Defect species in vitreous silica - a molecular dynamics simulation

The properties of overcoordinated defect species in vitreous silica were analyzed using the molecular dynamics computer simulation technique. The defect species are 3-fold- oxygen and 5-fold silicon. Frequency spectra, SiOSi bond angle distribution, and SiO bond lengths are presented for normally coordinated species and for defect species. Results indicate the presence of the C₃⁺ defect in vitreous silica. Also, a compression of the SiOSi bond angle and an expansion in the SiO bond length occur for the C₃⁺ oxygen defect.     

Novel luminescent rare earth hybrids covalently trapped through phthalic anhydride linkage

In the present work, a novel functional molecular bridge (PASi) is achieved through the modification of ortho phthalic anhydride (PA) by N-2-aminoethyl-3-aminopropyl-methyl-dimethoxylsiliane (abbreviated as AEAPMMS). The PASi plays the role of a functional bridge to link Eu³⁺ (Tb³⁺) through the oxygen atom to a Si–O network formed from tetraethoxysilane. Subsequently, a novel chemically bonded hybrid material with double chemical bonds (RE–O coordination bond and Si–O covalent bond) has been fabricated. The characteristic red luminescence of Eu³⁺ and green luminescence of Tb³⁺ indicate that an efficient energy transfer process exists within the hybrid systems.     

Ex situ XRD,TEM, IR,Raman and NMR spectroscopy of crystallization of lithium disilicate glass at high pressure

The structure of Li₂O · 2SiO₂ (LS₂) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 °C, respectively, using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si–O–Si bond angle ∼7° lower than that found in glass processed at 4.5 GPa. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa new high pressure form of lithium metasilicate crystallizes. This new phase, while having lithium metasilicate crystal symmetry, contains at least four different Si sites. NMR results for 6 GPa indicate the presence of Q⁴ species with (Q⁴)Si–O–Si(Q⁴) bond angles of ∼157°. This is the first reported occurrence of Q⁴ species with such large bond angles in alumina free alkali silicate glass. No five- or six-coordinated Si are found.     

Luminescence properties of nonbridging oxygen hole centers at the silica surface

Two variants of the surface-nonbridging oxygen hole center, (Si–O)₃Si–O^? and (Si–O)₂(H–O)Si–O^?, stabilized in porous films of silica nano-particles were investigated by time resolved luminescence excited in the visible and UV spectral range by a tunable laser system. Both defects emit a photoluminescence around 2.0 eV with an excitation spectrum evidencing two maxima at 2.0 and 4.8 eV, this emission decreases by a factor ～2 on increasing the temperature from 8 up to 290 K. However, the different local structure influences the emission lineshape, the quantum yield and the decay lifetime. Such peculiarities are discussed on the basis of the symmetry properties of these defects.     

Molecular assembly,luminescence and morphology of novel zinc/inorganic/organic nanohybrid material with covalent linkage

Polyethylene glycol (PEG) was first modified by an inorganic component of 3-(triethoxysilyl)-propyl isocyanate (TEPIC) to form the inorganic/organic polymeric functional precursor. The modified reagent with a functional group (–NHCOO–) further behaves as a bridge which can coordinate to Zn²⁺ through oxygen atom and further formed Si–O backbones after hydrolysis and polycondensation processes. Subsequently, the corresponding organic/inorganic molecular-based hybrids were assembled to behave as the structural polymeric ligands with the two components equipped with covalent bonds. Finally, ternary zinc/inorganic/organic polymeric hybrid materials with chemical bond (covalent bonds of –CO–NH– and Si–O, coordination bond of Zn–O–C) have been assembled. The resulting hybrids exhibit blue luminescence and nanometer morphology.     

