Physical properties of barium borate glasses determined over a wide range of compositions

Barium borate glasses are reported over an extended glass-forming range from R=0.2 (16.7 mol% BaO) to R=2.0 (66.7 mol% BaO), where R is the molar ratio of barium oxide to boron oxide. The density, T_g (glass transition temperature), T_x (glass recrystallization temperature), and optical cutoffs were determined. These data were compared with structural models for the glasses based on nuclear magnetic resonance results of Greenblatt and Bray. The barium borate data were also compared with similar studies of lithium and lead borate glasses. Key results observed include: (1) the density trends are understandable in terms of the abundance of the basic borate groups with the fraction of tetrahedral borons being the most important single factor, and (2) the T_gs are anomalously high when compared to either the alkali or lead borate systems.     

Local and medium range order in germanium chalcogenide glasses

The results of EXAFS measurements on GeX_x X = S, Se and 1.5 x 15) binary are reviewed. This structural study is completed by anomalous wide angle X-ray scattering experiments for vitreous GeSe₃ and GeSe₅. Thus, the local and the medium range order of these glasses have been charaterized.     

Refractive index and material dispersions of multi-component oxide glasses

Refractive index dispersion curves in the wavelength region of 0.40 to 5.03 μm are presented for multi-component oxide glass systems: borate, silicate, aluminate, germanate, tellurite, antimonate and heavy metal gallate. The material dispersion was determined using the refractive index data. Reflection spectra in the ultraviolet and infrared regions were measured to investigate the effects of electronic transitions and lattice vibrations on the material dispersion. Thallium tellurite, thallium antimonate and lead gallate glasses exhibit zero material dispersion wavelengths (ZMDWs) over 2.4 μm. The factors affecting the ZMDW are discussed.     

Refractive index and dispersion of glasses with different degrees of linking

The wavelength dependence of the refractive index of 20 chalcogenide glasses with different structures and different degrees of linking is analysed by the use of a 2-term Selmier dispersion formula. The infrared term, which characterizes the contribution of oscillation transitions to the refractive index, is to be neglected in the first approximation in contrast to the term describing the interband optical transitions. That means, that the refractive index dispersion can be described by a single-oscillator-model of the form n² ? 1 = A₁λ²/(λ²-λ²₁). For glass-forming compounds the resonance wavelength λ₁ detected by the dispersion calculations is in good agreement with the position of the strongest maximum of the ?₂-spectrum. In a first approximation the refractive index n and the materials dispersion dn/dλ are representable as linear functions of the parameters A₁, a measure of strength of the optical interband transitions and λ₁, the resonance wavelength of the “electron-oscillator”. The model used is equivalent to the single-effective-oscillator-model of Wemple and Di Domenico.     

Infrared conductivity of chalcogenide glasses

The general features of the conductivity spectra of chalcogenide glasses in the frequency range 10⁹ to 10¹³ Hz are briefly reviewed.     

Optical properties of chalcogenide glasses

Over the past two decades chalcogenide glasses have been researched in order to assess their suitability as passive bulk optical component materials for 3–5μm and 8–12μm infrared applications. This research has led to a greater understanding of the physical properties of these materials, and the present paper concentrates on the optical properties and applications of these bulk chalcogenide glasses. The various factors influencing the intrinsic and extrinsic optical loss mechanisms in materials are discussed, numerical data on the basic optical properties of chalcogenide glasses is presented and applications are discussed.     

