The structure of germanosilicate glasses,studied by X-ray photoelectron spectroscopy

O1s photoelectron spectra of several alkali silicate, germanate and germanosilicate glasses are given. The concentration ratio of bridging and non-bridging oxygen atoms is determined from these spectra. In contrast to silicate glasses, in germanate glasses non-bridging oxygen atoms are only observed when the alkali oxide mole fraction exceeds 0.18. The introduction of SiO₂ in the germanate glasses drastically alters the ratio of fourfold coordinated to sixfold coordinated germanium atoms.     

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.     

Competition between photorelaxation and photoexcitation in chalcogenide glasses and the effect of aging

Measurements of transient photodarkening and permanent Bragg reflector formation were performed on fragile As₁₈S₆₅Se₁₇ glass and indicate that photodarkening and photorefraction can only be induced in previously aged glasses while they are prevented in freshly annealed glasses. These phenomena are associated with the photorelaxation process that is revealed to be prominent in fresh glasses and is shown to effectively anneal out the defects produced during photoexcitation thereby resulting in no net permanent changes in the structure. On the other end, aged glasses do not undergo any photorelaxation and exhibit large transient and permanent changes associated with an increase in structural enthalpy as confirmed by Differential Scanning Calorimetry. These results demonstrate that photorelaxation can be a significant problem in fragile glass compositions as it cancels out the effect of photoexcitation and impedes the photodarkening and photorefractive process.     

Local order around tungsten atoms in tungstate fluorophosphate glasses by X-ray absorption spectroscopy

X-ray absorption spectroscopy was used to study the local environment of tungsten atoms in NaPO₃–BaF₂–WO₃ glasses and the results were compared with crystalline references Na₂WO₄ and WO₃. XANES measurements at the W-L₁ edge allowed to determine a distorted octahedral environment of tungsten atoms in these glasses similar to the local order of tungsten in monoclinic WO₃. Extended X-ray absorption fine structure (EXAFS) has been used as a local probe to monitor the effect of WO₃ concentration on the tungsten environment. Based on an analysis of the EXAFS data, we proposed a three-shell model of oxygen atoms around tungsten as in monoclinic WO₃. With increasing WO₃ concentration, it was found that R₂ decreases from 1.96 to 1.92 Å whereas R₃ increases from 2.07 to 2.12 Å.     

Structural investigation of SnO-B2O3 glasses by solid-state NMR and X-ray photoelectron spectroscopy

Local structure of the SnO-B₂O₃ glasses was investigated using several spectroscopic techniques. ¹¹B MAS-NMR spectra suggested that BO₄ tetrahedral units maximized at around the composition with 50 mol% SnO. The BO₄ units were still present at compositions with high SnO content (67 mol% SnO), suggesting that SnO acted not only as a network modifier but also as a network former. O1s photoelectron spectra revealed that the addition of small amounts of SnO formed non-bridging oxygens (NBO) (B-O?Sn) and the amounts of NBO increased with an increase in SnO content. ¹¹⁹Sn Mössbauer spectra indicated that Sn was present only as Sn(II) in the glasses. The structure of the SnO-B₂O₃ glasses was compared with that of conventional alkali borate glasses and lead borate glasses. The thermal and viscous properties of these glasses were discussed on the basis of the glass structure revealed in the present study.     

Local structural environment and intra-4f transition of rare-earth ion in chalcogenide glass: Comparison between Dy-doped Ge–As–S and Ge–Ga–S glasses

We have employed Dy L₃-edge extended X-ray absorption fine structure measurements to investigate the local structures of Dy doped in Ge–As–S and Ge–Ga–S glasses. It turned out that Dy–S distance on average is conspicuously longer in Ge–Ga–S glass despite the number of the nearest neighboring S atoms being nearly identical with each other. The enhanced rare-earth solubility of Ge–Ga–S glass is then explained in connection with decrease in covalence of Ga–S bonds compared with Ge–S or As–S bonds, and structural correlation between GaS₄ tetrahedra and Dy. The discrepancy in the local structural environments of trivalent Dy, however, results in no significant changes in the spectral lineshape and the mean energy of its intra-4f-configurational transitions.     

Defect configurations in Ge-S chalcogenide glasses studied by Raman scattering and positron annihilation technique

Defect configurations of Ge-S binary glasses have been studied systemically by Raman scattering technique and positron annihilation lifetime spectra (PALS). The correlations between the positron lifetime data, structural features, and chemical compositions of Ge-S binary glasses have been established, and also the identification of open volume originated from coordination defects has been carried out. The cognizance of defect configuration will be very helpful to further understand the unique photosensitivity of chalcogenide glasses.     

