Chemical Bond Approach Research Articles

Assessment of physical parameters for quaternary antimony substituted Ge-Se-Te alloys

New quaternary glassy compositions Ge16Se52Te32 − xSbx (x = 0, 2, 4, 6, 8) have been synthesized using melt quench technique. We have determined various physical parameters i.e. average coordination number, floppy modes, lone pair electrons, average heat of atomization, density, molar volume, compactness and cohesive energy. It is observed that on increasing Sb content density, cohesive energy, average heat of atomization and compactness increases while molar volume decreases. The chemical bond approach has been used to calculate the cohesive energy of the samples under investigation.

Effect of compositional variations on the optical properties of SbxSe60 -xS40 thin films

Thin films of SbxSe60-xS40(x=10, 20, 30, and 40) were deposited by thermal evaporation from the prepared bulk materials on glass substrates held at room temperature. The film compositions were confirmed by using energy dispersive X-ray spectroscopy. X-ray diffraction studies revealed that all the as-deposited films have amorphous structure. The optical constants (n, k, Eg, Ee, B1/2) of the films were determined from optical transmittance data, in the spectral range 500–1200nm, using the Swanepoel method. An analysis of the optical absorption spectra revealed an Urbach's tail in the low absorption region, while in the high absorption region an indirect band gap characterizes the films with different compositions. It was found that the optical band gap energy decreases as the Sb content increases. Finally, in terms of the chemical bond approach, degree of disorderness has been applied to interpret the decrease in the optical gap with increasing Sb content in SbxSe60-xS40 thin films. The changes in X-ray photo electron spectra and Raman shift in the films show compositional dependence.

Thermal and optical characterization of Ge–Sn–Se glass system

Thermal and optical characterization of Ge x Sn10Se90−x (x = 14, 18, 22, 26) glasses has been carried out using differential scanning calorimetry and UV–VIS–NIR spectroscopy, respectively. Bulk samples were prepared using conventional melt quenching technique. Amorphous nature of the prepared samples was confirmed using X-ray diffraction. The transmission spectra of as deposited films, for normal incidence, were recorded in the range 400–2,500 nm. Optical constants such as absorption coefficient, refractive index and extinction coefficients have been determined using the envelope method. Dispersion of refractive index has been analyzed using single-effective oscillator model. Optical band gap has been measured using Tauc plots. Effect of composition variation on thermal and optical properties has been discussed in view of chemical bond approach.

Theoretical prediction of physical parameters of GexSb20−x Te80 (x = 11, 13, 15, 17, 19) bulk glassy alloys

Abstract Physical properties of GexSb20−x Te80 (x = 11, 13, 15, 17, 19) bulk glassy alloys are examined theoretically. Lone pair electrons are calculated using an average coordination number (〈r〉) and the number of valence electrons, and are found to decrease with an addition of Ge. Mean bond energy (〈E〉) is proportional to glass transition temperature (Tg) and shows maxima near the chemical threshold. Cohesive energy of the system is calculated using chemical bond approach. A linear relation is found between cohesive energy, band gap (calculated theoretically and confirmed experimentally) and average heat of atomization. All these parameters are increasing with an increase in Ge content. A relation between average single bond energy and photon energy is discussed. Compactness of the structure is measured from the calculated density of the glass. An attempt is made to discuss the results in terms of structure of the glass or equivalently with average coordination number.

