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.     

Structural characterization and phase transformation kinetics of Se58Ge42−xPbx (x = 9, 12) chalcogenide glasses

The glass transition behavior and crystallization kinetics of Se₅₈Ge_42?xPb_x (x = 9, 12) have been investigated using Differential Scanning Calorimetry (DSC) at five different heating rates under non-isothermal conditions. It has been observed that these glassy systems exhibit single glass transition and double crystallization on heating. The XRD pattern revealed that the considered glasses get crystallized into GeSe₂ and PbSe/Se phases after annealing at 633–643 K for 2 h. The GeSe₂ and Se phases were found to crystallize in monoclinic structure while, PbSe phase crystallizes in cubic structure. Besides this, a mixed phase was also observed in DSC thermograms after annealing. The kinetic studies include determination of various parameters such as Avrami exponent (n), frequency factor (K_o), dimensionality of growth (m), the activation energy for glass transition (E_t) and for crystallization (E_c). The values of E_t increases while that of E_c decreases after annealing. Also, dimensionality of growth decreases to one dimension from two and three dimensions after annealing.     

Phase separation in Ag–Ge–Se glasses

Field emission-scanning electron microscopy (FE-SEM) and electric force microscopy (EFM) measurements were carried out on bulk Ag_x(Ge_0.25Se_0.75)_100−x glasses with x = 5 and 15 at.%. The presence of chemical contrasts and electrical inhomogeneity throughout the samples indicated that the glasses were phase separated. Moreover it appears that while silver-rich nodules of ∼1 μm were embedded in a silver-poor connecting phase in the glass containing 5 at.% Ag, it was the opposite that occurred in the glass containing 15 at.% Ag. Such an inversion explains the large difference of seven orders of magnitude in the conductivity of the two glasses.     

Preparation of TiO2–Na2O glass by sol–gel method and structural characterization

The applicability of sol–gel process in glass formation of binary system, (100 − x)TiO₂–xNa₂O (x = 10, 20, 30), was investigated and the glasses were prepared successfully by the sol–gel process for the first time. The process of glass formation was checked by using X-ray diffraction measurement and DTA–TG analysis. In the baking step, a DTA peak related to the crystallization of gel was found. The short-range structure of glassified samples was studied by neutron scattering measurement. It is found from the results of neutron scattering measurement that the coordination number of O atom around Ti atom is about 4, and the O atoms around Ti atom form a planer square rather than a regular tetrahedron.     

Ab initio calculation of vibrational frequencies of GexSe1−x glass

We have used the density functional theory to make the models of Ge_xSe_1?x glass for which the energy is a minimum. The clusters, Ge₂Se₂, Ge₂Se₃, Ge₃Se, Ge₃Se₂, Ge₄Se, GeSe₃, GeSe₄, chain mode zig-zag Ge₄Se₃, corner sharing GeSe₄, and edge sharing Ge₂Se₆, have been made successfully and their vibrational spectra have been calculated from the first principles. We are able to optimize the bond distances as well as the bond angles. The calculated values of the frequencies of vibrations of the various clusters have been compared with those obtained from the experimental Raman spectra of actual glasses, Ge_xSe_1?x(0 < x < 0.3). The local concentration, x within 0.25 nm is nonuniform in the amorphous material. When the same cluster occurs in two stable configurations, low frequency vibrations of frequency, ν < 100 cm^?1, are found. The corner sharing GeSe₄ has low frequency modes at 54 cm^?1 and 93 cm^?1 whereas these modes disappear in the pyramidal configuration. The low frequency modes are therefore associated with the breaking of C₄ symmetry of the pyramidal configuration. The computed vibrational frequencies of clusters Ge₃, Ge₄Se₃, Ge₂Se₃, GeSe₃ and Ge₃Se₂ are actually present in the Raman spectra of the glass, Ge_xSe_1?x(0 < x < 0.3).     