Diffusion and reactions of interstitial oxygen species in amorphous SiO2: A review

This article briefly summarizes the diffusion and reactions of interstitial oxygen species in amorphous SiO₂ (a-SiO₂). The most common form of interstitial oxygen species is oxygen molecule (O₂), which is sensitively detectable via its characteristic infrared photoluminescence (PL) at 1272 nm. The PL observation of interstitial O₂ provides key data to verify various processes related to interstitial oxygen species: the dominant role of interstitial O₂ in long-range oxygen transport in a-SiO₂; formation of the Frenkel defect pair (Si–Si bond and interstitial oxygen atom, O⁰) by dense electronic excitation; efficient photolysis of interstitial O₂ into O⁰ with F₂ laser light (λ = 157 nm, hν = 7.9 eV); and creation of interstitial ozone molecule via reaction of interstitial O₂ with photogenerated O⁰. The efficient formation of interstitial O⁰ by F₂ laser photolysis makes it possible to investigate the mobility, optical absorption, and chemical reactions of interstitial O⁰. The observed properties of O⁰ are consistent with the model that O⁰ takes the configuration of Si–O–O–Si bond. Interstitial O₂ and O⁰ react with dangling bonds, oxygen vacancies, and chloride groups in a-SiO₂. Reactions of interstitial O₂ and O⁰ with mobile interstitial hydrogen species produce interstitial water molecules and hydroperoxy radicals. Interstitial hydroxyl radicals are formed by F₂ laser photolysis of interstitial water molecules.     

High resolution X-ray Photoelectron Spectroscopy (XPS) study of K2O–SiO2 glasses: Evidence for three types of O and at least two types of Si

High resolution O 1s, K 2p and Si 2p XPS Spectra were collected for a series of potassium silicate glasses ranging in composition from 10 mol% to 35 mol% K₂O. The mole fraction of bridging oxygen (BO) has been accurately evaluated from the O 1s spectra. BO mole fractions of K-silicate glasses were calculated from Q-species distributions previously reported by ²⁹Si MAS NMR data. The mole fractions of BO are identical for the two techniques (within experimental error) in glasses containing 13 mol% to 25 mol% K₂O but in the compositional range between 25 mol% and 35 mol% BO mole fractions obtained by XPS are slightly greater than values derived from NMR data. The slight discrepancies between the two techniques at higher K₂O content have not been resolved. The experimental data between 13 mol% and 25 mol% K₂O indicate the presence of a third type of oxygen, O^2?, in these glasses. A thermodynamic analysis indicates O^2? is present at a few mol% in the glasses of low K₂O content, but increases monotonically with increased K₂O content.The O 1s XPS line widths for the BO peaks are highly variable. The variation in line widths may result from two types of BO contributing to the BO peak. As in the Na₂O–SiO₂ glass system, one type probably bridges two Si atoms (SiOSi) and the second type is O bonded to two Si atoms and one K atom.The Si 2p XPS spectra are distinctly non-symmetric, with low binding energy shoulders commonly present on the major peak, suggesting two contributions to the Si 2p signal. There is a strong correlation of Si 2p XPS peak and shoulder intensities with the abundances of the Q⁴ and Q³ species in glasses of the same composition suggesting that, with additional resolution, XPS may be capable of resolving individual Q-species in this system.     

An X-ray diffraction refinement of the crystal structure of natural apophyllite

The crystal structure of apophyllite (Andersberg — GDR), with a = 8.966(2), c = = 15.767(1) Å, P4/mnc, z = 2, Q = 2.35 gm · cm⁻³, has been refined (R = 0.035) by least squares with 976 reflections collected with a diffractometer. Apophyllite is a sheet structure with the bringing Si O bond lengths 1.6236 Å, while the non-bridging bond length Si O(3) is 1.5843 Å. The two independent Si O Si angles are 140.09, 140.76 degrees. The mean Si O bond length is 1.6138 Å. As it was not possible to locate unequivocally the H-atoms with the X-ray data, the interpretation of the of water in the structure was based on the charge balance approach of DONNAY and ALLMAN . Assuming that the fluorine ion is bonded to calcium ion (Ca-F = = 2.416 Å) and to H3 atom (F-H3 = 0.9145(1) Å) and water molecule hydrogen bonded to silicate framework. The average bond distances O-H are 0.962(2) Å and angle 105.24(1)°.     

