Structure of potassium germanophosphate glasses by X-ray and neutron diffraction. Part 1: Short-range order

It has been reported that the addition of K₂O or P₂O₅ to binary germanate glasses increases the Ge−O coordination numbers (N_GeO). The present work describes X-ray and neutron diffraction studies aimed at clarifying the concomitant effects of both oxides on the structures of ternary K₂O-GeO₂-P₂O₅ glasses. The Ge−O coordination numbers obtained range from 4.2 to 5.1, less than what is predicted according to a model which assumes all oxygen atoms form network bridges similar to those found in the related crystal structures. This implies that the glass structures must include terminal oxygen sites, likely associated with the PO₄ tetrahedra, that are neutralized by coordinating K⁺ ions. The shapes of the high resolution first-neighbor diffraction peaks do not indicate distinctly different species of P−O and Ge−O bonds. The model for the increase of N_GeO which is based on an increase of the fraction of GeO₆ units, at the expense of GeO₄ units, is supported by the analysis of the two main components of the Ge−O peak used in the fits. However, the existence of a GeO₅ fraction cannot be excluded from the present data sets. A linear relation between the total Ge−O coordination numbers and mean Ge−O distances exists, assuming end members of units GeO₄ and GeO₆ with bond lengths of ∼0.175 and ∼0.190 nm, respectively.     

Structure of potassium germanophosphate glasses by X-ray and neutron diffraction: 3. Reverse Monte Carlo simulations

Reverse Monte Carlo (RMC) is used to investigate the origin of the first sharp diffraction peaks (FSDP) found for K₂O–GeO₂–P₂O₅ glasses at very small scattering vector Q = ～7.5 nm^?1. Structures of the ternary glass with the greatest intensity of FSDP (KGeP5 – 25/50/25 mol% K₂O/GeO₂/P₂O₅), of the binary combinations of the three oxides and of vitreous GeO₂ are modeled. Results are deduced from comparisons of the partial structure factors and inspections of model sections. The P sites are uniformly distributed in the structure of KGeP5. The K⁺ ions interact more with the PO₄ units (via O_T-corners) than with Ge-centered units. Main component of the FSDP comes from the S_GeGe(Q) factor. The FSDP is due to separations of ～1 nm between the longish Ge-rich clusters which are visible in the corresponding models. Different to our tentative structural models reported before, the PO₄ tetrahedra possess a broad distribution of numbers of O_T corners. The FSDP’s of the binary K₂O–GeO₂ and K₂O–P₂O₅ glasses (～10 nm^?1) are due to a chemical order between network former and network modifier regions. The MRO of a mixed GeO₂–P₂O₅ glass of small P₂O₅ content (FSDP at ～16 nm^?1) shows great similarity to the MRO of vitreous GeO₂.     

The structure and optical properties of GeO2-GeS2 glasses

Oxy-chalcogenide glasses with compositions of xGeO₂-(100 − x)GeS₂, where 0 ? x ? 100 mol%, have been prepared and studied in terms of their structures and optical properties. X-ray fluorescence spectroscopy shows that Ge:S ratio can deviate from GeS₂ by ∼10 at.%, depending critically upon the preparation conditions. Raman scattering spectroscopy suggests that stoichiometric GeO₂-GeS₂ glasses have a heterogeneous structure in the scale of 1-100 nm. The optical gaps are nearly constant at 3.0-3.5 eV for glasses with 0 ? x ? 80 mol% and abruptly increase to ∼6 eV in GeO₂. This dependence suggests that the optical gap is governed by GeS₂ clusters, which are isolated and/or percolated. Composition-deviated glasses appear as orange and brown, and these glasses seem to have more inhomogeneous structures.     