High-temperature X-ray analysis of phase evolution in Sr-doped zinc-silicate glasses

Bone grafts are required in many clinical situations. Autografts are the traditional gold standard for treating conditions requiring bone grafts. However autografts have inherent drawbacks such as donor site morbidity, pain and increased operative time. An alternative for autografts are synthetic grafts. A series of strontium doped zinc silicate glasses were developed which were investigated using high temperature X-ray diffraction (HT-XRD) in order to establish phase transformations, which occur up to the first crystallization temperature, (T_p1), thus identifying the composition-structure relationships which arise during this thermal processing. In analysing BT110 it was observed that all glass material crystallised into 4 phases including strontium zinc silicate, sodium calcium silicate, calcium silicate and strontium silicate, leaving no residual glass phase. BT111 and BT112 were shown to contain a residual glassy phase alongside for BT111, sodium zinc silicate, larnite and silicon oxide and for BT112 strontium silicate, calcium silicate, sodium silicate and silicon oxide. In the case of BT111 the residual glass phase appears to be rich in strontium. The residual glass phase being Sr enriched with respect to the glass-ceramic may offer increased release of Sr²⁺ from the material; important for the regulation of osteoblastic and osteoclastic activity. BT113 crystallized to form strontium silicate, sodium silicate, and strontium zinc silicate. BT114 crystallized to form strontium silicate and sodium silicate. The biocompatibility of phases formed in BT113 and BT114 is as yet unknown. Further knowledge will be generated by later work examining the biocompatibility of these phases identified in this research. However, on the basis of these results, the materials (BT110-BT112) exhibit potential as a bone graft substitutes, whilst BT113-BT114 give rise to phases with unknown biocompatibility and so warrant further investigation.     

Structure of ultraphosphate glasses with small rare-earth ions by X-ray diffraction

Rare-earth ultraphosphate glasses with nominal R₂O₃ fractions of 10 and 15 mol% and small ionic radius (large atomic number) R³⁺ ions (R = Tb, Tm, Lu) are measured by X-ray diffraction at a synchrotron with photons of 119 keV (maximum scattering vector 260 nm^− 1). The total correlation functions T(r) show well-resolved R-O and O-O first-neighbor peaks. In contrast to all ultraphosphate RP₅O₁₄ crystals and the ultraphosphate glasses of larger R³⁺ ions, where RO₈ polyhedra (mean R-O coordination numbers of ~ 8 for the glasses) are observed, the R³⁺ ions in glasses with R = Lu, Tm, Tb have mean R-O coordination numbers of ~ 7.5. The R-O first-neighbor peaks extracted from the T(r) functions are compared with those obtained from atomic coordinates of related RP₅O₁₄ and RP₃O₉ crystals. The R-O distances of the ultraphosphate glasses studied are found to fall between those of the two crystals but with tails to the side of longer bonds.     

Structure of potassium germanophosphate glasses by X-ray and neutron diffraction: 2. Medium-range order

Characteristics of the medium-range order (MRO) of K₂O-GeO₂-P₂O₅ (KGP) glasses are obtained from X-ray and neutron scattering data. The behavior of the MRO is expressed in changes of pre-peaks and smooth contributions in the structure factors, S(Q), for Q < 12 nm^-1. Peaks at Q = (7.5 ± 0.5) nm^-1 (Q - magnitude of the scattering vector) are the outstanding features and reach maximum intensity for a glass of ∼25/50/25 mol% K₂O/GeO₂/P₂O₅. The pre-peaks are explained by a structural model in which K-rich and Ge-rich regions are separated by PO₄ units. The distance of repetition of similar regions (∼1 nm) is responsible for the pre-peak at ∼7.5 nm^-1. The intensity of this pre-peak is reduced and finally eliminated for glass compositions approaching the binary GeO₂-P₂O₅ system. The pre-peak is changed to a smooth scattering contribution and finally shifted to ∼9 nm^-1 for glass compositions approaching the binary K₂O-GeO₂ system. The strong tendency of the PO₄ units to coordinate K⁺ and Ge neighbors at their four corners is the source for the special MRO of the KGP glasses.     