A composition dependent thermal behavior of GexSe35−xTe65 glasses

Alternating differential scanning calorimetric (ADSC) studies have been performed to understand the thermal behavior of bulk Ge_xSe_35?xTe₆₅ glasses (17 ? x ? 25); it is found that the glasses with x ? 20 exhibit two crystallization exotherms (T_c1 & T_c2). On the other hand, those with x ? 20.5, show a single crystallization reaction upon heating. The exothermic reaction at T_c1 has been found to correspond to the partial crystallization of the glass into hexagonal Te and the reaction at T_c2 is associated with the additional crystallization of rhombohedral GeTe phase. The glass transition temperature of Ge_xSe_35?xTe₆₅ glasses is found to show a linear but not-steep increase, indicating a progressive, but a gradual increase in network connectivity with Ge addition. It is also found that T_c1 of Ge_xSe_35?xTe₆₅ glasses with x ? 20, increases progressively with Ge content and eventually merges with T_c2 at x ≈ 20.5 (〈r〉 = 2.41); this behavior has been understood on the basis of the reduction in TeTe bonds of lower energy and increase in GeTe bonds of higher energy, with increasing Ge content. Apart from the interesting composition dependent crystallization, an anomalous melting behavior is also exhibited by the Ge_xSe_35?xTe₆₅ glasses.     

Crystallization of barium magnesium phosphate glasses determined by differential thermal analysis and X-rays diffraction

The scope of this work is to determine the crystalline phases of devitrified barium magnesium phosphate glasses and the glass composition which presents the best resistance to crystallization. Barium magnesium phosphate glasses with composition xMgO · (1 ? x)(60P₂O₅ · 40BaO) mol% (x = 0, 0.15, 0.3, 0.4, 0.5, and 0.6) were analyzed by differential thermal analysis (DTA) to evaluate the thermal stability against crystallization, and X-ray diffraction (XRD) to identify the crystalline phases formed after devitrification. The glass transition temperature (T_g) increases as the MgO content increases. The maximum temperature attributed to the crystallization peak in the DTA curve (T_c) increases when x increases in the range 0 ? x ? 0.3, and it decreases for x > 0.3. The most thermally stable glass composition against crystallization is for x = 0.3. After the devitrification, the number of coexisting crystalline phases increases as the MgO content increases. For x = 0.3 there is the coexistence of γBa(PO₃)₂ and Ba₂MgP₄O₁₃ phases for devitrified glasses. The trend of the T_c is explained based on the assumptions of changes in the Mg²⁺ coordination number and the amphoterical features of MgO.     

Structure and crystallization behavior of mixed potassium-lithium niobiosilicate glasses

Potassium-lithium niobiosilicate (KLiNS) glasses with a composition of (27 ? x)K₂O · xLi₂O · 27Nb₂O₅ · 46SiO₂ (x = 0, 3, 12 and 20) have been synthesized by a melt-quenching method. The glass structure and devitrification behavior have been studied by Raman spectroscopy, DTA, and XRD. By increasing the lithium content, less distorted niobium octahedra increase, indicating a niobium clustering. This change strongly affects the crystallization behavior. In the glasses x = 0 and x = 3, just above T_g, only nanocrystals of an unidentified phase are formed, while for x = 12 and x = 20 potassium lithium niobate (KLN) solid solutions with tetragonal tungsten–bronze structure crystallize by bulk nucleation. In these glasses, LiNbO₃ crystallizes at higher temperature by surface nuclei. Ultimately, it is possible to produce nanostructured glasses based on KLN nanocrystals, by partial replacement of K by Li.     

A combined 77Se NMR and Raman spectroscopic study of the structure of GexSe1-x glasses: Towards a self consistent structural model