Effect of BaO addition on the structural aspects and thermophysical properties of sodium borosilicate glasses

Sodium borosilicate glasses containing different amounts of BaO were prepared by a conventional melt quench method and characterized for their structural aspects by ²⁹Si, ¹¹B NMR and IR spectroscopy. From ²⁹Si MAS NMR studies, it has been inferred that these glasses consist of Q² and Q³ structural units of silicon and that the addition of BaO results in the marginal conversion of Q³ to Q² structural units. There is no direct interaction between Ba²⁺ ions and boron structural units, as revealed by the identical values of the relative concentration of BO₃ and BO₄ structural units and quadrupolar coupling constant values for the BO₃ structural units. The identical values of glass transition temperature and vibrational frequencies corresponding to Si–O–Si/Si–O–B and B–O linkages of all the samples further support this. As the borosilicate network is unaffected, the systematic increase in the values of thermal expansion coefficient with increase in BaO content has been attributed to the increase in the relative concentration of less rigid Ba–O linkages compared to the more rigid Si–O and B–O linkages in the glass. Such studies will be useful for the development of matrices for the management of nuclear waste generated during the reprocessing of the spent fuel from thoria based reactors.     

Comparative infrared lattice vibration study of LiGaO2 and LiInO2 (II). Interatomic force constants

The frequencies of the wurtzite-like modes due to Li–O and Ga–O bond vibrations in LiGaO₂ and of the rocksalt-like mode due to Li–O bond vibrations in LiInO₂ are determined from the infrared reflectivity spectra of the compounds. The force constants for the Li–O and Ga–O bonds in LiGaO₂ follow the same trends as those found for the A^IB^IIIC₂^VI chalcogenides with tetrahedral cordination. The results for LiInO₂ are compared with measurements on the octahedrally coordinated modification of LiAlO₂.     

Radiation induced generation of non-bridging oxygen hole center in silica: Intrinsic and extrinsic processes

The generation of non-bridging oxygen hole center (Si–O) was investigated in a wide variety of natural (fused quartz) and synthetic silica samples exposed to different γ- and β-irradiation doses by looking at its optical bands. We distinguish two different generation processes: intrinsic associated with the cleavage of Si–O bond and characterized by a sublinear law and extrinsic due to the conversion of OH precursor characterized by a growth curve with a saturating tendency. The interplay between the two processes and the role of H are discussed.     

Studies on binary silicate glasses based on the SiKα and SiKß emission X-rays

The SiKα and Kβ X-ray emission spectra of binary silicate glasses of the Li₂O–SiO₂, Na₂O–SiO₂, K₂O–SiO₂, Cs₂O–SiO₂, B₂O₃–SiO₂, and GeO₂–SiO₂ systems were measured with an X-ray fluorescence spectrometer to examine the chemical effects on the spectra. The SiKα peak wavelengths for all the glasses agreed with that for SiO₂ glass, which corresponded to the concept that the coordination number of Si shouldbe four in all the glasses examined in this study. The SiKβ peak wavelength decreased with increasing alkali content in the alkali silicate glasses, indicating that the Si–O bond weakend in average as the alkali oxide was added to SiO₂ glass. On the other hand, no drastic shift in the SiKβ peak wavelength was observed in the B₂O₃–SiO₂ and GeO₂–SiO₂ systems, and this was interpreted as showing the constancy of Si–O bond strength in these glasses.     