Three-dimensional structure of fast ion conducting 0.5Li2S + 0.5[(1 − x)GeS2 + xGeO2] glasses from high-energy X-ray diffraction and reverse Monte Carlo simulations

A high-energy X-ray diffraction study has been carried out on a series of 0.5Li₂S + 0.5[(1 − x)GeS₂ + xGeO₂] glasses with x = 0.0, 0.1, 0.2, 0.4, 0.6 and 0.8. Structure factors were measured to wave vectors as high as 30 Å⁻¹ resulting in atomic pair distribution functions with high real space resolution. The three dimensional atomic-scale structure of the glasses was modeled by reverse Monte Carlo simulations based on the diffraction data. Results from the simulations show that at the atomic-scale 0.5Li₂S + 0.5[(1 − x)GeS₂ + xGeO₂] glasses may be viewed as an assembly of independent chains of (Li^+-S)₂GeS_2/2 and (Li^+-O)₂GeO_2/2 tetrahedra as repeat units, where the Li ions occupy the open space between the chains. The new structure data may help understand the reasons for the sharp maximum in the Li⁺ ion conductivity at x ∼ 0.2.     

Spectroscopic properties of Er-doped xGeO2-(80 − x)TeO2-10ZnO-10BaO glasses

Erbium-doped glasses with composition xGeO₂-(80 − x)TeO₂-10ZnO-10BaO were prepared by melt-quenching technique. The phonon sideband spectra and the optical absorption band edges for the host matrix were confirmed by means of the spectral measurements. Standard Judd-Ofelt calculations have been completed to these glasses. The dependence of up-conversion and infrared emission under 980 nm excitation on the glass composition was studied. The quantum efficiencies for the ⁴I_13/2 → ⁴I_15/2 transition of trivalent erbium in the glasses were estimated.     

Intermediate-range order in glassy GexSe1−x around the stiffness transition composition

X-ray scattering measurements on glassy Ge_xSe_1−x were performed in a concentration range x=0.07-0.333 in fine steps of 0.005-0.05, in order to explore the relation between the intermediate-range order (IRO) and the stiffness transition in this glassy system. The oscillations beyond the first peak around 20.5 nm⁻¹ in the structure factor, S(Q), remain almost unchanged or damp very slowly with decreasing the Ge concentration x, suggesting the preserve of the short-range order. On the other hand, the pre-peak around 10-12 nm⁻¹, indicating the existence of IRO, systematically changes with decreasing x; its Q position shifts towards the higher Q values and its area decreases. Especially near the onset of the stiffness transition, x=0.20, the peak position starts to deviate from the linear relation. The origin of the pre-peak is discussed in connection with results of a recent anomalous X-ray scattering measurement indicating two contributions of Ge-Ge and Se-Se correlations. Around the onset composition of the stiffness transition, the area of the pre-peak associated with the Ge-Ge correlation has a plateau-like gradual decrease with x followed by a rapid decrease at x<0.18, which is in good agreement with Raman data.     

Structural changes during the crystallization of the Bi4Ge3O12 glasses

Bismuth-germanate glass ceramics with the composition 40% Bi₂O₃-60% GeO₂ (in molar percents) were prepared through controlled crystallization of melt-quenched glass. The Raman and FTIR spectra recorded in the as-made glasses show broad bands at 240, 400, 780 cm^− 1 and 400, 745 cm^− 1 have been assigned Ge-O bonds which appear right after preparation. X-ray diffraction has shown that the as-made glasses are amorphous, but after annealing above the crystallization temperature at 558 °C, BGO nano-crystallites with a size of about 50 nm precipitate in the glass matrix. The Raman and FTIR spectra reveal sharp peaks associated to the “internal” and “external vibrations” of GeO₄ tetrahedral groups inside the BGO nano-crystallites. In the glass ceramic sample the transparency region is shifted at longer wavelengths compared to as-made glass, due to the Rayleigh scattering on the BGO nano-crystallites.     

Planar waveguides by ion exchange in Er-doped tellurite glass

This work reports the preparation of planar waveguides by Ag⁺ → Na⁺ ion exchange in Er³⁺-doped tellurite glass with a composition of 75TeO₂-2GeO₂-10Na₂O-12ZnO-1Er₂O₃ (mol%). The metric, of T_x − T_g, indicates that the glass has good thermal stability. Measurments of refractive index, absorption spectrum, luminescence and lifetime were made. The glass was chemically stable during the ion exchange process. Monomode and multimode planar waveguides in the tellurite glasses have been prepared. We determined the depth of the guides, effective diffusion coefficient and the activation energy. The depths of the waveguides could be controlled by varying ion exchange temperatures and times (250-280 °C, and 3-12 h were used).     