The short-range structures of stoichiometric and Se-deficient binary Ge_xSe_100 ?x glasses with 42 ≥ x ≥ 33.33 have been investigated using a combination of Raman and ⁷⁷Se Car–Purcell–Meiboom–Gill (CPMG) spikelet nuclear magnetic resonance (NMR) spectroscopy. When taken together, these spectroscopic results allow for self-consistent assignment of average ⁷⁷Se NMR isotropic chemical shifts to Se atoms in various coordination environments in Ge_xSe_100 ?x glasses. Analysis of the compositional variation of the relative concentrations of these Se environments indicates considerable violation of chemical order in the nearest-neighbor coordination environments of the constituent atoms in the stoichiometric Ge_33.33Se_66.67 glass. On the other hand, the presence of a random distribution of Ge―Ge bonds can be inferred in the Se-deficient glasses. Simulations of the previously published ⁷⁷Se NMR line shapes of Se-excess glasses on the basis of the revised structural assignments of ⁷⁷Se NMR chemical shifts obtained in this study conclusively indicate that the structure of these glasses is intermediate between a randomly connected and a fully clustered network of GeSe₄ tetrahedra and Se chains.     

FT-IR and Raman study of silver lead borate-based glasses

Glasses from the xMnO · (100−x)[3B₂O₃ · 0.9PbO · 0.1Ag₂O] system with 0 ⩽ x ⩽ 20 mol% have been prepared and studied by means of FT-IR absorption and Raman scattering. We interpreted the spectroscopic data in conjunction with the structural information obtained by X-ray diffraction and scanning electron microscopy (SEM). The X-ray patterns have showed homogenous glasses over the entire compositional range while the SEM pictures have detected metallic silver or Ag₂O clusters dispersed in the glass network. Acting as complementary spectroscopic techniques, both types of measurements, FT-IR and Raman, revealed that the network structure of the studied glasses is mainly based on BO₃ and BO₄ units placed in different structural groups, the BO₃ units being dominant. The influence of manganese-ion content (x), on the N_BO4/N_BO3 ratio evolution was investigated.     

Influence of composition in the crystallization process of Fe75−xNb10B15+x metallic glasses

The devitrification process of Fe_75−xNb₁₀B_15+x (x = 0, 5, 10) metallic glasses produced by melt-spinning has been analyzed by calorimetric, microstructural and magnetic measurements. The experimental results show large differences in the behavior of these alloys as a function of composition. The alloy x = 0 undergoes a primary crystallization process separated in two stages while only one is observed for alloys with x = 5 and x = 10. This difference, observed by DSC, is correlated with a microstructural change in the phases that precipitate. For alloys x = 5 and 10, Fe₂₃B₆ and bcc-Fe are identified after the first calorimetric peak. In the sample x = 0, bcc-Fe and an unidentified phase precipitate in the first peak but massive crystallization of bcc-Fe is observed after the second stage. Finally the thermal dependence of magnetization has been measured and the Curie temperatures for the metallic glasses are determined. The change of these quantities with heat treatment and composition is discussed.     

Micro-structural studies of GeSe2–Ga2Se3–MX (MX = CsI and PbI2) glasses using Raman spectra

Microstructure of the chalcohalide glasses: GeSe₂–Ga₂Se₃–CsI and GeSe₂–Ga₂Se₃–PbI₂ ternary system were investigated by Raman spectra, lifetime of Dy³⁺ infrared emission and glass transition temperature (T_g). The evolution of the Raman spectra shows that the fundamental structural groups of these studied glasses consist of [Ge(Ga)Se₄] tetrahedral and some complex structure units [Ge(Ga)I_xSe_4?x](x = 1–4). The x value varied when the different iodide was added in Ge–Ga–Se matrix. For GeSe₂–Ga₂Se₃–CsI glasses, the [Ge(Ga)I_xSe_4?x](x = 1–4) mixed-anion tetrahedral and [Ga₂I₇]^? units occurred. For GeSe₂–Ga₂Se₃–PbI₂ glasses, the [Ge(Ga)I₂Se₂], [Ge(Ga)I₃Se] units can be formed. The changes of Dy³⁺ infrared emission lifetime and T_g support the results. Additionally, [PbI_n] structural units will be formed in GeSe₂–Ga₂Se₃–PbI₂ glasses due to high form-ability of these units when the PbI₂ content is high.     

