Se Te System Research Articles

Exploring the Structure and Properties of VwSeyTe2−y Mixed Crystals in the VTe2–VSe2 System

Vanadium (IV) chalcogenide materials are of increasing interest for use in catalysis and energy conversion-related applications. Since no ternary compounds are yet known in the V–Se–Te system, we studied ternary VwSeyTe2−y (w = 1.10, 1.13; y = 0.42, 0.72) phases crystallizing in space group P3¯m1 (no. 164). Two single-crystal specimens with differing compositions of a solid solution were obtained using the ceramic method. All products were characterized by either single-crystal or powder X-ray diffraction. The lattice parameters increase with rising tellurium content in accordance with the larger ionic radius of the tellurium anion compared to selenium. The chemical compositions were confirmed by energy-dispersive X-ray spectroscopy. Furthermore, magnetic measurements mostly revealed antiferromagnetic properties. Simultaneous differential scanning calorimetry/thermogravimetric analyses in a nitrogen atmosphere showed endothermic decomposition accompanied by the formation of VN. The decomposition of VSe and VTe was observed in an argon atmosphere. The results of this work can serve as a basis for the synthesis of new phases in the V–Se–Te and related vanadium chalcogenide systems.

The influence of mercury vapor on the electrical resistance of chalcogenide amorphous films

Using the planar structures "Ni layer - chalcogenide amorphous film - Ni layer" and "graphite probe - chalcogenide amorphous film graphite probe" samples, the influence of mercury vapor on the electrical resistance of amorphous films of the Se-Te, Se-Sb and Se-As systems was investigated. It was established that exposure of samples in mercury vapor leads to a decrease in their electrical resistance by 4-7 orders of magnitude. As the temperature and mercury concentration increase, the transition time from a high-resistance state to a low-resistance state decreases. When introducing Te, Sb, and As into amorphous selenium and increasing their concentration in the composition of the films, the transition time increases, and the value of the change in resistance decreases. It was established that the change in resistance is mainly determined by the change in surface conductivity of chalcogenide films. A decrease in the electrical resistance of selenium-containing amorphous films modified with mercury is caused by the formation of HgSe crystalline inclusions in their matrix.

Phase diagrams of Bi–Sb–Se–Te system

Bi–Sb–Se–Te is one of the most important material systems for thermoelectric applications. Phase diagrams of its constituent binary and ternary systems are reviewed and assessed. The Bi–Sb–Se–Te isothermal section tetrahedron at 400 °C and liquidus projection tetrahedron were proposed. Ternary compounds are only found in the Bi–Sb–Se system. There are eight three-phase regions at 400 °C and seven primary solidification phases in the Bi–Sb–Se system, including (Bi,Sb), (Bi2)m(Bi2Se3)n, Bi2Se3, Se, Sb2Se3, Bi3Sb5Se2, Bi3Sb12Se15. In the Bi–Sb–Te system, there are four three-phase regions at 400 °C. The (Bi,Sb)2Te3 and (Bi,Sb) are continuous solid solutions. There are six primary solidification phases, including (Bi,Sb)2Te3, (Te), γ, δ, (Bi,Sb), and (Bi2)m(Bi2Te3)n. In the Bi–Se–Te system, there is one three-phase region. (Bi2)m(Bi2Se3)n and (Bi2)m(Bi2Te3)n form a continuous solid solution phase at 400 °C. There are four primary solidification phases, including Bi, (Bi2)m(Bi2(Se,Te)3)n, Bi2(Se,Te)3 and (Se,Te). In the Sb–Se–Te system, there are five three-phase regions and six primary solidification phases, including (Sb), δ-(Sb2Te), γ-(SbTe), Sb2Te3, and (Se,Te). A wide range of (Bi,Sb)2(Se,Te)3 single-phase region was observed in the Bi–Sb–Se–Te quaternary system.

A DFT study of Se n Te n clusters.

First principles calculations have been performed to study the characteristic properties of SenTen (n = 5–10) clusters. The present study reveals that the properties of these small clusters are consistent with the properties of Se–Te glassy systems. Several hundred equilibrium structures obtained from a genetic algorithm based USPEX code are relaxed to their minimum energy using the VASP code. Most of the lowest energy buckled ring-like structures are formed from Se–Te heteropolar bonds. Detailed structural analysis and distance energy plots unveil that many equilibrium structures are close in energy to their global minimum. The computed Raman and IR spectra show the dominance of Se–Te heteropolar bonds, consistent with earlier simulation and experimental findings in Se1−xTex glass materials. Low frequency vibrational modes observed in small clusters are characteristic features of amorphous materials. Non-bonding orbitals (lone pair) are observed in the HOMO, whereas the LUMO is formed from purely antibonding orbitals. The dielectric functions corroborate the bonding mechanism and slightly polar nature of SenTen clusters. The energy loss and absorption coefficient indicate the presence of π–plasmons in the UV-visible region. Furthermore, it is ascertained that the use of a hybrid functional (B3LYP) does not affect the properties of small clusters appreciably, except causing a blue shift in the optical spectra. Hence, we find that the small clusters have bearing on the formation of glassy Se–Te systems.

