Low temperature elastic behaviour of As-Sb-Se and Ge-Sb-Se glasses

The ternary glasses of arsenic and germanium with antimony and selenium can be prepared in large sizes for optical purposes. The elastic behaviour of eight compositions of each glass has been studied down to 4.2 K using a 10 MHz ultrasonic pulse echo interferometer. The glasses have a normal elastic behaviour, with the velocities gradually increasing as the temperature is lowered. An anharmonic solid model of Lakkad satisfactorily explains the temperature variations. The elastic moduli of Ge _x Sb₁₀Se_90?x glasses increase linearly as the Ge content is increased up to 25 at. % and beyond this the increase is nonlinear. (AsSb)₄₀Se₆₀ glasses show a linear increase in elastic moduli with increasing Sb content. The elastic moduli of As _x Sb₁₅Se_85?x glasses exhibit a drastic change near the stoichiometric composition As₂₅Sb₁₅Se₆₀. These behaviours have been qualitatively explained on the basis of the structural changes in glasses.     

Ionic conduction pathways in noble metal chalcohalide glasses

High-energy X-ray diffraction, neutron diffraction and extended X-ray absorption fine structure experiments have been carried out for (MI)_x(As₂Se₃)_1−x glasses (M: Ag or Cu). The addition of a large amount of MI does not significantly affect short-range orderings of the host network matrix. The interatomic distance of M–I pairs is very similar to the case of crystalline MI. However, the coordination number of M–I pairs in the present glasses seems to be somewhat smaller than that in the crystalline MI, suggesting a heavy distortion of tetrahedral coordinates of MI units or structural disorder of the I–I sub-cages in the glass state. It would be predictable that the structure model for MI–As₂Se₃ glasses is proposed to be a pseudobinary mixture of the As(Se_1/2)₃ network matrix and MI-related conduction pathways.     

Effect of replacement of Se by S on structural and physical properties of Ge–Sb–As–Se–S chalcogenide glasses

In the present study, the structural and opto-mechanical properties of Ge–Sb–As–Se–S chalcogenide glasses have been investigated. For this purpose, different bulk glasses of Ge₂₀Sb₅As₁₅Se_60?xS_x (0 ≤ x≤50) were prepared by conventional melt quenching technique in quartz ampoule and different characteristics of prepared glasses such as glass transition temperature, density, hardness, transmittance, optical band gap energy and refractive index were determined. The value of hardness and glass transition temperature of prepared glasses were found to increase with increasing the sulfur content as a result of formation of GeS₄ tetrahedral units and increasing the network connectivity and average bonding energy. The optical energy gap (according to Tauc’s relation), transmittance and refractive index of prepared glasses are in direct relation with sulfur content. In this study, the highest value of transmittance (about 70%) and lowest value of refractive index (2–2.3) was achieved in Ge₂₀Sb₅As₁₅Se₄₀S₂₀ and Ge₂₀Sb₅As₁₅Se₁₀S₅₀ glasses, respectively.     

Calorimetric study of characteristic temperatures and crystallization behavior in Ge-As-Se-Te glass system

Results of differential scanning calorimetry of high purity Ge_xAs_40−xSe₄₀Te₂₀ (x=0-40) chalcogenide glasses are reported. The glass transition temperatures and crystallization behavior were studied under non-isothermal conditions at different heating rates (2.5-35 K/min). The glass transition temperature changes from 140 °C up to 320 °C with increasing the Ge content in Ge_xAs_40−xSe₄₀Te₂₀ glass. The studied glasses with x≤35 have no exothermal peaks of crystallization, indicating their high glass-forming ability. The glass of Ge₄₀Se₄₀Te₂₀ composition has one-stage glass transition and double-stage crystallization process during phase change. The activation energy of the glass transition (E_g), the activation energy of crystallization (E_c), the Avrami exponent (n), the frequency factor (K₀) and the crystallization criteria of Ge₄₀Se₄₀Te₂₀ glass were determined.     

Multiphonon absorption in As2S3As2Se3 glasses

Multiphonon absorption in As₂S₃ and As₂Se₃ glasses is well explained by a molecular model in terms of combination bands of high frequency vibrational modes of pyramidal AsY₃ and bent AsYAs groups (Y = SorSe). Multiphonon absorption coefficients in mixed As₂S₃As₂Se₃ glasses are nearly additive in terms of the pure components, suggesting a high degree of non-random mixing.     