Nano-heterogeneous structure of (1-x)KNbO3–xSiO2 glasses in the low glass-forming oxide content range 0.05 ≤ x ≤ 0.3

Glasses with a high content of niobium oxide are of significant interest for electro-optics and nonlinear optics. In the present paper we report the results of the investigation of the submicroscopic structure and nonlinear optical properties of (1-x)KNbO₃–xSiO₂ (KNS) glasses (x = 0.05–0.30) by XRD, SANS, electron microscopy and second harmonic generation (SHG) technique. Vitreous samples were fabricated by rapid melt cooling, via pressing the melt by steel plates, quenching between rotating metal rolls or splat cooling in air or nitrogen flow. Glasses with x < 0.15 are shown to possess a micro-inhomogeneous structure with regions enriched by SiO₂. On the contrary, as-quenched glasses with x > 0.15 are found by SANS to be homogeneous, but form nanostructures enriched by SiO₂ after heat-treatment. At temperatures below ~(T_g + 50 °C), SiO₂-enriched regions grow slightly, whereas their chemical composition shifts considerably closer to SiO₂. The data on the nano-inhomogeneous structure enables clarifying the complicated T_g(x) dependence of KNS glasses. SHG-active KNbO₃ phase precipitates at later stages of crystallization when the glass starts to lose its transparency, and crystallization of perovskite-like KNbO₃ is accompanied by the enhancement of SHG efficiency by several orders of magnitude.     

Glass forming region in the GeSe2–GeTe–PbTe system and some physicochemical properties of glassy alloys

New chalcogenide glasses from the GeSe₂–GeTe–PbTe system were synthesized. The glass forming region was determined using visual, X-ray diffraction and scanning electron microscopy analyses. It is extended towards the GeSe₂ and lies partially on the GeSe₂–GeTe (0–58 mol% GeTe) and GeSe₂–PbTe (20.0–57.5 mol% PbTe) sides. No glasses were obtained in the GeTe–PbTe system. The investigated physicochemical properties vary between 4.04–6.21 g/cm³ (density, d); 97–121 kgf/mm² (microhardness, HV); ?0.187 ÷ 0.007 (compactness, C) and 14.3–17.8 GPa (elasticity modulus, E), respectively.     

Conductivity percolation transition of Agx(Ge0.25Se0.75)100−x glasses

The ionic conductivity of several chalcogenide glasses increases abruptly with mobile ion addition from values typical of insulating materials (10⁻¹⁶–10⁻¹⁴ Ω⁻¹ cm⁻¹) to values of fast ionic conductors (10⁻⁷–10⁻¹ Ω⁻¹ cm⁻¹). This change is produced in a limited concentration range pointing to a percolation process. In a previous work [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259] the transition from semiconductor to fast ionic conductor of Ag_x(Ge_0.25Se_0.75)_100−x glasses was detected at x¹ ≅ 10 at.% in the form of a steep change in the conductivity. Ag_x(Ge_0.25Se_0.75)_100−x glasses with x ⩽ 25 at.%, prepared by a melt quenching method, are investigated by impedance spectroscopy in the frequency range 5 Hz–2 MHz at different temperatures, T, from room temperature to 363 K and by DC measurements at room temperature. The conductivity of the glasses, σ, was obtained as a function of silver concentration and temperature. For x ⩾ 10 at.% our results are in agreement with those reported by Kawasaki et al. [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259]. The percolation transition was observed in the range 7 ⩽ x ⩽ 8. The temperature dependence of the ionic conductivity follows an Arrhenius type equation σ = (σ_o/T) · exp(−E_σ/kT). The activation energy of the ionic conductivity, E_σ, and the pre-exponential term, σ_o, are calculated. The results are discussed in connection with other chalcogenide and chalcohalide systems and linked with the glass structures.     

Characterization of B2O3 and/or WO3 containing tellurite glasses

Characterization of B₂O₃ and/or WO₃ containing tellurite glasses was realized in the 0.80TeO₂–(0.20 ? x)WO₃ ? xB₂O₃ system (0 ≤ x ≤ 0.20 in molar ratio) by using differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectrometry techniques. Glasses were prepared with a conventional melt-quenching technique at 750 °C. To recognize the thermal behavior of the glasses, glass transition and crystallization temperatures, glass stability value, glass transition activation energy, fragility parameter were calculated from the thermal analyses. Density, molar volume, oxygen molar volume and oxygen packing density values were determined to investigate the physical properties of glasses. Fourier transform infrared spectra were interpreted in terms of the structural transformations on the glass network, according to the changing B₂O₃ and/or WO₃ content. Crystallization behavior of the glasses was investigated by in situ X-ray diffraction measurements and microstructural characterization was realized by scanning electron microscopy and energy dispersive X-ray spectrometry analyses.     