Determination of sulfur environments in borosilicate waste glasses using X-ray absorption near-edge spectroscopy

Sulfur can be the waste-loading limiting constituent for vitrification of sulfur-bearing radioactive wastes due to low solubility in silicate melts. Methods to improve sulfur loading would benefit from improved understanding of the structural aspects of sulfur incorporation in borosilicate and other glasses. To this end, sulfur XANES spectra were collected for eight crystalline standards and twenty-four glasses, including borosilicate, phosphate, and borate compositions. Spectra for the standards show a systematic energy shift of the sulfur K-edge from 2469 to 2482 eV, as sulfur valence increases from 2− (in sulfides) to 6+ (in sulfates). Most crucible glasses investigated have simple edges near 2482 eV that indicate sulfur in the form of sulfate only. Other glasses, some synthesized under reducing conditions, have complicated edges, indicating sulfate, sulfite, and more reduced species that may include S, S-S doublets, or short polysulfide chains. Sulfide species (S²⁻) were not dominant in any of the samples over the range of redox conditions investigated. These results indicate that sulfur incorporation is considerably more complex than would be suggested by the conventional interpretation of the redox-dependence of sulfur solubility, which considers only sulfate and sulfide species. Raman data indicate that several of the glasses investigated are not homogeneous with regard to all sulfur species.     

Molecular dynamics simulation of radiation damage in glasses

Molecular dynamics simulations of the ballistic effects arising from displacement cascades in glasses have been investigated in silica and in a SiO₂-B₂O₃-Na₂O glass. In both glasses the T-O-T′ angle (where T and T′ are network formers) diminishes, despite radiation causes opposite effects: while the ternary glass swells and silica becomes denser. We show that radiation-induced modifications of macroscopic glass properties result from structural change at medium/range, reflecting an increasing disorder and internal energy of the system. A local thermal quenching model is proposed to account for the effects of ballistic collisions. The core of a displacement cascade is heated by the passage of the projectile, then rapidly quenched, leading to a process that mimics a local thermal quenching. The observed changes in both the mechanical and structural properties of glasses eventually reach saturation at 2 10¹⁸ α/g as the accumulated energy increases. The passage of a single projectile is sufficient to reach the maximum degree of damage, confirming the hypothesis postulated in the swelling model proposed by J.A.C. Marples.     

Tin valence and local environments in silicate glasses as determined from X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) was used to characterize the tin (Sn) environments in four borosilicate glass nuclear waste formulations, two silicate float glasses, and three potassium aluminosilicate glasses. Sn K-edge XAS data of most glasses investigated indicate Sn⁴⁺O₆ units with average Sn-O distances near 2.03 Å. XAS data for a float glass fabricated under reducing conditions show a mixture of Sn⁴⁺O₆ and Sn²⁺O₄ sites. XAS data for three glasses indicate Sn-Sn distances ranging from 3.43 to 3.53 Å, that suggest Sn⁴⁺O₆ units linking with each other, while the 4.96 Å Sn-Sn distance for one waste glass suggests clustering of unlinked Sn⁴⁺O₆ units.     

Structural study of thiosaccharin by single crystal X-ray diffraction and infrared spectroscopy

The structure of thiosaccharin (1,2-benzisothiazol-3(2H)-thione 1,1-dioxide) has been investigated by X-ray diffraction and infrared spectroscopic methods. The compound crystallizes in the orthorhombic space groupFdd2 witha=26.591(3),b=25.058(3),c=4.934(5) Å,Z=16. The structure consists of thiosaccharin molecules bonded to each other through N–H...O intermolecular hydrogen bonds. The infrared spectra of the thiosaccharin at room and liquid-nitrogen temperature were recorded. The spectral features in the N–H stretching region were correlated with the crystallographic data on the geometry of the N–H...O hydrogen bonding. In an attempt to assign the bands due to the SO₂ stretching modes, the spectrum of thiosaccharin was compared with that of saccharin (1,2-benzoisothiazole-3(2H)-one, 1,1-dioxide).     

Depth‐profile analysis from X‐ray photoelectron spectroscopy on As‐implanted ZnO activated in ozone ambient

Chemical bonds of As‐implanted ZnO annealed in ozone molecular (O₃) ambient were analyzed through the x‐ray photoelectron spectroscopy as a function of the etching depth. With the etching depth increased to 25 nm from surface, the peaks of Zn2p and O1s core levels shifted toward the low‐binding energy, and the bonding formation of As 3d core level gradually varied from As₂O₅ to As₂O₃. The new peaks were observed, which were posited to high‐binding energy of 4.4 eV from Zn2p and O1s core levels. Finally, the chemical bonds of As‐based oxides were found to consist of Zn(AsO₃)₂, As₂O₅, and As₂O₃. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Local order and non-linear optical properties in bulk nanostructured niobiosilicate glasses

This communication presents and discusses an experimental proof of the correlation among local structure and second harmonic generation (SHG) in bulk nanostructured potassium niobiosilicate (KNS) glasses. In particular, SHG shows a maximum in correspondence of the early stages of nanostructuring, that are characterized by the segregation within the amorphous matrix of nanosized inhomogeneities. EXAFS experiments indicate that these inhomogeneities are determined by the maximum size reached by the niobium second coordination shell combined with the sudden inclusion of potassium atoms in it. Such rearrangement at the local level determines the maximum fluctuation of the bulk glass refractive index and in turn its maximum SHG activity.     