Development and characterizations of BaO-CaO-Al2O3-SiO2 glass-ceramic sealants for intermediate temperature solid oxide fuel cell application

Several compositions based on BaO-CaO-Al₂O₃-SiO₂ (BCAS) glass system have been studied in this investigation to see their applicability as sealant for solid oxide fuel cell (SOFC). The glasses as well as the corresponding glass-ceramics have been systematically characterized by differential thermal analysis, dilatometry, X-ray diffractometry, electron microscopy and impedance analysis to examine their suitability as sealant. While the glass transition temperature (T_g) determined from DTA are within 600-665 °C, the coefficient of thermal expansion (CTE) can be tailored between 9.5 and 13.0 × 10⁻⁶ K⁻¹. These glasses are found to be well adhered with metallic interconnects, such as commercial ferritic steel (Crofer22APU), at an optimum sealing temperature of 850 °C. The shrinkage behavior of the developed glasses in their pellet form has also been investigated. The resistivities of the glass-ceramics, as obtained from impedance analysis, are found to be within 10⁴-10⁶ Ω cm at 800 °C. Under sandwiched condition between two metals, some of the developed compositions are found to maintain this high resistivity even after 100 h of operation. One of the glass compositions has shown a low leak-rate of the order of ∼10⁻⁷ Pa m² s⁻¹.     

On a qualitative model for the incorporation of fluoride nano-crystals within an oxide glass network in oxy-fluoride glass-ceramics

We report on a Raman scattering study of oxy-fluoride glass-ceramics with a typical composition 32SiO₂:9AlO_1.5:31.5CdF₂:18.5PbF₂:5.5ZnF₂:3.5ErF₃ (in mol%), which indicates a narrow size distribution of β-PbF₂ fluoride nano-crystals, typically of 13 ± 1 nm, in the silica-based glass network and gives insight on how then nano-crystals are incorporated into the glassy network. The Raman spectra indicate the presence of Q⁰, Q¹ and Q²-like units, which are SiO_xF_4−x tetrahedra with zero, one and two bridging oxygens, respectively. The frequency, width and depolarization ratio of Raman bands corresponding to these Qⁿ-like units (n=0, 1, 2) indicate that Q⁰ units are mostly symmetric SiO₄ tetrahedra, Q¹ are SiO₄ tetrahedra where one F may substitute for O and Q² are SiO₄ tetrahedra where one or two F may substitute for O. We argue that the non-bridging O atoms belonging to Qⁿ tetrahedra, mostly to Q⁰ tetrahedra, are located near the interface between the nano-crystalline and glassy phases, allowing an easy accommodation of fluoride nano-crystals with a spherical shape, in the oxide glass network. This was in agreement with the very low-frequency Raman features found between ∼4 and 8 cm⁻¹, due to acoustic vibrations of the nano-crystals embedded in the glassy matrix.     

Studies of Er doped germanate-oxyfluoride and tellurium-germanate-oxyfluoride transparent glass-ceramics

An erbium doped germanate-oxyfluoride glass 60GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (GPOF) and a tellurium-germanate-oxyfluoride glass 30TeO₂ · 30GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (TGPOF) were prepared in the bulk form. By appropriate heat treatment of the as-prepared glasses above, transparent glass-ceramics were obtained with the formation of β-PbF₂ nanocrystals in the glass matrix confirmed by X-ray diffraction. Optical absorption and photoluminescence measurements were performed on as-prepared glass and glass-ceramics. The luminescence of Er³⁺ ions in transparent glass-ceramics revealed sub-band splitting generally seen in a crystal host. The intensity of red and near infrared luminescence significantly increased in transparent glass-ceramic compared to that in as-prepared glass. Two luminescence bands at 758 nm from ⁴F_7/2 → ⁴I_13/2 and at 817 nm from ²H_11/2 → ⁴I_13/2 transitions were observed from transparent glass-ceramic but cannot be seen from the corresponding as-prepared glass. These results are attributed to the change of ligand field of Er³⁺ ions and the decrease of effective phonon energy when Er³⁺ ions were incorporated into the precipitated β-PbF₂ nanocrystals.     