Network structure of multi-component sodium borosilicate glasses by neutron diffraction

Neutron diffraction structure study has been performed on multi-component sodium borosilicate based waste glasses with the composition of (65 − x)SiO_2. · xB₂O₃ · 25Na₂O · 5BaO · 5ZrO₂, x = 5–15 mol%. The maximum momentum transfer of the experimental structure factor was 30 Å⁻¹, which made available to determine the distribution function with high r-space resolution. Reverse Monte Carlo modelling was applied to calculate several partial atomic pair correlation functions, nearest neighbor distances and coordination numbers have been revealed. The characteristic features of Si–O and Si–Si distributions are similar for all glassy samples, suggesting that the Si–O network consisting of tetrahedral SiO₄ units is highly stable even in the multi-component glasses. The B–O correlations proved to be fairly complex, two distinct first neighbor distances are present at 1.40 Å and 1.60 Å, the latter equals the Si–O distance. Coordination number distribution analyzes has revealed 3 and four-coordinated boron atoms. The O–O distribution suggests a network configuration consisting of boron rich and silicon rich regions. Our findings are consistent with a structure model where the boron rich network contains mostly trigonal BO₃ units, and the silicon rich network is formed by a mixed continuous network of ^[4]Si–O–Si^[4] with several different ^[4]B–O–Si^[4] and ^[3]B–O–Si^[4] linkages.     

Laser-induced defects in fused silica by UV laser irradiation

The structure of fused silica with irradiation of the third harmonic of the Nd:YAG laser was investigated by Fourier transform infrared, Raman and Photoluminescence spectroscopy. Some variation in the Si/O stoichiometry of silica in the ablated spot was induced. The primary defect species are oxygen deficient centers and oxygen interstitials. The frequency shift of the Si–O–Si vibration proves that the central force constant between oxygen and silicon atoms, and the band angle of Si–O–Si increases in the UV-laser ablation spot. Small silicon clusters within SiO_x appear to be a possible explanation for the 564 nm Fluorescence peak, and the 181 cm⁻¹ Raman peak.     

Ca–Mg mixing in aluminosilicate glasses: An investigation using 17O MAS and 3QMAS and 27Al MAS NMR

To better understand non-framework cation mixing in Ca–Mg aluminosilicate glasses, ¹⁷O MAS and 3QMAS NMR studies were done on glasses on the Mg₃Al₂Si₃O₁₂–Ca₃Al₂Si₃O₁₂ join as well as several compositions with lower Al/Si ratios. While not all of the oxygen sites are fully resolved, the non-bridging oxygen associated with two Ca and one Mg cations is under-represented relative to that predicted by a model assuming random Ca/Mg mixing. Therefore, local non-bridging oxygen environments are rich in Mg, and extra Ca must be associated with charged bridging oxygens such as Al–O–Si. The deviation from random mixing increases as the Al/Si ratio decreases and as X_Mg approaches 0.50, suggesting a reduction in configurational entropy that may be compensated for by other sources, including mixing of network forming cations. Al–O–Al is detected, and appears to increase with increasing X_Mg and increasing Al/Si. High field ²⁷Al MAS NMR also shows that these glasses all have substantial concentrations of ^[5]Al, but no detectable ^[6]Al (0.5% detection limit). The amount of ^[5]Al increases non-linearly as X_Mg increases and very slightly as Al/Si increases.     

Synthesis and Crystal Structure of μ-oxo-bis[(octaethyloxoporphinato)iron(III)] Tetrafluoroborate

Exposure of dichloromethane solution of [OEOPFe(BF₄)], where OEOP is the monoanion of octaethyloxoporphyrin, to dioxygen results in its transformation into the μ-oxo bridged compound, [(OEOPFe)₂O)](BF₄)₂. The molecular structure of the title compound, [(OEOPFe)₂O](BF₄)₂, was determined by single-crystal X-ray diffraction. It contains a binuclear centrosymmetric [(OEOPFe)₂O]²⁺ cation (the bridging O atom lies on an inversion centre) and two tetrafluoroborate anions. The Fe atom is five-coordinate to four N atoms of the porphyrin ring and to one bridging O atom. The compound is characterized by an average Fe–N bond length of 2.064 Å. The Fe–O bond distance is 1.7665(11) Å and the Fe–O–Fe bond angle is 180.0° and the two porphyrin rings are parallel. Crystal data: crystal system, monoclinic, a = 8.867(3), b = 26.104(9), c = 15.748(6) Å, β = 105.40(3)°, space group, P2₁/c, V = 3514(2) Å³, Z = 2.     