Ti production under ionizing radiation in aluminoborosilicate glasses studied by EPR spectroscopy

Reduction processes under irradiation of Ti⁴⁺ ions in aluminoborosilicate glasses have been studied by EPR spectroscopy at 20 K. Different parameters like the [Na]/[Ti] ratio and the integrated dose were analyzed in this work. Simulation of the Ti³⁺ ion EPR spectra has shown three different Ti³⁺ environment attributed to one ^[VI]Ti³⁺ and two ^[V]Ti³⁺ environment (square pyramid and trigonal bi-pyramid). The ^[VI]Ti³⁺ ion environment is observed only for higher [Na]/[Ti] ratios although the two others are observed for all values of the [Na]/[Ti] ratio considered.     

The structure of simple silicate glasses in the light of Middle Infrared spectroscopy studies

The aim of the work is to determine the internal structure of simple silicate glasses based on structural studies. Due to the absence of the long-range order, the X-ray methods usually applied in the studies of crystalline materials are of low applicability in the investigations of glasses. Therefore, spectroscopic methods, such as Middle Infrared (MIR), which make it possible to ‘see’ the short- and the middle-range orders are extremely suitable in their studies. MIR investigations have shown that the glasses studied exhibit domain composition, which corresponds to the order of certain crystalline phases. Analysis of the MIR spectra of simple silicate glasses and their mathematical decomposition allowed us to identify the bands characteristic for ring systems as well as those originating from Si-O⁻, Si = O defects. Appearance of the bands characteristic for pseudolattice ring vibrations (740-600 cm^− 1) in the MIR spectra of glasses is an evidence of the existence of over-tetrahedral order.     

Structural investigations of tungsten silver phosphate glasses by solid state NMR, vibrational and X-ray absorption near edge spectroscopies

Glasses were prepared in the pseudo-binary system (1 − x)AgPO₃-xWO₃ (0≤ × ≤ 0.6 mol%). The structural evolution of the vitreous network was studied as a function of composition by thermal analysis, Fourier Transform Infrared spectroscopy (FTIR), Raman scattering, high resolution ³¹P solid state NMR and XANES at the W-L₁ absorption edge. For compositions with x ranging from 0 to 0.5 a pronounced increase in the glass transition temperature is observed, suggesting a significant increase in the glass network connectivity. At the same time Raman spectra indicate that tungsten atoms are linked to non-bridging oxygen atoms (W-O- or W=O bonded species) whereas the ³¹P MAS-NMR spectra indicate the successive formation of new P-O-W linkages. The formation of some anionic tungsten sites (if these are revealed by the presence of W-O terminal bonds) implies an increase in the average degree of polymerization of the phosphate network, which is consistent with the compositional evolution of the ³¹P MAS-NMR spectra at low x values. For higher x-values, the Raman spectra indicate the presence of W-O-W linkages. W-L₁ XANES data indicate that at least 90% of tungsten atoms are octahedrally coordinated.     

Bonding and ion-ion interactions of Mn ions in fluoride-phosphate and boro-silicate glasses probed by EPR and fluorescence spectroscopy

Electron paramagnetic resonance (EPR) and fluorescence spectroscopy are sensitive and selective methods for probing coordination and bonding of Mn²⁺ ions in glasses. Both methods provide additional information on Mn-Mn ion interactions and cluster formation. Mn²⁺ was found to be tetrahedrally coordinated in boro-silicate glasses of high optical basicity, and octahedrally coordinated in low alkaline boro-silicate glasses (duran-type) as in fluoride-phosphate glasses. Broad emission bands and multicomponent fluorescence decay curves in duran glasses indicate very strong Mn-Mn ion interactions and the presence of multiple Mn²⁺ sites. Site distribution is more homogenous in metaphosphate glasses, though concentration quenching is apparent at high Mn-levels. As the Mn-content increases the EPR spectra show exchange narrowing due to a decrease in the Mn-Mn distances in the duran series, but show extreme linewidth broadening due to increased cluster sizes at constant Mn-Mn distances for metaphosphate glasses. For the fluoride-phosphate and boro-silicate systems investigated, fluorescence lifetimes are found to decrease as the wavelength of the emission maximum increases and with increasing g-values of the sextet at g = 2. For octahedral coordination of Mn²⁺ ions the EPR hyperfine splitting constant decreases linearly with increasing optical basicity, as a result of an increasing covalent character of the Mn²⁺-ligand bond.