High niobium oxide content in germanate glasses: Thermal,structural, and optical properties

Niobium alkali germanate glasses were synthesized by the melt‐quenching technique. The ternary system (90‐x)GeO₂–xNb₂O₅–10K₂O forms homogeneous glasses with x ranging from 0 to 20 mol%. Samples were investigated by DSC and XRD analysis, FTIR and Raman spectroscopy, and optical absorption. Structural and physical features are discussed in terms of Nb₂O₅ content. The niobium content increase in the glass network strongly modifies the thermal, structural and optical properties of alkali germanate glasses. DSC, Raman and FTIR analysis suggest niobium addition promotes NbO₆ groups insertion close to GeO₄ units of the glass network. XRD analysis also pointed out that samples containing high niobium oxide contents exhibit preferential niobium oxide‐rich phase after crystallization after heat treatment, which is similar to orthorhombic Nb₂O₅. Absorption spectra revealed high transmission range between 400 nm to 6.2 μm, added to a considerably decreased hydroxyl group content as the addition of niobium in the alkali germanate network. The niobium oxide‐rich phase crystallization process was studied and activation energy was determined, as well as nucleation and crystal growth temperatures and time for obtaining transparent glass‐ceramics.     

Synthesis of bulk silicon oxynitride glass through nitridation of SiO2 aerogels and determination of Tg

The development of transparent glass for use in high-temperature applications is continuing. In this study, we synthesized bulk silicon oxynitride glasses (a-Si(O,N)_x) through the nitridation of SiO₂ aerogels containing methyl (CH₃-) groups and evaluated their bulk properties, including their glass transition temperature (T_g). Tetramethyl orthosilicate and methyltrimethoxysilane were added into the precursor gels, and those gels were subjected to a supercritical CO₂ drying process. The presence of CH₃-group in the gel avoided cracking during ammonolysis at 750°C–1400°C, and the transparency of the gel was remained even after ammonolysis at 1300°C. The ammonolysis successfully introduced nitrogen into the gels even at relatively low temperatures, for example, 750°C, and the highest nitrogen content (11.7 mass%) was achieved in the gel after ammonolysis at 1300°C. As the nitrogen-related signals in electron spectroscopy indicated presence of nitride ions (N³⁻) after ammonolysis and the infrared absorption signals attributed to Si–N bonds were enhanced with the increase of nitrogen concentration, we successfully obtained oxynitride glasses. Those oxynitride glasses showed increase of T_g with their nitrogen concentration.     

X‐ray photoelectron spectroscopy analysis of Ge–Sb–Se pulsed laser deposited thin films

Pulsed laser deposition was used to prepare amorphous thin films from (GeSe₂)_100?x(Sb₂Se₃)_x system (x = 0, 5, 10, 20, 30, 40, 50, and 60). From a wide variety of chalcogenide glass‐forming systems, Ge–Sb–Se one, especially in thin films form, already proved to offer a great potential for photonic devices such as chemical sensors. This system has a large glass‐forming region which gives the possibility to adjust the chemical composition of the glasses according to required physical characteristics. The chemical composition of fabricated thin films was analyzed via X‐ray photoelectron spectroscopy (XPS) and compared to energy dispersive spectroscopy (EDS) data. The results of both techniques agree well: a small deficiency in chalcogen element and an excess of antimony was found. The structure of as‐deposited thin films has been investigated by XPS. The presence of the two main structural units, [GeSe₄] and [SbSe₃] proposed by Raman scattering spectroscopy data analysis, was confirmed by XPS. Moreover, XPS core level spectra analysis revealed the presence of M–M bonds (M = Ge, Sb) in (Ge,Sb)–Ge–(Se)₃ and (Ge,Sb)–Sb–(Se)₂ entities that could correspond to Ge‐based tetrahedra and Sb‐based pyramids where one of its Se atoms at corners is substituted by Ge or Sb ones. The content of depicted M–M bonds tends to increase with introduction of antimony in the amorphous network of as‐deposited thin films from x = 0 to x = 40 and then it decreases. XPS analysis of as‐deposited thin films shows also the presence of the (Ge,Sb)–Se–(Ge,Sb) and Se–Se–(Ge,Sb) entities.     