Germanosilicate and alkali germanosilicate glass structure: New insights from high-resolution oxygen-17 NMR

We present here triple-quantum, magic-angle spinning (3QMAS) NMR spectra for ¹⁷O in a SiO₂–GeO₂ binary glass, and for two sodium germanosilicate glasses, all with Si/Ge ratios of 1. In the binary germanosilicate, three NMR peaks are partially resolved, and correspond to the three types of bridging oxygens, Si–O–Si, Si–O–Ge, and Ge–O–Ge. Peak areas indicate that the relative abundances of these species are close to those expected for random mixing of the Si and Ge in the network. In a sodium germanosilicate glass with a relatively low Na content (Na₂O ≈ 8 mol%), the spectra demonstrate the formation of significant fractions of both nonbridging oxygens bonded to Si, and of oxygens bonded to Ge in five- or six-coordination. At higher Na content (Na₂O ≈ 31%), most or all Ge is four-coordinated and network modification is dominated by the formation of NBO on Si and on Ge. Models of physical properties of alkali germanosilicates, in which modifier oxides are distributed between the Si and Ge components of the network in proportion to the Si/Ge ratio, are thus supported, as is extensive mixing of Si and Ge.     

Improved method for preparation of anhydrous silica by microwave irradiation with spectroscopic characterization and toxicity assay

Amorphous anhydrous silica SiO_{2 (mw)} (99.99%) is successfully synthesized through microwave irradiation technique and time of reaction is reduced up to 1 h. The dehydration phase study of Si–water, Si–OH, Si–O–Si networking, elemental analysis and surface morphology was carried out by FTIR, FTNIR, SEM and EDAX spectroscopic techniques. The broad absorption stretching and bending of Si–OH and H₂O at 3695.38–2832.96 cm^? 1, 1638 cm^? 1 and 1191.20–1017.14 cm^? 1 completely disappeared and appearance of new bands at 946.93 and 797.63 cm^? 1 confirmed the amorphous anhydrous silica with Si–O–Si networking. The SEM images of SiO_{2 (mwc)} described the smooth and fine particle texture and confirmed 99.99% Si–O–Si networking of anhydrous silica. The 99.99% purity was verified by EDAX spectra which exhibited sharp signals only for oxygen and silicon. Toxicity against Monomorium minimum and Tribolium castaneum with 100% mortality and LT₅₀ 91 min and 7.5 h respectively is being reported. It can be used for long storage of grains in the future.     

Crystal and Molecular Structure of an Acridinedione

The title compound, 10‐(4‐hydroxybenzoylamino)‐3,4,6,7,9,10‐hexahydro‐1,8‐(2H,5H)‐acridinedione monohydrate,C₂₀H₂₂N₂O₄.H₂O, consists of partially hydrogenated acridine moiety with one benzoylamino substituent on the central ring. The compound crystallizes in monoclinic system with P2₁/c space group and the unit cell constants are: a = 11.142(4), b = 12.266(2), c = 13.320(2) Å; β = 91.76(2)° and V = 1819.6(8) ų.The central ring (B) adopts boat and the outer rings (A and C) adopt sofa conformations. The water molecule takes part in OW‐H...O and N‐H...OW hydrogen bond formation with acridinedione and benzoylamino group. The oxygen atom O1 interacts through O–H...O bond with the hydroxyl group and in a head‐to‐tail link up of molecules that results in the formation of an infinite “supra molecular“ chain.