Infrared properties of AsχTe1−χ glasses

Infrared reflection and transmission measurements in As_χTe_1?χ glasses (45?×?55) show well-defined Gaussian vibrational absorption peaks. Comparisons with spectra observed in crystalline and glassy As₂S₃ and As₂Se₃ indicate that the local order in As_χTe_1?χ glasses is not like that found in crystalline As₂Te₃, but rather it consists primarily of AsTe₃ pyramids which are probably linked together in a fashion similar to that found in As₂S₃ and As₂Se₃.     

Magnetic resonance study of arsenic bonding sites in ternary chalcogenide glasses

⁷⁵As NQR and high-field NMR experiments have been performed on Ge_xAs_ySe_1−x−y glasses. Evolution of As bonding structure from arsenic sites with axially symmetric distribution of the electric field gradient (EFG) to highly asymmetric As surroundings has been revealed. Arsenic atoms form pyramidal structural units in Ge₂As₂Se₇ with no evidence of significant concentration of homopolar bonds. In Ge₂As₂Se₅ most of arsenic atoms form structural units with two As-As bonds per atom and asymmetric EFG distribution. Arsenic bondings become more complicated in Ge_0.33As_0.12Se_0.55 where all arsenic sites are highly distorted. The combination of NQR and NMR data provide valuable information on arsenic bonding dynamics in these glasses.     

Vitrification and transport properties in AgBr-doped chalcogenide systems

Vitrification and transport properties measurements for (AgBr)_x(As₂Se₃)_1−x glasses have been carried out in order to investigate the ion conduction phenomena of these systems. Glass samples were successfully obtained in a composition range of 0 ≤ x ≤ 0.65. The addition of AgBr leads these systems to an exponential increase of electrical conductivity. The ionic component of the electrical conductivity is dominant in highly AgBr-doped glasses. The vitrification brings the fast ion conduction to the present glass systems.     

Spectral dependence of the optical absorption temperature coefficient of CdS1−xSex nanocrystallites embedded in a silicate glass

The absorption temperature coefficient of CdS_1−xSe_x nanocrystallites embedded in a silicate glass has been studied in the temperature range above room temperature at different technological regimes and sizes of nanocrystals. To understand the optical properties of silicate glasses with semiconductor nanocrystallites, especially that at the initial stage of their formation, it is necessary to include the structural changes occurring in the nanocrystals during the heat treatment.     

Temperature and pressure dependences of the thermal conductivity of As2(Se1-xTex)3 solid solutions

The effect of temperature and pressure on the thermal conductivity of solid solutions based on the As₂(Se_{1 - x}Te_x)₃ system was investigated in glassy and polycrystalline samples at 273–450 K and hydrostatic pressures of up to 0.35 GPa. The compound As₂Se₃ was studied in a temperature range of 300–760 K. Analysis showed that the short-cange order structure in As₂Se₃ remains unchanged upon the glass—liquid transition right up to 760 K.     

Elastic properties of Se and As2Se3 glasses under pressure and temperature

The sound velocities and their pressure and temperature variations of Se and As₂Se₃ glasses have been determined by means of a pulse superpositions method, and other elastic constants and their pressure and temperature derivatives were calculated from these data. The bulk modulus was found to be 94·6 kbar for Se glass and 143·7 kbar As₂Se₃ glass, both of which are higher than the values calculated from the previous compression data. No anomaly was observed in any of the pressure and temperature dependence of elastic behavior of these glassses. Furthermore, the comparison of the pressure and temperature derivatives of the bulk modulus indicates that the thermodynamic self-consistency is satisfied on these materials. The bulk moduli of these glasses and crystalline As and Se were used to obtain an empirical bulk modulus-volume relationship for compounds in the As?Se system. The acoustic Grüneisen parameter was calculated and compared with the thermal Grüneisen parameter.     

Thermal conductivity of GexSb(As)ySe100‐x‐y glasses measured by Raman scattering spectra

We systematically measured thermal conductivity of Ge_xSb(As)₁₀Se_90−x, Ge_xSb₁₅Se_85−x, and Ge_xSb(As)₂₀Se_80−x chalcogenide glasses by measuring their Stokes and anti‐Stokes Raman scattering spectra and estimating the temperature raised by laser irradiation via the ratio of Stoke and anti‐Stokes scattering cross‐section. We aimed at demonstrating the viability of Raman scattering method for thermal conductivity measurements, and understanding the role of chemical composition in determining thermal conductivity of the chalcogenide glasses. We found that, while the values of the thermal conductivity measured in the paper are in a range from ~0.078 to 1.120 Wm^‐1K^‐1 that are in agreement with those reported data in the literatures, thermal conductivity increases before it reaches a maximum at the glass with chemically stoichiometric composition, and then decreases with increasing Ge content. We ascribed the threshold behavior of the thermal conductivity to the demixing of the structural units like GeSe₂, As₂Se₃ and Sb₂Se₃ from the main glass network. The present study demonstrated that Raman scattering method is simple and easy to measure thermal conductivity of the material. Copyright © 2014 John Wiley & Sons, Ltd.     