Structural origin of the weak germanate anomaly in lead germanate glass properties

Binary PbO–GeO₂ glasses have been studied in detail from 5 to 75 mol% PbO using high-resolution neutron diffraction, high-energy X-ray diffraction, 207-Pb NMR, pycnometry, and thermal analysis. The Ge–O coordination number displays a broad maximum n_GeO = 4.14(3) close to 27 mol% PbO. This is smaller than the maximum n_GeO = 4.3 reported in CaO–GeO₂ glasses but occurs at a similar composition. This structural behavior appears to explain the relatively weak germanate anomaly manifest in lead germanate glasses, for example as a maximum in the measured atom number density and a plateau in the glass transition temperatures. The structural role of Pb(II) is complex. On the one hand, short covalent Pb–O bonds and small Pb–O coordination numbers of ∼3 to 4 indicate glass network former character for Pb(II), associated with a stereochemically active electron lone pair. On the other hand, the presence of some GeO₅ or GeO₆ units, in addition to the majority GeO₄ tetrahedral species, indicates some modifier character of Pb(II) at low PbO contents, giving rise to the observed weak germanate anomaly, as well as elongation and enhanced ionicity of the Pb–O bonds. Overall, the observed structural behavior of Pb(II) in lead germanate glasses appears as intermediate between that observed in lead silicate and lead borate glasses. Despite rapid quenching, at low PbO contents, the glasses studied exhibited nanoscale heterogeneity, evidenced by small-angle X-ray scattering consistent with the early stages of spinodal decomposition.     

The influence of the mixed alkaline earth effect on the structure and properties of (Ca,Mg)–Si–Al–O–N glasses

Alkaline earth oxynitride glasses of (Ca, Mg)–Si–Al–O–N with different CaO/(CaO + MgO) molar ratios (0, 0.25, 0.5, 0.75, and 1) were successfully prepared using the sol-gel method, and their structural compositions were characterised by Raman and FT-IR techniques. The glass dynamic properties of thermal expansion coefficient, glass transition temperature (T_g), and static properties of density, molar volume, Vickers hardness and compressive strength were systematically measured and analysed. The results showed that the static properties exhibited an overall regular change as the CaO/(CaO + MgO) ratio gradually increased, while the dynamic properties had an obvious mixed alkaline earth effect, which represented the appearance of an extreme value point in CaO/(CaO + MgO) mole ratios of 0.25 and 0.75, respectively. The typical thermal expansion coefficient and T_g of mixed alkaline earth oxynitride glasses deviated far from the linear connection between single alkaline earth oxynitride glasses. Raman spectra and infrared spectra revealed that the ratio value of the Q³/(Q²+Q⁴) decreased (Qⁿ: n = no. of bridging anions joining SiO₄ tetrahedra) in the mixed alkaline earth oxynitride glasses with increasing the amount of Ca, confirming that Ca decreased the crosslinking between individual tetrahedra via the transformation of Q³ species into Q² and Q⁴ species.     

Topological model of alkali germanate glasses and exploration of the germanate anomaly

Germanate glasses are of particular interest for their excellent optical properties as well as their abnormal structural changes that appear with the addition of modifiers, giving rise to the so-called germanate anomaly. This anomaly refers to the nonmonotonic compositional scaling of properties exhibited by alkali germanate glasses and has been studied with various spectroscopy techniques. However, it has been difficult to understand its atomic scale origin, especially since the germanium nucleus is not easily observed by nuclear magnetic resonance. To gain insights into the mechanisms of the germanate anomaly, we have constructed a structural model using statistical mechanics and topological constraint theory to provide an accurate prediction of alkali germanate glass properties. The temperature onsets for the rigid bond constraints are deduced from in situ Brillouin light scattering, and the number of constraints is shown to be accurately calculable using statistical methods. The alkali germanate model accurately captures the effect of the germanate anomaly on glass transition temperature, liquid fragility, and Young's modulus. We also reveal that compositional variations in the glass transition temperature and Young's modulus are governed by the O–Ge–O angular constraints, whereas the variations in fragility are governed by the Ge–O radial constraints.     