Effect of Sb additive on structural and optical properties of Se–Te–Sb thin films

Amorphous Se85−x Te15Sb x (x = 2.5, 5 and 7.5 at.%) thin films were deposited on glass substrates by thermal evaporation. The absorption coefficient was determined from the transmittance, and reflectance spectra in the wavelength range 400–2500 nm. Various optical constants have been calculated for the studied compositions. The mechanism of the optical absorption follows the rule of non-direct transition. It was found that the optical energy gap, E g, decreases from 1.42 to 1.36 eV with increasing Sb content from 2.5 to 7.5 at.%. These results can be interpreted on the basis of the chemical bond approach proposed by Bicermo and Ovshinsky. On the other hand, the optical constants such as refractive index, n, and extinction coefficient, K ex, of the films were studied. The dispersion of the refractive index was discussed in terms of single-oscillator Wemple–DiDomenico model. The influence of Sb content on the structure for the annealed films was investigated using X-ray diffraction (XRD) and scanning electron microscopy. The average particle size of the crystalline size for Te0.04Se0.96 phase deduced from XRD pattern. This average decreases with increasing Sb content.

Characterization of new quaternary Ge20Se60Sb20 − xAgx (0 ≤ x ≤ 20 at.%) glasses

The present paper reports the effect of replacement of antimony by silver on the physical parameters of new quaternary Ge20Se60Sb20−xAgx (where x=0, 5, 10, 15 and 20at.%) glasses. The average coordination number and the glass transition temperature decrease while the density, the compactness, the average heat of atomization, the cohesive energy and the overall mean bond energy increase with increasing Ag content. The overall mean bond energy increases from 59.20 to 60.99kcal/mol with an increase of the Ag content from 0 to 20at.%. The increasing cohesive energy with an increase of the Ag content has been interpreted according to the chemical bond approach. The observed dependence of the physical parameters on composition for the Ge20Se60Sb20−xAgx (where x=0, 5, 10, 15 and 20at.%) glasses can be explained on the basis of a chemically ordered network arrangement.

Dispersion and optical properties of thermally deposited Se91 − xTe9Px (x = 0, 10) films

Se91Te9 and Se81Te9P10 bulk glasses have been prepared by melt-quench technique. The amorphous nature of the as-prepared alloys is confirmed by using X-ray diffraction patterns. Thin films of Se91Te9 and Se81Te9P10 are prepared on chemically cleaned glass substrates by thermal evaporation under vacuum. Differential scanning calorimetery revealed a higher glass transition temperature for Se91Te9 in comparison to Se81Te9P10 bulk glasses. Transmission spectra of the as-prepared films are measured at normal incidence in the wavelength range of 300–2500nm. Refractive index and the film thickness were determined from optical transmittance data using Swanepoel envelope method. It was found that the refractive index dispersion data obeys the single oscillator Wemple and DiDomenico model from which the dispersion parameters were determined. The single oscillator energy, the dispersion energy, the high frequency dielectric constant and the ratio of free charge carrier concentration to the effective mass are then estimated as a function of alloy composition. Analysis of the spectral behavior of the absorption coefficient revealed an allowed indirect optical band gap in this alloy system. With phosphorus incorporation, the optical band gap is found to decrease along with a subsequent increase in the refractive index. The ensemble of these results is interpreted in terms of the chemical bond approach, electronegativity difference of the atoms, cohesive energy and overall mean bond energy.

Composition effect on the structure and optical parameters of Ge–Se–Te thin films

The present work reported the influence of Ge content variation on the optical properties of GexSe50Te50-x (x=0, 5, 15, 20, 35 at%). Vacuum thermal evaporation technique was employed to prepare amorphous GexSe50Te50−x thin films. The stoichiometry of the chemical composition was checked by energy dispersive X-ray spectroscopy (EDX), whereas the thin films structure was determined by an X-ray diffraction and a scanning electron microscope (SEM). The optical absorption measurements were performed at room temperature in the wavelength range of 200–900nm. Many optical constants were calculated for the studied thin films utilizing the optical absorption data. It was observed that the optical absorption mechanism follows the rule of the allowed direct transition. The optical band gap was found to increase from 2.31 to 2.60eV as the Ge content increases from 0 to 35 at%. This result was explained in terms of the chemical bond approach.

