3-Tl2Te3 system
(glass formation area obtained in slow cooling mode 1 and in liquid nitrogen quenching mode 2)

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Figure 2. Diffractograms of alloys of the As2S3-Tl2Te3 system.
1-As2S3, 2-Tl2As2S3Te3, 3-Tl2Te3.
diffraction patterns differs from those for the initial components. This indicates that a new Tl2As2S3Te3 phase exists in the system. According to the results of X-ray phase analysis, it was found that the Tl2As2S3Te3 compound crystallizes in a tetragonal sygony with lattice parameters: a=11.09; c=9.60 Å, z=5, ρpucn/=6.28 g/cm3, ρX-ray=6.30 g/cm3. The results of X-ray diffraction analysis of the Tl2As2S3Te3 compound are given in Table 3.

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Table 3. Crystallographic data of the Tl2As2S3Te3 compound

In the As2S3-Tl2Te3 system based on As2S3, solid solutions reach 1.0 mol. %, and almost no solid solutions were found based on Tl2Te3. The liquidus of the As2S3-Tl2Te3 system consists of four branches of primary crystallization: α-solid solutions based on As2S3, Tl2As2S3Te3, TlTe and Tl2Te3. α-phase and Tl2As2S3Te3 form a eutectic of composition 25 mol. % Tl2Te3 and a temperature of 210°C. In the concentration range of 50-100 mol. % Tl2Te3 there is a process of eutectic equilibrium and peritectic transformation.
Within concentrations of 0-50 mol. % Tl2Te3 below the malt line, α-phases and two-phase alloys α+ Tl2As2S3Te3 crystallize. The Tl2Te3 compound decomposes at temperatures above 238oC according to the following reaction: Tl2Te3↔L + TlTe. Therefore, in the concentration range of 50-100 mol %   Tl2Te3 he three-phase fields  (L + TlTe + Tl2As2S3Te3) and (L + TlTe + Tl2Te3) are located above the solidus line. At a temperature of 195°C, peritectic  processes L + TlTe ↔Tl2Te3 occur in this region. In the concentration range of 50–100 mol % Tl2Te3, two-phase alloys (TlTe + Tl2As2S3Te3) crystallize below the solidus line. 
 

Conclusion

Thus, the phase diagram of the As2S3-Tl2Te3 system was constructed. It has been established that the As2S3-Tl2Te3 system is partially quasi-binary. A congruently melting compound at 287°С of composition Tl2As2S3Te3 is formed in the system. The resulting compound is glassy and participates in the system as a glass former. Therefore, an extensive region of glass formation is formed in the As2S3-Tl2Te3 system. According to the results of X-ray phase analysis, it was found that the Tl2As2S3Te3  compound crystallizes in a tetragonal syngony with lattice parameters: a = 11.09; c = 9.60 Ǻ, z=4, ρpucn/ 6.28 g/cm3, ρX-ray.= 6.30 g/cm3. The system based on As2S3 has a limited solubility range up to I mol % Tl2Te3. It has been established that in the system upon slow cooling, the region of glass formation extends up to 80 mol % Tl2Te3, and quenching in liquid nitrogen is about 100 mol % Tl2Te3.

References

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