Structure and physical properties of Ge15Sb20Se65‐xSx glasses

We explored the structure and physical properties of Ge₁₅Sb₂₀Se_65‐xS_x (with x = 0, 16.25, 32.5, 48.75, and 65) glasses in order to screen the best compositions for the applications in photonics, since the laser damage thresholds in Se‐based glasses are too low although their optical nonlinearities are high. We found that, linear and nonlinear refractive index of the glasses decreased, but glass transition temperature T_g, optical bandgap E_g and the laser damage threshold increased with increasing S content. We further employed Raman scattering and high‐resolution X‐ray photoelectron spectra to probe the structure of the glasses. Through the analysis of the evolution of the different structural units in the glasses, it was concluded that, the heteropolar bonds (Ge–Se/S, Sb–Se/S) were dominated in these glasses. With the increase in chalcogen Se/S ratio, the number of the Se‐related chemical bonds (Ge–Se, Sb–Se and Se–Se) increased and that of S‐related chemical bond (Ge–S, Sb–S and S–S) decreased gradually, and Ge was prior to bond with S rather than Se. The elemental substitution thus had negligible effect on the glass structure. The change of the physical properties was mainly due to the difference of the strength of the chemical bonds between S–Ge(Sb) and Se–Ge(Sb).     

Network Rearrangement in AgI‐Doped GeTe4 Glasses

The structure of (GeTe₄)_1?x(AgI)_x (x = 0.15 and 0.25) glasses has been investigated by X‐ray and neutron diffraction as well as extended X‐ray absorption spectroscopy (EXAFS) and Raman spectroscopy. Large‐scale structural models have been obtained by fitting simultaneously the experimental datasets in the framework of the reverse Monte Carlo simulation technique (RMC). Short‐range order parameters have been calculated and compared with that of GeTe₄. Doping with AgI affects the structure of the host GeTe₄ matrix in two ways. First, while Te is essentially twofold coordinated in GeTe_4, its coordination number is as high as ~2.9 ± 0.3 for x = 0.25. The change is mainly due to the increased fraction of Te–Te bonds. Second, Ge atoms remain fourfold coordinated but the tetrahedral symmetry is distorted due to the elongation of some Ge–Te bonds. The incorporation of AgI in the GeTe₄‐based host covalent matrix and the Te coordination increase explains the enhanced thermal stability of (GeTe₄)_1?x(AgI)_x in the supercooled liquid‐state hindering the crystallization of Te found in case of GeTe₄ glass.     

Structural dependence of crystallization in glasses along the nepheline (NaAlSiO4) ‐ eucryptite (LiAlSiO4) join

Lithium and sodium aluminosilicates are important glass‐forming systems for commercial glass‐ceramics, as well as being important model systems for ion transport in battery studies. In addition, uncontrolled crystallization of LiAlSiO₄ (eucryptite) in high‐Li₂O compositions, analogous to the more well‐known problem of NaAlSiO₄ (nepheline) crystallization, can cause concerns for long‐term chemical durability in nuclear waste glasses. To study the relationships between glass structure and crystallization, nine glasses were synthesized in the Li_xNa_1‐xAlSiO₄ series, from x = 0 to x = 1. Raman spectra, nuclear magnetic resonance (NMR) spectroscopy (Li‐7, Na‐23, Al‐27, Si‐29), and X‐ray diffraction were used to study the quenched and heat‐treated glasses. It was found that different LiAlSiO₄ and NaAlSiO₄ crystal phases crystallize from the glass depending on the Li/Na ratio. Raman and NMR spectra of quenched glasses suggest similar structures regardless of alkali substitution. Li‐7 and Na‐23 NMR spectra of the glass‐ceramics near the endmember compositions show evidence of several differentiable sites distinct from known Li_xNa_1‐xAlSiO₄ crystalline phases, suggesting that these measurements can reveal subtle chemical environment differences in mixed‐alkali systems, similar to what has been observed for zeolites.     