Topological analysis and glass kinetics of Se-Te-Ag lone pair semiconductors

The physical properties of Ag doped Se based ternary glassy alloys have been examined theoretically. (Se 70 Te 30 )100−x Ag x (0.0 ≤ x ≤ 8.0) chalcogenide alloys have been prepared by quenching method. The results show that the system is in floppy mode having constant average coordination number 2. The estimated density shows an increase with the Ag content. The mean bond energy and glass transition temperature vary in a similar fashion with an increment in Ag atomic % in the Se-Te system. The value of cohesive energy (1.91 eV mol−1 to 1.97 eV mol−1) and average bond strength (24.19 kcal mol−1 to 25 kcal mol−1) has shown increasing trend in the system. From the results of the investigated samples, the covalency parameter > 90%. So, it can be assumed that compositions are stable glass formers and may be used in the infrared system.

Phase diagram of Pb–Se–Te system I: Experimental study

Pb–Se–Te ternary system is of significant importance for thermoelectric applications. However, no systematic study of its phase diagram has been carried until now. Samples of Pb–Se–Te ternary alloys were prepared, their phase equilibrium phases were determined, and the Pb–Se–Te isothermal sections at 350 °C and 500 °C were proposed based on the experimental results. No ternary compounds were found. There is one three phase field, Pb(Se,Te)+liquid(Se,Te)+(Se,Te) at 350 °C and no three phase field at 500 °C. PbSe and PbTe form a continuous solid solution at both temperatures.

The study of the Pb–Se–Te phase diagram: Part 2 – The thermodynamic assessment of the Se–Te and Pb–Se–Te systems

Pb–Se–Te ternary system is of significant importance for thermoelectric applications. In spite of this, no systematic experimental or theoretical study of its phase diagram has been carried out up to now. The CALPHAD type theoretical assessment was done in the scope of this work for the Se–Te and Pb–Se–Te systems based on our own experimental work and the experimental results from literature. It was found out that the reassessment of the existing Se–Te phase diagram assessments is necessary because of some discrepancies between the experimental results and calculations for this system. This new dataset was consequently used for the new assessment of the Pb–Se–Te system and very good agreement was reached with existing experimental data.

Effect of thermal annealing on the optical stability of amorphous Ge–Se–Te films

Ge–Se–Te chalcogenide films with Ge content from 10% to 27% were prepared using thermal evaporation. The films were annealed with different times, and the changes of the linear refractive index and optical band gap of the films with different chemical compositions were investigated. It was found that, after 30 h of annealing, the Ge20Se8.5Te71.5 film exhibits the smallest change ratio in terms of linear refractive index (<0.5%), optical band gap (<1.5%), and thickness (<2.5%). Therefore, this component has the best optical stability in the Ge–Se–Te system studied in this paper. The optical band gap of Ge20Se8.5Te71.5 is about 0.8 eV, and the refractive index exceeds 3.4, which is beneficial to the applications in Te-based optical waveguide devices.

Thermodynamic assessment of the Ga–Se–Te system

In this study, the Ga–Te binary system was reassessed by means of the CALPHAD method using a modified lattice stability parameter for Te as well as experimental data for this binary system. The two-sublattice ionic solution model was applied for the liquid phase, and the intermediate phases were described by the sublattice model. A set of self-consistent thermodynamic parameters was optimized for all the phases in the Ga–Te binary system, which reproduced the phase diagram and the thermodynamic properties well. Using the reevaluated Ga–Te system, previously assessed Ga–Se system, and modified Se–Te system, a critical evaluation of the Ga–Se–Te ternary system was performed. The calculated vertical sections, isothermal sections, and liquidus projection agreed reasonably well with the experimental data. Immiscibility in the liquid phase was observed, and the origin of this behavior is discussed from a thermodynamic perspective.