Thermal stability and crystallization kinetics of quaternary Sb–Se–Ge–In chalcogenide glasses

Thermal analysis plays an important role to know the characteristic behavior of alloys for various applications. Differential thermal analysis has been carried out on Sb10Se65Ge25−yIny (y=0, 3, 6, 9, 12, 15) alloys under non-isothermal conditions at different heating rates. We report the analysis of thermal parameters, viz. glass transition temperature, crystallization temperature and melting temperature. The thermal stability factor, reduced glass transition temperature, and heating rate dependence of glass transition and crystallization peaks have been investigated. The dependence of activation energies of glass transition and crystallization with In content have been discussed in terms of energy barrier and chemical bond approach respectively.

Role of 4p-3d orbital hybridization on band gap engineering of heavy metal glass for optoelectronic applications

Optoelectronic properties of glasses can be engineered by understanding the electronic structure and the symmetry of electronic states across the band gap where the chemical bonding is the origin of such electronic structure. Thus, series of heavy metal lead borate glasses of the composition 0.25B2O3–0.75PbO is prepared by melt quenching technique for Vis–IR photonics applications. Hybridization of p- and d-block elements, through co-substitution of Cr2O3 and SeO2, by B2O3, is used to tune effectively the glass electronic structure characteristics such as; band gap energy, Fermi level, and Urbach exciton–phonon coupling. Two-photon absorption coefficient is determined to elucidate the glass nonlinear sub-interband transitions. Chemical bond approach is applied to analyze and explain the obtained glass properties. The excitons excitation is discussed by applying Elliott’s model which indicates direct interband transition nature of the glass that is assisted by the existence of stable Frenkel excitons.

Optical properties of Se–Te–Sb thin films

Amorphous thin films of Se80Te(20–X)SbX (X = 0, 4, 8, and 10 at. %) were deposited onto clean quartz substrates by vapour deposition (thermal evaporation) under a vacuum of 10−5 Torr. The transmission spectra at normal incidence in the spectral range of 250–2500 nm were measured at room temperature. The optical energy gap Eg was found to be indirect. The value of Eg was found to decrease from 1.69 to 1.43 eV with increasing Sb content. The composition dependence of the optical energy gap is discussed in terms of the chemical bond approach. The band tail width of localized states Ee was found to increase from 0.0297 to 0.0551 eV with increasing Sb content.

Thermal analysis and physical properties of Bi–Se–Te chalcogenide glasses

The present work reported the effect of variation of bismuth content on the glass transition, thermal stability and crystallization kinetics of Bix(Se20Te80)100−x (0⩽x⩽12) glasses. Measurements of the heat flow as a function of temperatures and different heating rates (β) for Bix(Se20Te80)100−x glasses were carried out by using differential scanning calorimetry (DSC). Both the heating rate and Bi content are found to affect the values of the characteristic temperatures such as the glass transition temperature (Tg), the peak temperature of crystallization (Tp) and the melting temperature (Tm). From the heating rate dependence of Tg and Tp, values of the activation energy for glass transition (Eg) and for crystallization (Ec) were deduced. Afify’s method was used to derive the values of Ec without any dependence on the kinetic exponent (n). The compositional dependence of Eg and Ec was discussed in terms of the chemical bond approach. Based on the values of Tg, Tp, Tm and Ec the thermal stability criteria (ΔT, Hr, Hg, S, K(T) and Kr(T)) of the studied glasses was evaluated. It was found that, increasing the Bi additions leads the decrease of the glass thermal stability i.e. the glass without any Bi content is the most stable one.