Effect of BaF2 Content on Luminescence of Rare‐Earth Ions in Borate and Germanate Glasses

Rare‐earth‐doped oxyfluoride germanate and borate glasses were synthesized and next studied using spectroscopic methods. Influence of fluoride modifier on luminescence properties of rare earths in different glass hosts was examined. The excitation and emission spectra of Pr³⁺ and Er³⁺ ions in the studied glasses were registered. The emission spectra of Pr³⁺ ions in germanate and borate glasses are quite different and depend strongly on the glass host. In samples doped with Er³⁺ ions emission bands located around 1530 nm corresponding to the main ⁴I_13/2→⁴I_15/2 laser transition were registered, independently of the glass host. Quite long‐lived near‐infrared luminescence of Er³⁺ ions was observed for germanate glasses with low BaF₂ content, while in borate glass systems influence of barium fluoride on luminescence lifetimes is not so evident. The Judd–Ofelt calculations were used in order to determine quantum efficiencies of excited states of rare‐earth ions in germanate and borate glasses.     

Structure-mechanical properties correlation in bulk LiPON glass produced by nitridation of metaphosphate melts

The glassy solid electrolyte Lithium phosphorous oxynitride (LiPON) has been widely researched in thin film solid state battery format due to its outstanding stability when cycled against lithium. In addition, recent reports show thin film LiPON having interesting mechanical behaviors, especially its ability to resist micro-scale cracking via densification and shear flow. In the present study, we have produced bulk LiPON glasses with varying nitrogen contents by ammonolysis of LiPO₃ melts. The resulting compositions were determined to be LiPO_3-3z/2N_z, where 0 ≤ z ≤ 0.75, and the z value of 0.75 is among the highest ever reported for this series of LiPON glasses. The short-range order structures of the different resulting compositions were characterized by infrared, Raman, ³¹P magic angle spinning nuclear magnetic resonance, and X-ray photoelectron spectroscopies. Instrumented nano-indentation was used to measure mechanical properties. It was observed that similar to previous studies, both trigonally coordinated (N_t) and doubly bonded (N_d) N co-exist in the glasses in about the same amounts for z ≤ 0.36, the limit of N content in most previous studies. For glasses with z > 0.36, it was found that the fraction of the N_t increased significantly while the fraction of N_d correspondingly decreased. The incorporation of nitrogen increased both the elastic modulus and hardness of the glass by approximately a factor of 1.5 when N/P ratio reaches 0.75. At the same time, an apparent embrittlement of the glass was observed due to nitridation, which was revealed by nanoindentation with an extra sharp nanoindenter tip.     

Surface Analysis of Cr2O3‐, CoO‐, and Al2O3‐Doped Iron–Phosphate Glasses at High Temperature

Surface structures of iron–phosphate glasses were examined using X‐ray photoelectron spectroscopy (XPS). Cr₂O₃, CoO, and Al₂O₃ were introduced to the glass by the replacement of a part of Fe₂O₃, and the simulated fission products are also added. The obtained glasses showed high chemical durabilities by MCC‐1 test. In situ high‐temperature and room‐temperature XPS measurements were conducted on the polished sample surfaces and also those after 1‐week chemical durability test. Unique trends were observed in XPS spectra on heating and after the chemical durability test, respectively. Nature of the glass surface of iron–phosphate glasses was explained from the point of view of surface energy, and the origin of high chemical durability and the effect of chromium ions were discussed based on the changes on surface composition and valence states of transition‐metal ions.     

Enhanced Thermostability,Thermo‐Optics,and Thermomechanical Properties of Barium Gallo‐Germanium Oxyfluoride Glasses and Glass‐Ceramics

Serial substitutions of BaF₂ for BaO in BaO–Ga₂O₃–GeO₂ glasses were performed, and the effects of the substitutions on the glass properties were investigated. The glass transition temperature, density, refractive index, thermo‐optics coefficient, and figure of merit for thermal shock decreased with the replacement of oxygen by fluorine. On the other hand, the glass‐forming ability increased. Fluorine substitution removed the absorption band of hydroxyl near 2.9 μm. Raman scattering spectroscopy was used to characterize the fluorogermanate glasses. The crystallization process of the glass‐ceramics under different heat‐treatment conditions was also investigated using differential scanning calorimetry, X‐ray diffraction, scanning electron microscopy, and atomic force microscopy. The thermal and mechanical properties were improved by controlling the crystal size of the near‐ and middle‐infrared transparent glass‐ceramics.     