Effect of Bi addition on optical properties of Se–Te–Bi films

The present paper reports the optical properties of Se80−xTe20Bix (0 ≤ x ≤ 12) films. Melt quenching method has been employed to prepare amorphous bulk samples, and thermal evaporation method has been used to deposit films of the samples under the vacuum of 10−5 torr. The structural characterization of films was carried out using X-ray diffraction and transmission electron microscopy. The X-ray diffraction studies showed that samples are amorphous in nature, while surface morphology results indicated the formation of nanorods of diameter 20–100 nm. The absorption and transmission spectra were recorded in the wavelength range 400–2500 nm, and data were used to determine refractive index, thickness of film, extinction coefficient and band gap of film samples. It was found that refractive index increases from 1.76 to 1.94 on increase in Bi wt% in the samples. Extinction coefficient also shows the same trend. This is due to increased polarizability of the larger Bi atoms. Further, the results show that energy band gap decreases on increase in Bi concentration in Se–Te–Bi nanostructured system. The value of Eg for x = 0 is 1.43 eV, while for x = 12, it decreases to 0.27 eV. The decrease in band gap is due to increase in the number of localized states, which eventually narrows the optical band gap. The values of band tail width (Ee) were also found to increase from 0.064 to 0.460 on Bi addition to Se–Te system. The small value of band gap and extinction coefficient makes these nanostructured films suitable for nano-electronic devices and anti-reflection multi layers.

Electrical conduction mechanism in films of Se80−xTe20Bix (0 ≤ x ≤ 12) glassy alloys

This paper reports the study of DC electrical conductivity of films of Se80−xTe20Bix (0 ≤ x ≤ 12) glasses prepared using physical vapor deposition method. The films were structurally characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM results indicate the formation of nanorods within the films. The electrical conductivity of the samples was studied using Keithley electrometer in the temperature range 303–373 K. The results show that conduction in these samples takes place via thermally assisted tunnelling and variable range hopping of charge carriers corresponding to higher and lower temperature ranges, respectively. Further, it was found that the conductivity increases with increase in Bi concentration in Se–Te system. This has been explained on the basis of chemically ordered network model. It was also found that nanorod formation improves the electrical conductivity of Se–Te–Bi system compared to bulk Se–Te–Bi system.

Combining Ultrafast Calorimetry and Electron Microscopy: Reversible Phase Transformations in SeTeAs Alloys.

Reversible amorphous–crystalline phase transitions are studied using complementary ultrafast differential scanning calorimetry and transmission electron microscopy techniques, which together allow a wealth of thermal and structural properties to be determined. The SeTe(As) system is investigated because these chalcogenide based materials have favorable properties as a phase-change memory material and in optical systems. Using calorimetry, we find that the addition of 10 at. % As to SeTe alloys strongly increases their glass forming ability, increasing both glass transition and crystallization temperatures while reducing critical quench rate. Ex situ investigation of SexTe90–xAs10 using electron microscopy and elemental mapping reveals a two-phase lamellar segregation mechanism, where a trigonal SeTe-phase and an amorphous As-rich phase are formed. These findings demonstrate the power of combining thermal and structural analysis techniques.

Correlation between structural and thermodynamic properties of some selenium based phase-change materials

In this study, we prepared novel selenium rich multi-component glasses by incorporating In, Cd and Sb as foreign elements in an Sn containing SeTe system in order to study their metal-induced effects on the thermal properties of the parent ternary glass. In particular, we determined the thermodynamic parameters of Se80Te18Sn2 and Se80Te8Sn2M10 (M = Cd, In, Sb) glassy semiconductors in a non-isothermal environment using the differential scanning calorimetry. Calorimetric measurements were obtained in the glass transition regions for Se80Te18Sn2 and Se80Te8Sn2M10 (M = Cd, In, Sb) glasses to determine their thermodynamic parameters such as the specific heat, enthalpy, and entropy during glass transition. We analyzed the variation in the specific heat before and after the heat capacity jump in these alloys. The metal-induced effects of foreign elements on the thermodynamic properties of the parent glass were also investigated in terms of the influence of the elemental specific heat of the added elemental metal as well as the thermal stability and glass-forming ability of the glasses.

Investigations of crystallization kinetics of Se82-xTe18Sbx (x =0, 4, 8, 12) glassy alloys by iso-conversional approach

At different heating rates, the crystallization temperature (Tc) are obtained from the non-isothermal differential scanning calorimetry (DSC) measurements of bulk Se82–xTe18Sbx (x = 0, 4, 8 and 12) chalcogenide glasses prepared by melt quench technique. It was observed that the crystallization temperatures increase with increasing heating rates and also by increasing Sb concentration in Se–Te system. The activation energy of crystallization (Ec), the Avrami exponent (n) and rate of crystallization (Kp) have been determined from the different models. Glassy nature of as prepared samples has been confirmed using DSC techniques.

Melt–gas phase equilibria and state diagrams of the selenium–tellurium system

The partial pressures of saturated vapor of the components in the Se–Te system are determined and presented in the form of temperature–concentration dependences from which the boundaries of the melt–gas phase transition are calculated at atmospheric pressure and vacuums of 2000 and 100 Pa. The existence of azeotropic mixtures is revealed. It is found that the points of inseparably boiling melts correspond to 7.5 at % of Se and 995°C at 101325 Pa, 10.9 at % at 673°C and 19.5 at % at 522°C in vacuums of 2000 and 100 Pa, respectively. A complete state diagram is constructed, including the fields of gas-liquid equilibria at atmospheric and low pressures, the boundaries of which allow us to assess the behavior of selenium and tellurium upon distillation fractionation.