Compositional dependence of the optical properties of amorphous Se80 − xTe20Bix thin films using transmittance and reflectance measurements

Optical properties of amorphous Se80−xTe20Bix thin films with different compositions (x=0, 2, 4, 6 and 8at.%) deposited by evaporation technique have been investigated by optical spectrophotometry measurements in the wavelength range of 400–2500nm. An optical characterization method for uniform films based on Swanepoel's ideas (envelope method) has been employed to extract the refractive index n and film thickness d, with accuracies better than 1%. It was found that the refractive index increases with increasing the concentration of Bi at the expense of Se content. The increasing in refractive index n is interpreted in terms of polarizability and the mean coordination number. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple–DiDomenico model. The fundamental parameters for the investigated semiconducting thin films Se80−xTe20Bix e.g. absorption coefficient and band gap are calculated in the strong absorption region of transmittance and reflectance spectra. The possible optical transition in these films is found to be allowed indirect transition with energy gap Egoptdecreases from 1.467 to 1.035eV with increasing Bi content at expense of Se. Finally, the decrease in the optical band gap with increasing Bi content in Se80−xTe20Bix has been explained in terms of the chemical bond approach.

Far-infrared investigation of ternary Ge–Se–Sb and quaternary Ge–Se–Sb–Te chalcogenide glasses

Chalcogenide glasses are remarkable materials for remote sensors, infrared imaging and infrared fibers due to their transparency in Far-infrared (IR) region. Far-IR absorption spectra of Ge19Se81−xSbx (x=0, 4, 8, 12, 16, 17.2, 20) and Ge19−ySe63.8Sb17.2Tey (y=0, 2, 4, 6, 8, 10) have been obtained in the range of 30–350cm−1. Three possible bond arrangement approaches viz. chain-crossing model (CCM), random-covalent-network model (RCNM) and chemical bond approach (CBA) have been used to discuss the formation of bonds. Results are explained on the basis of GeSe4, GeSe2 and SbSe3 isolated structural units which have appeared in Ge–Se–Sb system while Se6Te2 rings, GeTe4 tetrahedron, GeTe2, GeSe4 and GeSe2 structural units have appeared for Ge–Se–Sb–Te system.

Calorimetric study of thermal crystallization kinetics in Se78-xTe20Sn2Pbx(0 ≤ x ≤ 6) alloys

Glassy samples of Se 78− x Te 20 Sn 2 Pb x (0 ≤ x ≤ 6) system are prepared by melt quenching method. For non-isothermal study of crystallization kinetics, DSC scans have been taken at the heating rates 5, 10, 15 and 20 K/min in non-isothermal mode. Activation energy of crystallization ( E c ) has been calculated using Kissinger method, Matusita-Sakka method and Augis-Bennett method. Various kinetic parameters of crystallization kinetics like peak crystallization temperature ( T c ), Rate constants ( K ) and order parameter ( n ) are determined using these DSC scans. Thermodynamic parameters such as crystallization enthalpy ( ΔH c ) and entropy change during the crystallization ( ΔS ) are also evaluated. Results are discussed using chemical bond approach.

The effect of indium additive on the structural relaxation of Se–Sb–Sn semiconducting glasses

Differential scanning calorimetry (DSC) has been used to study the kinetics of structural relaxation in the glass transition region of Se80Sb12Sn8−yIny (y=0, 0.5, and 1) chalcogenide glasses, which were previously annealed at 394K below the glass transition temperature. Based on the Kovacs–Aklonis–Hutchinson–Ramos model, the dependence of the endothermic peak temperature (Tp) on heating rate has been used to evaluate the relaxation parameter. From the knowledge of this parameter the structural relaxation activation energy (Δh) has been obtained. The shift in Tp with heating rate and annealing time (ta) has been used to calculate the structural parameter, which controls the contribution of temperature and structure to the relaxation time. Results reveal that Δh decreases with In content due to the replacement of some SnSe4/2 structural units of strong bond energy by Se3In2 of weak bond energy. This reduces the cross-linking energy as well as the overall mean bond energy of the studied glasses as predicted by the chemical bond approach. The values of Δh were used to estimate the fragility index which is found to be in the range 29.7–31.7 indicating that the glasses under investigation were obtained from strong glass-forming liquids. The excess enthalpy (δH) has also been calculated, for different annealing times ta, from the knowledge of the excess specific heat. The values of δH were plotted as a function of ln(ta) and exhibit a linear behavior which indicates that the studied glasses were not completely relaxed.