Dielectric relaxation study of chalcogenide glasses

The paper reports dielectric measurements carried out for a variety of threshold and memory alloys of glassy AsGeTe and SeGeTe at different temperatures (83 to 373 K) and various frequencies (0.2, 0.5, 1.0, 2.6 and 5.0 MHz). It is found that the glassy system of chalcogenides exists in the form of molecular dipoles which remain frozen at low temperatures and, as the temperature is increased, the molecules attain freedom of rotation at temperatures which are sometimes as low as 253 K. All the materials displayed dielectric dispersion in the radio frequency range. Gevers' formula has been used to calculate the dielectric loss (?′') and loss-angle (tan δ) from the measured values of the real part of dielectric constant (?′). The curves: log ?′ versus temperature, ?′ versus log ω, ?″ versus log ω, tan δ versus log ω and tan δ versus temperature, gave a direct evidence of the existence of a Debye-type relaxation having a wide distribution of relaxation times.Cole-Cole diagrams have been used to determine the distribution parameter (α) and the molecular relaxation time (τ). The temperature dependence of α and τ for all the alloys is consistent with the “molecular relaxation mechanism”. The paper also reports accurate values of the static and optical dielectric constants for all the alloys.Eyring's relaxation rate equations have been used to determine the free energy of activation (ΔF), and enthalpy of activation (ΔH) for all the alloys. These results indicate the existence of a stronger intermolecular interaction for SeGeTe alloys. mott's concept of “dangling bonds” has also been used to explain the existence of a stronger intermolecular interaction, and hence a greater density of defect states in case of SeGeTe as compared with AsGeTe alloys.It has been finally concluded that the dielectric behaviour of chalcogenide glasses, in general, can be successfully explained by using the theory of molecular relaxation.     

The structure of some chalcogenide glasses

A study using replica electron microscopy, scanning electron microscopy, electron diffraction, X-ray diffraction, sputter etching and differential thermal analysis of the structural properties of glasses having a range of compositions within the AsTeGeSi quaternary system has shown that phase separation generally occurs in the bulk material. The second phase is dispersed as inclusions in the non-crystalline matrix and is apparently non-crystalline away from the boundary of the glass forming region but is crystalline near to the high Te boundary. The crystalline second phase is probably Te. This quaternary system, AsTeGeSi is often used as source material for the fabrication of devices studied for their monostable electrical switching behaviour and the phase separation observed in these materials needs to be considered in the interpretation of the switching properties and for failure analysis of such devices.     

Electrical characterizations of silver chalcogenide glasses

We present a study of the electrical properties of silver chalcogenide glasses ‘40AgI’–30Ag₂S–30GeS₂, 45AgI–27.5Ag₂S–27.5GeS₂ and 50AgI–25Ag₂S–25GeS₂ in the 77–400 K temperature and the 20 Hz to 1 MHz frequency ranges. In our temperature range, a large variation of the real permittivity is observed, in relation with an electrodes polarization effect. As the amount of silver iodide increases in the Ag₂S–GeS₂ matrix, the glass transition temperature and the activation energies decrease, the electrical conductivity increases and reaches 4 Ω⁻¹ m⁻¹ at room temperature for the glass with 50% AgI. The study of the conductivity shows a behavior due to a high ionic conductivity, thermally activated with E_dc = 0.21 eV, E₁ = 0.075 eV (40AgI–30Ag₂S–30GeS₂, 45AgI–27.5Ag₂S–27.5GeS₂), E_dc = 0.17 eV, E₁ = 0.055 eV for 50AgI–25Ag₂S–25GeS₂. For these glasses, we have seen three conductivity regimes. The first two terms are thermally activated. The third term cannot be actually clearly identified because either it is thermally activated with a very low activation energy and frequency dependent, or it is almost non-thermally activated and frequency dependent.     

Non-linear properties of chalcogenide glasses and fibers

High purity chalcogenide glasses were prepared in the series As₂S_(3?x)Se_x where x = 0 to 3. The measured third order non-linearities increase with the value of x, and are up to about 1000 times larger than silica for As₂Se₃ glass. We show that the anharmonic oscillator model, using the normalized photon energy, gives an excellent fit to the data over three orders of magnitude. Single mode optical fibers based on As₂S₃ and As₂Se₃ glasses have been fabricated using the double crucible technique and the Stimulated Brillouin Scattering (SBS) investigated. The threshold intensity for the SBS process was measured and used to estimate the Brillouin gain coefficient. Preliminary results indicate record high values for the figure of merit and theoretical gain, compared to silica, which bodes well for slow-light based applications in chalcogenide fibers.     