Mass fractal characteristics of sonogels prepared from sonohydrolysis of tetraethoxysilane with additions of dimethylformamide

Wet silica gels with ∼1.4 × 10⁻³ mol SiO₂/cm³ and ∼90 vol.% liquid phase were prepared from the sonohydrolysis of tetraethoxysilane (TEOS) with different additions of dimethylformamide (DMF). Aerogels were obtained by CO₂ supercritical extraction. The samples were studied mainly by small-angle X-ray scattering (SAXS) and nitrogen adsorption. Wet gels exhibit a mass fractal structure with fractal dimension D increasing from 2.23 to 2.35 and characteristic length ξ decreasing from ∼9.4 nm to ∼5.1 nm, as the DMF/TEOS molar ratio is increased from 0 to 4. The supercritical process apparently eliminates some porosity, shortening the fractality domain in the mesopore region and developing an apparent surface/mass fractal (with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3) in the micropore region. The fundamental role of the DMF addition on the structure of the aerogels is to diminish the porosity and the pore mean size, without, however, modify substantially the specific surface area and the average size of the silica particle of the solid network.     

Structural characteristics of silica sonogels prepared with different proportions of TEOS and TMOS

Sonohydrolysis of mixtures of tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) with different TMOS/(TMOS + TEOS) molar ratio R was carried out to obtain ∼2.0 × 10⁻³ mol SiO₂/cm³ and ∼86%-volume liquid phase wet gels. Aerogels were obtained by supercritical CO₂ extraction in autoclave. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The structure of the wet gels can be described as a mass fractal structure with fractal dimension D ∼ 2.2 and characteristic length ξ increasing from ∼4.6 nm for pure TEOS to ∼6.4 nm for pure TMOS. A fraction of the porosity is eliminated with the supercritical process. The fundamental role of the TMOS/(TMOS + TEOS) molar ratio on the structure of the aerogels is to increase the porosity and the pore mean size as R changes from pure TEOS to pure TMOS. The supercritical process increases the mass fractal dimension and shortens the fractality domain in the mesopore region. A secondary structure appearing in the micropore region of the aerogels can be described as a mass/surface fractal structure with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3.     

Structural study of PbO-MoO3-P2O5 glasses by Raman and NMR spectroscopy

Glasses in the ternary system PbO-MoO₃-P₂O₅ were prepared in three compositional series (100 − x)[0.5PbO-0.5P₂O₅]-xMoO₃ (A), 50PbO-yMoO₃-(50 − y)P₂O₅ (B) and (50 − z)PbO-xMoO₃-50P₂O₅ (C) and their structure was studied by Raman and ³¹P NMR spectroscopies. In the compositional series (100 − x)[0.5PbO-0.5P₂O₅]-xMoO₃ homogeneous glasses were prepared in the concentration region of 0-70 mol% MoO₃. Their glass transition temperature increases with increasing MoO₃ content having a maximum at x = 50 mol% MoO₃. ³¹P MAS NMR spectra reveal that in the glass series (A) the incorporation of MoO₃ results in the shortening of phosphate chains and gradual transformation Q² units into Q² and Q⁰ units, prevailing in glasses with a high MoO₃ content. Octahedral structural units MoO₆ dominate in most glass compositions and they are present also in the structure of Pb(MoO₂)₂(PO₄)₂ compound corresponding to the glass composition 50Pb(PO₃)₂-50MoO₃. The analysis of Raman spectra of glasses of the (B) series with a high MoO₃ content showed the transformation of octahedral MoO₆ units into tetrahedral MoO₄ units.     

Structural investigations of magnesium silicate glasses by Si 2D Magic-Angle Flipping NMR