Effect of Sn addition on the optical constants of Ge–Sb–S thin films based only on their measured reflectance spectra

In this work, different compositions of homogenous Snx(Ge1Sb2S7)100−x (0⩽x⩽9at.%) thin films were prepared by thermal evaporation method. The film reflectance R(λ) was measured in wavelength range 400–2500nm. A straight forward analysis proposed by Minkov based on the maximum and minimum envelopes of the reflection spectra have been applied to drive the complex index of refraction and film thickness with high accuracy. Addition of Sn content to Snx(Ge1Sb2S7)100−x thin films was found to affect the refractive index and the extinction coefficient for these films. The dispersion of the refractive index was discussed in terms of the single oscillator Wemple–DiDomenico model. The absorption mechanism of the prepared films obeys the rule of the allowed non-direct optical transitions. The refractive index increases while the optical band gap decreases with increasing Sn content. The compositional dependence of the optical band gap for the Snx(Ge1Sb2S7)100−x thin films was discussed in terms of the chemical bond approach. The plasma frequency, dielectric loss, dissipation factor and the optical conductivity of these films have been investigated.

Optical properties of Cd20Se80−xMx (M: Zn, In, and Sn) thin film alloys

Amorphous Cd20Se80−xMx (M: Zn or In or Sn and x=0 & 10) thin films were deposited on glass substrates by thermal evaporation. The spectral dependence of the absorption coefficient was determined from the transmittance and reflectance spectra, in the wavelength range 200–2500nm. Both direct and indirect electronic transitions were responsible for the optical properties of Cd20Se80 thin films while only the direct allowed transition was active for the other films alloys. The variation in the optical band gap (Eg) with the addition of the third element was discussed in terms of the width of localized states (Ee) and chemical bond approach model (CBA). The optical constants (refractive index (n) and absorption index (k)) of the films were studied and the dispersion of the refractive index was discussed using the Wemple–DiDomenico single oscillator model. The real and imaginary parts of the dielectric constant in addition to the volume (VELF) and surface (SELF) energy loss functions of the films were also determined. Generally, the addition of Zn was found to have the opposite effect to that of In or Sn addition.

The effect of composition on structural and optical properties of ZnSe alloys

Bulk of ZnxSe100−x (x=10, 20 and 40at%) was prepared by the usual quench melt technique. Thin films of these compositions were prepared by thermal evaporation. Differential thermal analyses (DTA) on powder samples under non-isothermal conditions show that the glass transition temperature Tg increased and crystallization temperature Tp decreased with increasing Zn content. The average particle size of the crystalline ZnSe phase deduced from X-ray diffraction (XRD) patterns show a decrease with increasing Zn content. These results were confirmed with the results obtained by scanning electron microscopy (SEM).The optical properties of ZnxSe100−x have been studied at a wavelength range (300–900nm), and it was found that the optical band gap (Eg) increases with increasing Zn concentration in the ZnxSe100−x system. The results can be interpreted on the basis of the chemical bond approach proposed by Bicermo and Ovshinsky. The value of the extinction coefficient (k) decreases with increasing wavelength and Zn content.On the other hand, the optical energy gap (Eg) decreased with increasing the annealing temperature for Zn10Se90 thin films. These results can be interpreted by the Davis and Motte model.

The shape of order in glasses

AbstractThe chemical bond approach to the glassy state is discussed. It is based on two concepts: Ovshinsky's idea of bond option as a necessary condition for amorphization, and Dembovsky's notions about hypervalent bonds (HVB) as alternative bonding state responsible for glass specificity and glass formation at all. The third idea is self‐organization of HVBs in the form of a bond wave, which provides an interconnected interpretation of three structural levels: short‐range order that is different at around HVB and far outside it, medium‐range order in the wavefronts populated with HVB, and long‐range order due to the bond wave itself. Experimental evidences and practical consequences of the bond‐wave picture are discussed.