An evidence for a microscopic phase separation in AsSeAg glasses revealed by thermal and structural investigations

Alternating Differential Scanning Calorimetric (ADSC) studies show that Se rich As₂₀Se_80-xAg_x (0 ≤ x ≤ 15) glasses exhibit two endothermic glass transitions and two exothermic crystallization peaks; the observed thermal behavior has been understood on the basis that As₂₀Se_80-xAg_x glasses are microscopically phase separated, containing Ag₂Se phases embedded in an As-Se backbone. With increasing silver concentration, the Ag₂Se phase percolates in the As-Se matrix, with a well-defined percolation threshold at x = 8. A signature of this percolation transition is shown up in the thermal behavior, as sudden jumps in the composition dependence of non-reversing enthalpy, ΔH_nr obtained at the second glass transition reaction. Scanning Electron Microscopic (SEM) studies also confirm the microscopic phase separation in these glasses. The super-ionic conduction observed earlier in these glasses at higher silver proportions, is likely to be associated with the silver phase percolation.     

Structural relaxation and rigidity transition in aged and rejuvenated AsxSe100−x glasses

Differential scanning calorimetry (DSC) measurements were performed to investigate the kinetics of the structural relaxation in aged and unaged (rejuvenated) As_xSe_100−x glasses with 0≤x≤40. The activation energy of the glass transition (E_a) of the aged and rejuvenated glasses was determined from the variation of the glass transition temperature (T_g) with the heating rates (β). Significant effect of prolonged aging of the glasses on the values E_a was observed. Evidence of transition from floppy to rigid phase is presented. The observed significant physical aging in samples with composition x<40 indicates the absence of the intermediate phase. The compositional dependence of T_g for aged and rejuvenated data was analyzed using the stochastic agglomeration theory.     

Optical band gap and refractive index dispersion parameters of As x Se70Te30−x (0≤x≤30 at.%) amorphous films

Amorphous As _x Se₇₀Te_30?x thin films with (0≤x≤30 at.%) were deposited onto glass substrates by using thermal evaporation method. The transmission spectra T(λ) of the films at normal incidence were measured in the wavelength range 400–2500 nm. A straightforward analysis proposed by Swanepoel based on the use of the maxima and minima of the interference fringes has been used to drive the film thickness, d, the complex index of refraction, n, and the extinction coefficient, k. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model (WDD). Increasing As content is found to affect the refractive index and the extinction coefficient of the As _x Se₇₀Te_30?x films. With increasing As content the optical band gap increases while the refractive index decreases. The optical absorption is due to allowed indirect transition. The chemical bond approach has been applied successfully to interpret the increase of the optical gap with increasing As content.     

The effect of addition of gallium on the thermal stability and crystallization kinetic parameters of GaxSe100−x glass system

Differential scanning calorimetry (DSC) technique was used to study the kinetics of amorphous to crystalline transformation for Ga_xSe_100−x glass system (x=0, 2.5 and 5 at%). The kinetic parameters of Ga_xSe_100−x glass system under non-isothermal conditions are analyzed by the model-free and model-fitting models at different constant heating rates (5-50 K/min). A strong heating rate dependence of the effective activation energy of crystallization was observed. The analysis of the present data shows that the effective activation energy of crystallization is not constant but varies with the degree of crystallization and with temperature as well. The crystallization mechanisms examined using the local Avrami exponents indicate that one mechanism (volume nucleation with one-dimensional growth) is responsible for the crystallization process for heating rates 5-50 K/min for Se glass and two mechanisms (volume nucleation with two- and one-dimensional growth) are working simultaneously during the amorphous-crystalline transformation of the Ga_2.5Se_97.5 and Ga₅Se₉₅ glasses (5-50 K/min). The reaction model that may describe crystallization process of all the compositions of Ga_xSe_100−x glass system is Avrami-Erofeev model (g(α)=[−ln(1−α)]^1/n) with n=2 for Se glass. While for Ga_2.5Se_97.5 and Ga₅Se₉₅ glasses, the values of n are equal to 3 and 2 for the heating rates 5-20 and 35-50 K/min, respectively. A good agreement between the experimental and the reconstructed (α-T) curves has been achieved. The transformation from amorphous to crystalline phase in Ga_xSe_100−x glass system demonstrates complex multi-step involving several processes.     