Conversion of Alumina to Aluminum Nitride Using Polymeric Carbon Nitride as a Nitridation Reagent

Polymeric carbon nitride, which was synthesized by polymerization of dicyandiamide at 500°C, was used as a nitridation reagent in the conversion of δ‐alumina (δ‐Al₂O₃) to aluminum nitride (AlN). The products obtained at various reaction temperatures were characterized by powder X‐ray diffraction, ²⁷Al magic‐angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, Raman spectroscopy, and X‐ray photoelectron spectroscopy (XPS). δ‐Al₂O₃ began to convert to AlN at 900°C, which is the lowest temperature reported for the formation of AlN from Al₂O₃, and completely converted to AlN at 1400°C. The occurrence of reaction intermediates during nitridation was confirmed by ²⁷Al MAS NMR and XPS. The change in Raman spectra with reaction temperatures indicated that lattice defects in AlN were reduced by calcining at higher reaction temperatures.     

Nature of Pd and Ti Metals in the Structure of CMAS Glass and Ceramics

The structure of electrically conductive CMAS‐TiO₂‐Pd glass and ceramics was investigated by transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), X‐ray photoelectron spectroscopy (XPS), X‐ray absorption near‐edge spectroscopy (XANES), and extended X‐ray absorption fine structure spectroscopy (EXAFS). The XANES spectra of Ti do not show any significant difference between the glasses ceramized in air or in a reducing “forming” gas, as well as between Pd‐containing versus Pd‐free samples, nor between surface versus bulk of the glass‐ceramic samples. However, EXAFS and XANES data recorded at the Pd K‐edge show significant dependences on whether the glass‐ceramic was ceramized in air or in “forming” gas. The XPS spectra of Ti 2p core‐level electrons for glasses ceramized in air or “forming” gas also show a strong difference depending on whether the samples did or did not contain Pd. STEM mapping confirms the existence of grains in the form of main crystalline phases identified with XRD, and also reveals the existence of Pd nanoparticles in glasses ceramized in both air and in “forming” gas.     

Correlation Between Crystallization Behavior and Network Structure in GeS2–Ga2S3–CsI Chalcogenide Glasses

Diagram of the phase transformation behavior of GeS₂–Ga₂S₃–CsI glasses is realized in this article and the structure‐property dependence of the chalcogenide glasses is elucidated using differential scanning calorimetry and Raman spectroscopy. We observe the compositional threshold of crystallization behavior locates at x = 6–7 mol% in (100?x)(0.8GeS₂–0.2Ga₂S₃)–xCsI glasses, which is confirmed by the thermodynamic studies. Structural motifs are derived from the Raman result that [Ge(Ga)S₄], [S₂GeI₂], [S₃GaI], and [S₃Ga–GaS₃] were identified to exist in this glass network. Combined with the information of structural threshold, local arrangement of these structural motifs is proposed to explain all the experimental observations, which provides a new way to understand the correlation between crystallization behavior and network structure in chalcogenide glasses.     

First Identification of Rare‐Earth Oxide Nucleation in Chalcogenide Glasses and Implications for Fabrication of Mid‐Infrared Active Fibers

Gallium (Ga) helps solubilize rare‐earth ions in chalcogenide glasses, but has been found to form the dominant crystallizing selenide phase in bulk glass in our previous work. Here, the crystallization behavior is compared of as‐annealed 0–3000 ppmw Dy³⁺‐doped Ge–As–Ga–Se glasses with different Ga levels: Ge_16.5As_(19?x)Ga_xSe_64.5 (at.%), for x = 3 and 10, named Ga₃ and Ga₁₀ glass series, respectively. X‐ray diffraction and high‐resolution transmission electron microscopy are employed to examine crystals in the bulk of the as‐prepared glasses, and the crystalline phase is proved to be the same: Ge‐modified, face centered cubic α‐Ga₂Se₃. Light scattering of polished glass samples is monitored using Fourier transform spectroscopy. When Ga is decreased from 10 to 3 at.%, the bulk crystallization is dramatically reduced and the optical scattering loss decreases. Surface defects, with a rough topology observed for both series of as‐prepared chalcogenide glasses, are demonstrated to comprise Dy, Si, and [O]. For the first time, evidence for the proposed nucleation agent Dy₂O₃ is found inside the bulk of as‐prepared glass. This is an important result because rare‐earth ions bound in a high phonon–energy oxide local environment are, as a consequence, inactive mid‐infrared fluorophores because they undergo preferential nonradiative decay of excited states.     