The two-dimensional Magic Angle Flipping Nuclear Magnetic Resonance (2D MAF NMR) experiment on ²⁹Si nuclei is used to determine the distribution of Q⁽ⁿ⁾ sites in two ²⁹Si-enriched magnesium silicate glasses with compositions 2MgO·SiO₂ and MgO·SiO₂. A significant degree of polymerization is observed in the 2MgO·SiO₂ glass, supporting previous studies using Raman and ²⁹Si NMR spectroscopy. Relative abundances of 0.629 ± 0.001 for Q⁽⁰⁾ and 0.371 ± 0.001 for Q⁽¹⁾ were obtained from spectral fits of the 2D MAF spectrum of the 2MgO·SiO₂ glass. Mole fractions for the free oxygen anion and each Q⁽ⁿ⁾-species were calculated and used in a thermodynamic model of Q⁽ⁿ⁾ disproportionation to calculate an equilibrium constant of k₀ = 0.04 ± 0.02 in 2MgO·SiO₂. In the MgO·SiO₂ glass relative abundance of 0.014 ± 0.001 for Q⁽⁰⁾, 0.191 ± 0.003 for Q⁽¹⁾, 0.530 ± 0.004 for Q⁽²⁾, 0.252 ± 0.003 for Q⁽³⁾, and 0.014 ± 0.001 for Q⁽⁴⁾ were measured. The mole fractions for the free oxygen anion and each Q⁽ⁿ⁾-species in MgO·SiO₂ were used to calculate corresponding disproportionation equilibrium constants of k₁ = 0.19 ± 0.02, k₂ = 0.174 ± 0.009, and k₃ = 0.11 ± 0.01. A comparison of k₃ values from previous MAF studies of various alkali and alkaline earth silicate glasses indicate an exponential increase in k₃ with the increasing modifying cation potential. Using the van't Hoff relation, we show that differences in both thermal history and modifier cation potential contribute to this spread in k₃ values. Nuclear shielding tensor anisotropy, ζ, and asymmetry, η, values of ζ = 0.0 ppm and η = 0.0 for Q⁽⁰⁾ and ζ = 33.0 ± 0.1 ppm, and η = 0.4 ± 0.1 for Q⁽¹⁾ in 2MgO·SiO₂ glass were determined from its 2D MAF spectrum. These values were used in obtaining the remaining values of ζ = − 36.0 ± 0.5 ppm and η = 0.99 ± 0.01 for Q⁽²⁾, and ζ = − 27.5 ± 0.5 ppm and η = 0.45 ± 0.11 for Q⁽³⁾, ζ = 0.0 ppm and η = 0.0 for Q⁽⁴⁾ in the MgO·SiO₂ glass from its 2D MAF spectrum. The magnitude of ζ values observed are lower than those reported in previous MAF studies of alkali and alkaline earth silicate glasses containing different modifier cations, consistent with previously reported trends in ζ versus modifying cation potential.     

Near infrared and blue cooperative emissions in Yb-doped GeO2-PbO glasses

In previous years there has been great interest in new materials for photonic devices operating at infrared (IR) and visible (VIS) regions. We report here near infrared and blue cooperative luminescence properties for Yb³⁺-doped GeO₂-PbO glasses. Luminescence and lifetime measurements in the VIS and near-IR regions were performed to investigate the spectroscopic characteristics of the glasses. Intense emissions around 507 and 1010 nm were observed using 980 and 808 nm excitation, respectively. The VIS lifetimes (∼0.4 ms) are about half of their respective near infrared ones (∼0.8 ms), as expected for materials in which the VIS emission is caused by the cooperative effect. Regarding the IR emission, the glasses exhibited a high absorption cross-section (1.2 × 10⁻²⁰ cm²) at 978 nm and an emission cross-section of 0.6 × 10⁻²⁰ cm², at 1010 nm, with a minimum pump intensity of 2.8 kW/cm². These results suggest this glass composition as a potential material to be used in devices operating in the VIS and IR spectral range, such as 3-D displays and infrared lasers.     

High frequency acoustic attenuation of vitreous silica: New insight from inelastic x-ray scattering

We present new experimental results on the propagation and damping of the high frequency acoustic-like modes in vitreous silica. The new data are measured by means of the inelastic x-ray scattering technique down to an exchanged wavevector Q ∼ 0.9 nm ^− 1, at the limit of the instrument capabilities. Thanks to the continuous development of the technique, the new spectra are characterized by a very high signal to noise ratio when compared to previous experiments. The higher data quality finally allows for a reliable determination of the position and width of the inelastic excitations. The new data show that the sound damping Γ is marked by a frequency dependence compatible with the Rayleigh law, Γ ∼ ν⁴, for frequencies below the position of the excess vibrational modes at the boson peak. We show that the new data are in good agreement with estimates of the acoustic mean free path from the thermal conductivity, which take into account the peculiar plateau at a few Kelvin. The connection between the boson peak and the Rayleigh law is further confirmed by a comparison of the present data with literature data for the sound attenuation in a permanently densified silica sample.     