Universal structural phase transition in network glasses

Raman and Mössbauer spectroscopy provide evidence for a transition from a molecular cluster network at x=0 to a continuous network at x=0.35 in Ge_1?xSn_xSe₂ alloy glasses. The nature of this morphological transition involves a reformation of molecular cluster surfaces in the heterogeneous phase to yield a homogeneous phase. The transition is believed to be a universal property of the easy glass formers and can be effected in one of several ways.     

Electronic structure of new iron-based superconductors: From pnictides to chalcogenides and other similar systems

The electronic spectra of new iron-based high-temperature superconductors and a number of other chemically similar compounds have been discussed and compared with the focus on iron chalcogenide K_{1 ? x} Fe_{2 ? y} Se₂ and isostructural pnictide BaFe₂As₂ (122). It has been shown that the Fermi surfaces in K_{1 ? x} Fe_{2 ? y} Se₂ are significantly different from those in pnictides. The LDA + DMFT and LDA’ + DMFT calculations have demonstrated that the effect of electron correlations in K_{1 ? x} Fe_{2 ? y} Se₂ on the electronic structure is much stronger than that in the most studied 122 system. The electronic structure of several multiband superconductors similar in chemical composition to iron-based high-temperature superconductors, but having a relatively low T _c value (such as SrPt₂As₂, APt₃P (A = Sr, Ca, La), and (Sr, Ca)Pd₂As₂), and the non-superconducting compound BaFe₂Se₃ has also been discussed. It has been shown that the electronic structure of these systems is significantly different from previously studied iron pnictides and chalcogenides. The T _c value in these systems can be understood within the simple Bardeen-Cooper-Schrieffer model.     

Compositional modification of Se-Ge-Sb chalcogenide glasses by addition of arsenic element

The modification of structural, thermal and optical properties of Se-Ge-Sb glasses by addition of arsenic element was the goal of this study. In this regards, six different glasses of Se₆₀Ge_40-xSb₅As_x (0 ≤ x ≤ 15) were prepared by conventional melt quenching technique in quartz ampoule. The produced samples were characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), UV–Vis–NIR spectrophotometer, Fourier transform infrared (FTIR) and Raman spectroscopy. The fundamental absorption edge for of the glasses was analyzed in terms of the theory proposed by Davis and Mott. Based on the obtained results, the glass transition temperature, optical energy gap and Urbach energy of prepared glasses in this alloying system were in the range of 325–380 °C, 1.43–1.64 eV and 0.03–0.3547 eV, respectively. The as prepared glasses show anomalous behavior at 5–7.5 mole% of arsenic for the glass transition temperature, transmittance, absorption edge, optical energy gap and Urbach energy. Based on the Raman spectra, the structural analysis indicates that, increasing the network connectivity upon increasing the arsenic content up to 7.5 mole% is the main reason of anomalous behavior in Se₆₀Ge_40-xSb₅As_x (0 ≤ x ≤ 15) system.     

A model for the temperature dependence of the viscosity in Cu–As–Se system

The microscopic bonding behavior of amorphous Cu_x(As₂Se₃)_1?x (x = 0.00, 0.01, 0.05, 0.10 and 0.20) is studied from the point of view of the viscosity. The model used in this analysis describes the temperature dependence of the viscosity in terms of the mean bond strength E₀, the mean coordination number Z₀, and their fluctuations ΔE, ΔZ of the structural units that form the melt. The model reproduces quite well the temperature dependence of the viscosity of Cu_x(As₂Se₃)_1?x observed experimentally. According to the theory, the fragility of the system increases as the fluctuation of the total bond strength increases. It is shown that the viscous flow is accompanied by the cooperative movements of the structural units and occurs by breaking selectively the weaker parts of the bonds between the structural units. This notion is related to the concept of Cooperatively Rearranging Region (CRR) in the theory of Adam–Gibbs. By analyzing the model, the fluctuation of the total bond strength in Cu–As–Se system is evaluated quantitatively, and the composition dependence of the fragility is discussed in terms of bond strength and the coordination number between the structural units. The analysis suggests that in the Cu_x(As₂Se₃)_1?x system, there must be a strong composition dependence in the bond strength and a weak composition dependence in the fragility and coordination number fluctuation.