Evaluation of basic physical parameters of quaternary Ge–Sb-(S,Te) chalcogenide glasses

New quaternary chalcogenide Ge_xSb_40?xS₅₀Te₁₀ (x = 10, 20 and 27 at.%) and Ge_xSb_40?xS₅₅Te₅ (x = 20 and 27 at.%) glasses have been synthesized and the compositions have been characterized applying prompt gamma-ray activation analyses, neutron diffraction, and material density measurements. Using the experimental data, the basic physical parameters, such as average atomic volume, packing density, compactness, average coordination number, number of constrains, average heat of atomization and cohesive energy, of the synthesized glasses are evaluated and the results are discussed in a function of glass composition.     

Crystallization kinetics of a-Se75Te25 − xGax (x = 0, 5, 10 and 15 at wt %) glassy system

Bulk glasses of a-Se₇₅Te_25 ? xGa_x (x = 0, 5, 10 and 15 at wt %) have been prepared by melt quenching technique. These samples were structurally characterized by using X-ray diffraction. Kinetic of crystallization in these glasses was studied under non-isothermal conditions using differential thermal analysis (DTA). DTA is performed at different heating rates of 5, 10, 15, 20 and 30 °C/min. The values of glass transition (T_g) and crystallization peak temperature (T_p) are found to be composition and heating-rate dependent. The obtained results have been analyzed in terms of activation energy of glass transition (E_g) using Kissinger's and Mahadevan et al. relations. Values of E_g obtained by the two relations are in agreement with each other. The results indicate that the crystallization process is a three-dimensional growth.     

Structural study of AsS2–Ag glasses over a wide concentration range

(As_0.33S_0.67)_100-xAg_x (0 ≤ x ≤ 28) bulk glasses showing micro-phase separation in a wide concentration range have been studied by X-ray diffraction, neutron diffraction and extended X-ray absorption fine structure measurements. The AsAgS₂ composition, which forms a homogeneous glass, is modeled with the reverse Monte-Carlo simulation technique. It is established that Ag prefers the environment of S; Ag―As bonding cannot be observed. Similarly to the AsAgS₂ crystalline modifications smithite and trechmannite, the main structural units of the glass are AsS₃ pyramids. The covalent network of As and S atoms becomes fragmented in the glassy AsAgS₂ unlike in the glassy AsS₂. The environment of Ag atoms in the AsAgS₂ glass differs from that in the crystalline state. In average, each Ag atom has four nearest neighbors, three of them being S and one being Ag.     

Preparation of transparent lithium-mica glass-ceramics

In order to fabricate transparent and machinable mica-type glass-ceramics, parent glasses having chemical compositions corresponding to Li_(1+x)Mg₃AlSi_3(1+x)O_10+6.5xF₂ (x = 0.15, 0.5, 0.65, 1.0 and 1.2) were crystallized. Lithium-micas were precipitated in all the specimens as the main crystal at 650 °C though the content was little. And the micas were changed from trisilicic type to tetrasilicic type as the x increased. After the precipitation of the micas, a large amount of β-eucryptite solid solution was precipitated as the main crystal at higher temperatures and even at 650 °C after a certain period of time. The parent glasses having the spinodal phase separation, of which the x was 1.0 and 1.2, were transparent and colored in light blue. And at 650 °C, the fine mica crystals with size of about 20 nm were precipitated in one continuous glass phase of the spinodal phase separation, which made the phase separation structure become finer. Consequently, the heated specimens showed higher transmittance of visible ray and became colorless. Moreover, because the mica crystals formed the continuous phase, the specimens showed the machinability.