Polyhedration of the Pb–S–Se–Te system

Polyhedration of the Pb–S–Se–Te system

Role of Sb Substitution on Electrical Properties of Se-Te Glasses

Objectives: This paper reports the influence of Sb addition on the electrical properties on bulk samples of Se80-xTe20Sbx(x=0,6, 12) glasses. Methods/Statistical Analysis: The amorphous samples have been prepared using melt quenching method and the pellets of the bulk sample of thickness 1mm and diameter 5mm has been formed using pelletiser. The I-V measurements of pellets have been recorded on Keithley electrometer in the temperature range 298-398 K. The data obtained is analysed to calculate activation energy of conduction and temperature dependent dc electrical conductivity. Findings: The observations suggest that conductivity decreases due to formation of highly crosslinked structure when Sb is added in less amount i.e. 6 at.wt% whereas on addition of Sb upto 12 at. wt. %, conductivity increases significantly due to increase in ring structure and decrease of steric hindrance in system. Earlier reports on the electrical properties of Se-Te-Sb system provided information upto 10 at. wt. % of Sb addition to Se-Te system, whereas in present study Sb is added upto 12 at.wt.% to get its influence on electrical properties of Se-Te system. Applications/Improvements: The higher content of Sb in Se-Te system increases the electrical conductivity which makes it a potential candidate for application in electronic devices.

Experimental investigation and thermodynamic modeling of the Se–Sn–Te system

Vertical section at the SnSe–Te joint in the Se–Sn–Te system was investigated using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) and differential thermal analysis (DTA). Combined with the present experimental results and experimental information of vertical sections, isothermal sections and liquidus projection in literature, the phase diagram and the thermodynamic property of the Se–Sn–Te system were modeled using the CALPHAD (CALculation of PHAse Diagrams) technique. The liquid was described as (Se, Sn, Te, SnSe, SnTe) through an associate model. The terminal solid solution phases of Se and Te were treated as hex (Se, Te), and Sn as bct (Sn). The intermetallic compounds, α/βSnSe and SnSe2, were treated as (Sn, Te)0.5(Se, Te)0.5 and (Sn, Te)0.3333(Se, Te)0.6667, respectively, on the basis of the thermodynamic model in the Se–Sn binary system in literature and experimental solid solubility of Te in these compounds in Se–Sn–Te system. The compound SnTe was modeled as (Sn, Se)0.5(Se, Te)0.5 according to thermodynamic model in the binary Se–Te system and experimental solid solubility of Se in the Se–Sn–Te system. A set of self-consistent thermodynamic parameters of individual phases was obtained.

Non-equilibrium Phases Formed in Cu–In–Se–Te System Synthesized by Melt-Quench Method

Non-equilibrium phases formed in melt-quenched CuIn(Se x Te1−x )2 system, where x = 0.1, 0.2, 0.4, 0.5, 0.6, 0.8 and 0.9, have been studied using Rietveld refinement of the crystal structure and Raman spectroscopy. Results of structure refinement have showed that all the samples, except the CuIn(Se0.1Te0.9)2, are heterogeneous. All the observed non-equilibrium phases are quaternary system and are found to have chalcopyrite structure ( $$ I\bar{4}2d $$ ), in accordance with the CuInTe2–CuInSe2 phase diagram. The lattice constants deduced from the refinement have showed linear variation with Se content. A detailed analysis of the characteristic A1 modes present in the Raman spectrum of individual sample has corroborated the results obtained from the structure analysis. The position of A1 mode of individual phase is found to vary linearly with Se content, which suggests that CuIn(Se x Te1−x )2 system exhibits single-mode behaviour.

Synthesis and characterization of Bi doped Se–Te nanostructured thin films

The nanostructure formation has been revealed in different types of materials and therefore an increasing interest has been witnessed in recent years to study the amorphous materials. Nano-structured materials show dramatic changes in their structural, optical and electrical characteristics due to the decrease in size below excitonic limit and provide new physical and chemical properties. In the present research work amorphous Se80Te20−xBix (x=3 and 6) was prepared by melt quenching techniques. Nano-structured thin films of amorphous Se80Te20−xBix have been obtained using physical vapor condensation process on glass/Si wafer substrate. The morphology of amorphous Se80Te20−xBix films was scanned by employing field emission scanning electron microscope (FESEM). The optical absorption and reflection spectra of nano-structured films were inspected by UV/VIS/NIR spectrophotometer. The optical absorption analysis indicates the non-direct transitions predominate in nano-structured Se80Te20−xBix films. The extinction coefficient and absorption coefficient are found to decrease while optical energy band gap and the refractive index increase with Bi contents in Se–Te system.