Structure of chalcogenide glasses by neutron diffraction

The purpose of this work is to study the change in the structure of the Ge–Se network upon doping with Ag. We report here a neutron diffraction study on two glasses of the system Ag_x(Ge_0.25Se_0.75)_100−x with different silver contents (x = 15 and 25 at.%) and for two different temperatures (10 and 300 K). The total structure factor S(Q) for the two samples has been measured by neutron diffraction using the two-axis diffractometer dedicated to structural studies of amorphous materials, D4, at the Institut Laue Langevin. We have derived the corresponding radial distribution functions for each sample and each temperature, which gives us an insight about the composition and temperature dependence of the correlation distances and coordination numbers in the short-range. Our results are compatible with the presence of both GeSe_4/2 tetrahedra and Se–Se bonds. The Ag atoms are linked to Se in a triangular environment. Numerical simulations allowing the identification of the main peaks in the total pair correlation functions have complemented the neutron diffraction measurements.     

Infrared absorption of some high-purity chalcogenide glasses

New compouning techniques were devised to prepare high-purity Ge₂₈Sb₁₂Se₆₀ (TI 1173)and Ge₃₃As₁₂Se₅₅ TI 20). The methods were based on the combination of the reactant purification and compounding steps. The goal of the program was to establish the absorption limit for the glasses and to lower the absorption at 10.6 μm. At the present purity level, the GeSbSe glass is found to have an absorption level of about 0.01 cm^?1 at 10.6 μm while the absorption level for the GeAsSe glass is 0.05 cm^?1. Underlying causes for the limits are discussed along with the possibilities for improvement.     

The thermal conductivity of some chalcogenide glasses

The temperature-dependent thermal conductivities of several chalcogenide glasses were determined above 300 K. At 300 K, the thermal conductivity for most of these glasses is about 3 mW/cm · deg.     

Neutron thermodiffractometry study of silver chalcogenide glasses

Silver-containing chalcogenide glasses are of considerable interest for applications in optical and electrical recordings and as solid electrolytes. Therefore, insight in their structural properties is important. A neutron thermodiffraction study of Ag_x(Ge_0.25Se_0.75)_100−x glasses with x = 15 and 25 at.% was carried out. The amorphous samples were heated in situ from room temperature up to 350 °C, while the neutron diffraction spectra were collected every degree. For both glasses the Ag₈GeSe₆ phase appeared first, followed by the crystallization of GeSe₂. The crystallization process for the Ag-rich glass was more complex with the appearance of a non-identified third phase. Rietveld refinement helped us in studying the evolution of the lattice constants of each phase upon heating.     

A Study on the phase transition in decamethylosmocene crystal over a wide temperature range

The reversible structural phase transition accompanied by the doubling of the unit cell parameter c and a change in the space group is found in the decamethylosmocene crystal Cp ₂ ^* at a temperature of 243 ± 5 K. In the high-temperature (HT) phase (space group P2₁/m, Z = 2), the molecule adopts an eclipsed conformation and lies in the mirror symmetry plane. In the low-temperature (LT) phase (space group P2₁/c, Z = 4), the molecule occupies the general position with a small rotation of the ligands. The structural transformations from the high-temperature phase into the low-temperature phase at 120 K are described by the deviation of the molecular axis of fivefold symmetry by 3.2(2)° and the displacement of the center of mass of the molecule by 0.173(2) Å with respect to their positions in the high-temperature phase, and also by the rotation of the Cp* ligands in the same direction by 3.0(2)° and 5.4(2)°, respectively.     

Microstructural modification of chalcogenide glasses by femtosecond laser

With the irradiation by femtosecond (fs) laser with high repetition rate, GeS₂ micro/nano-crystalline formation and microstructural modification occurred in pseudo-binary GeS₂–In₂S₃ glass, while almost no similar change was observed in GeS₂ glass. The addition of In₂S₃ is beneficial for the precipitation of GeS₂ micro/nano-crystals. It is expected that functional micro/nano-crystals can be controllably prepared in chalcogenide glasses by fs laser irradiation through glass composition design.     

Temperature dependence of dielectric losses in chalcogenide glasses

The frequency (2 × 10⁴?2 × 10⁷ Hz) and temperature (250–600 K) dependence of the imaginary part of the permittivity, ?′', of As₂Se₃ and Tl₂SnSe_8.61 is explained within a theoretical model. The dielectric premittivities of these compounds are correctly explained by means of a model of a hopping process over a potential barrier between localized sites.We have found that the glassy system of chalcogenide can exist in the form of dipoles. The theory shows, in agreement with the experimental results, that ?′' = Aω^m, where m is a function of temperature.