Submicron rectangular hollow tube crystals of rutile-type GeO2

Single crystals of rutile-type GeO₂ having a structure equivalent to that of TiO₂, a well-known photocatalyst, have been grown for the first time in supercritical oxygen at approximately 5 GPa and 3000 K. The obtained crystals exhibit a rectangular hollow tube structure with submicron size (cross section with sides of ∼500 nm, wall thickness of ∼20 nm, and longitudinal length of ∼5 μm). These single crystals were grown within 1 s and along the c-axis surrounded by the (1 1 0) faces. The crystal growth mechanism strongly depends on the growth mechanism of rutile-type oxides, and the extremely short growing time is an important factor in the formation of hollow tube crystals.     

Phase separation and crystallization in the system SiO2-Al2O3-P2O5-B2O3-Na2O glasses

The phase separation and crystallization behavior in the system (80 − X)SiO₂ · X(Al₂O₃ + P₂O₅) · 5B₂O₃ · 15Na₂O (mol%) glasses was investigated. Glasses with X = 20 and 30 phase separated into two phases, one of which is rich in Al₂O₃-P₂O₅-SiO₂ and forms a continuous phase. Glasses containing a larger amount of Al₂O₃-P₂O₅ (X = 40 and 50) readily crystallize and precipitates tridymite type AlPO₄ crystals. It is estimated that the phase separation occurs forming continuous Al₂O₃-P₂O₅-SiO₂ phase at first, and then tridymite type AlPO₄ crystals precipitate and grow in this phase. Highly transparent glass-ceramics comparable to glass can be successfully obtained by controlling heat treatment precisely. The crystal size and percent crystallinity of these transparent glass-ceramics are 20-30 nm and about 50%, respectively.     

Effect of P2O5 content in two series of soda lime phosphosilicate glasses on structure and properties - Part I: NMR

The effect of the variation in phosphate (P₂O₅) content on the structure of two series of bioactive glasses in the quaternary system SiO₂-Na₂O-CaO-P₂O₅ was studied. The first series (I) was a simple substitution of P₂O₅ for SiO₂ keeping the Na₂O:CaO ratio fixed (1.00:0.87). The second series was designed to ensure charge neutrality in the orthophosphate , therefore as P₂O₅ was added the Na₂O and CaO content was varied to provide sufficient Na⁺ and Ca²⁺ cations to charge balance the orthophosphate present. The glass network connectivity (NC) was calculated for each glass and a modification for the presence of a separate P₂O₅ phase was included (NC′). ³¹P and ²⁹Si magic-angle-spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was performed on glass series I and II to determine the structural units present and their relation to glass properties. ³¹P MAS-NMR spectra of series I resulted in a broad resonance around 9 ppm corresponding to orthophosphate in an amorphous environment. The 9.25 mol% P₂O₅ glass shown to be partially crystalline by X-ray diffraction was heat treated, and the ³¹P MAS-NMR spectrum showed a sharp peak around 3 ppm corresponding to calcium orthophosphate or sodium pyrophosphate and overlapping broader peaks at 8.5, 10.5 and 14 ppm possibly corresponding to two mixed calcium-sodium orthophosphate phases and amorphous sodium orthophosphate respectively. ³¹P MAS-NMR spectra of series II resulted in a broad resonance around 10.5 ppm corresponding to orthophosphate in an amorphous environment. ²⁹Si MAS-NMR spectra of glasses from series I showed a shift in the resonance peak from around −78 to −86 ppm indicating an increase in Q³ species in the glass and a reduction in Q² with phosphate addition confirming the presence of orthophosphate. The heat treated sample showed a sharp ²⁹Si-NMR resonance at −88 ppm, indicating a crystalline Q² six-membered combeite (Na₂O · 2CaO · 3SiO₂) silicate-type phase, which was confirmed by powder X-ray diffraction. ²⁹Si MAS-NMR spectra of glasses from series II showed no shift in the resonance at around −78 ppm across the series, confirming an orthophosphate environment.