Correlating Structure with Threshold Behavior of Chalcogenide Glasses Within Ge–Sn–Se Ternary System

A systematic investigation of the optical and structural properties of chalcogenide glasses in Ge–Sn–Se ternary system is presented. We have found a threshold behavior of optical property, namely, existence of transitional composition of the Ge–Sn–Se glasses, with progressive replacement of Se by Sn. Calculation of mean coordination number indicates that the transition‐like feature of optical property is associated with the evolution of chemical ordering of the Ge–Sn–Se network. Analysis of Raman spectra of the glasses explains that the interaction between Se–Se bonds, Sn(Se_1/2)₄ tetrahedra, and Sn–Sn homopolar bonds is the origination of such optical phenomenon.     

Effect of CeO2 on the Glass Structure of Sodium Germanate Glasses

Germanate glasses have potential applications as optical fibers. Materials doped with rare earth ions are good candidates for optical, lasing, and magnetic applications. Based on the ternary system, CeO₂–Na₂O–GeO₂ a series of six glasses were fabricated using powder fusion, and varying the Na₂O content from 0 to 45 mol%, and a CeO₂ content constant at 3 mol%. The glasses were analyzed by FT‐IR, Raman and X‐ray photoelectron (XPS) spectroscopies to obtain information about the glass structure, cerium oxidation's state and how it is introduced in the glass network. FT‐IR and Raman spectra revealed the presence of GeO₆ and GeO₄ groups as well as Q² and Q³ units in the glasses with alkali low content. XPS spectra analysis revealed that the cerium ions were reduced from Ce⁴⁺ to Ce³⁺. The nonbonding to total oxygen ratio was estimated from the curve fitting of the O 1s core level spectra. Density and elastic parameters showed a nonlineal tendency in the change of the physical properties as a function of Na₂O content. Finally, photoluminescence spectroscopy confirmed the presence of Ce³⁺ ions. The characteristic 4f → 5d electronic transitions at 360 nm were detected, when a 280 nm excitation line of pulsed laser was used as excitation source.     

Compositional dependence of crystallization in Ge–Sb–Se glasses relevant to optical fiber making

For fiber‐optic mid‐infrared bio‐ and chemical‐sensing, Ge–Sb–Se glass optical fibers are more attractive than Ge–As–Se because of: (i) lowered toxicity and (ii) lower phonon energy and hence transmission to longer wavelengths, with potential to reach the spectral “fingerprint region” for molecular sensing. There is little previous work on Ge–Sb–Se fibers. Here, fibers are fabricated from two glass compositions in the Ge_xSb₁₀Se_90?x atomic (at.) % series. Both glass compositions are of similar mean‐coordination‐number, lying in the overconstrained region, yet of different chemical composition: stoichiometric Ge₂₅Sb₁₀Se₆₅ at. % and non‐stoichiometric Ge₂₀Sb₁₀Se₇₀ at. %. Thermal analysis on bulk glasses has previously shown that the former exhibited the maximum glass stability of the series. However, during fiber‐drawing of Ge₂₅Sb₁₀Se₆₅ at. %, the preform tip is found to undergo surface‐devitrification to monoclinic GeSe₂ alone, the primary phase, no matter if the preform is an annealed, as‐melted rod or annealed, extruded rod. The heating rate of the preform‐tip to the fiber‐drawing temperature is estimated to be up to ~100°C/min to ~490°C. Lower heating rates of 10°C/min using thermal analysis, in contrast, encourage crystallization of both Sb₂Se₃ and GeSe₂. The non‐stoichiometric: Ge₂₀Sb₁₀Se₇₀ at. % composition drew successfully to low optical loss fiber, no matter whether the preform was an annealed, as‐melted rod or annealed, extruded rod.