Diffraction studies of glass structure: (V). The structure of some arsenic chalcogenide glasses

X-ray and neutron diffraction experiments have been performed on vitreous arsenic sulphide close to the composition As₂S₃ and on a series of vitreous arsenic selenides including compositions both arsenic- and selenium-rich with respect to As₂Se₃. The data have been analysed by direct examination of the diffraction patterns, by Fourier analysis and by comparison both with quasi-crystalline calculations based on the orpiment structure and with models of the first neighbour correlations using the experimental peak functions. Sheet structures occur in all the glasses, the sheet separations being larger than in the corresponding orpiment structures, and the intersheet correlation perpendicular to the sheets extends for 15 Å in arsenic sulphide and 20 Å in arsenic selenide. For the selenide glasses the sheet structure becomes more pronounced with increasing arsenic concentration. For all the glasses the data are consistent with full satisfaction of the respective two- and threefold covalency requirements of chalcogenide and arsenic atoms, excess atoms being incorporated into the structure by the formation of covalent bonds between like atoms. The distribution of arsenic-chalcogen bond lengths is shown to be symmetrical about the average values of 2.24 Å and 2.44 Å for sulphide and selenide respectively. In the selenide glasses close to the As₂Se₃ composition, the average first SeSe distance is 3.65 Å. The advantages and limitations of the experimental and analytical techniques employed are discussed and a simple model based on steric considerations is advanced to account for the influence of arsenic concentration on the inter-sheet correlation.     

Diffraction studies of rare-earth phosphate glasses

A series of lanthanide oxides was incorporated in a vitreous phosphate host network. Molar constituents of the glasses were typically (La₂O₃)₁₀(R_xO_y)₁₀(Al₂O₃)₅(P₂O₅)₇₅. Each glass had a different lanthanide (R atom) from the series; La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er and the values of x and y depended on the valency of the rare-earth atom. Both X-ray and neutron diffraction were employed in examining their structures. The results indicate that the basic PO₄ tetrahedral unit remains unaltered with an average P–O distance of and predominant Q² linkages to its neighbouring units so as to form a continuous network while accommodating the included lanthanides. In accordance with this model, the average distance of rare-earth (comprising La and a second type of R atom) to oxygen decreased from 2.44 to 2.26 , a trend to be expected from the lanthanide contraction. The average oxygen coordination around the rare-earth was found to vary in the range of 6–8. With these average parameters, a small (74 atom) hand-built model was made to check the feasibility of constructing a continuous random network. Optical transmission measurements show all these glasses to absorb strongly in the UV region and to have marked absorption resonances in the visible region of 400–1000 nm except for the La, Ce, Eu, Tb containing glasses which have low or negligible absorption in the latter range.     

Diffraction methods analysis of the approximation of the real glass structure

X-ray and electron diffraction methods are discussed and the results of an analysis of glass structure are reported.Experimental methods were employed to analyse the structures of a series of complex oxide glasses of the type:

$\text{SiO}{}_2\text{(～72\%)+}\text{Na}{}_2\text{O}\text{(～14\%)+}\text{CaO}\text{(～8\%) +}\text{MgO}\text{(～4\%)+R}{}_2\text{O}{}_3\text{(～2\%)}$

.In calculating the structural parameters of the real glass structure the assumed effective molecule was approximated which included from two (SiO₂) to five (SiO₂Na₂OCaOMgO) groups of various atoms. The structural parameters obtained allowed the real internal structure to be established and were employed in the construction of a space model of the glass.The most probable structural models are presented, obtained by successive approximation with individual oxide groups added one after another. The models' structural and physicochemical parameters were chosen to be consistent with those of the nodal model which is near to the real glass structure. The parameters required to construct the nodal model in question were established by analysing radial distribution functions and by the differentiation of the adequate radial distribution functions by subtraction and successively approximating the effective molecule including the oxide groups (SiO₂, Na₂O, CaO and MgO) added one after another.An aperiodic model of a nodal lattice is presented in a scheme, stressing the structural parameters and local orderings in groups describing the internal structure of complex oxide glasses. Also, an approximate nodal space model of great importance for the construction of a structural model of glass is presented and discussed with regard to its utility and accuracy in the determination of its parameters.     

Structural investigation of sodium borate glasses and melts by Raman spectroscopy. III. Relation between the rearrangement of super-structures and the properties of glass

The structure of the intermediate range order (IRO) of sodium borate glasses and melts were quantitatively investigated by the analysis of high-temperature Raman scattering spectra. Raman bands at the middle frequency region of 700-950 cm⁻¹ were normalized using the bands at high frequency spectra and their intensities were compared among the spectra collected from the melts with different composition at various temperature. Bands at 805, 780, 750 and 720 cm⁻¹ were focused and their intensity changes were quantified. The exceeds of temperature over the glass transition temperature did not necessarily cause the decrease of all the band intensities. At Na₂O<15 mol%, 805 cm⁻¹ band was the most sensitive to temperature, while at 15<Na₂O<30, it was switched to 780 cm⁻¹ band and 805 cm⁻¹ band became insensitive. When Na₂O concentration exceeded 30 mol%, 750 and 720 cm⁻¹ bands were decreased with temperature. Accompanying the previous analysis on the structures of short range-order (SRO) of boron atoms [J. Non-Cryst. Solids, in this issue], some models of the structural rearrangement along with temperature were proposed. The combination of the obtained structural informations of IRO and SRO was found to explain the mechanisms causing various characteristic properties of borate glasses and melts, especially immiscibility and boron oxide anomaly of thermal expansion coefficient from the microscopic and dynamic points of view of the vitrification process in melt.     

Manifestation of thermodynamic glass transition by structure and picosecond dynamics in alkali tellurite glasses

The Raman spectra of the 0.1Cs₂O–0.9TeO₂ melt were measured and analyzed over a broad temperature range including the glassy, supercooled and molten state in an effort to follow the varying structural and dynamical aspects caused by temperature and alkali modifier. The network structure of the glass/melt is formed by mixing TeO₄ trigonal bipyramid and TeO₃ trigonal pyramid units. Changing alkali content and/or temperature results to conversion of the TeO₄ units to TeO₃ units with a varying number of non-bridging oxygen atoms. The low-frequency Raman spectra reveal a well-resolved Boson peak whose frequency also depends on temperature. The variation of the maximum of the Boson peak has been determined and discussed in the framework of current phenomenological models. The short-time dynamics of the system experiences drastic changes when approaching the glass-to-liquid transition. The temperature dependent plot of the correlation times extrapolates to a crossover value, which we assign as spectral evidence of the system's known thermodynamic glass transition temperature. Similar behavior exhibit several spectral features, such as the maximum of the Boson peak, the exponent of the susceptibility and the intensity ratio related to the structural transformation from TeO₄ tbp to TeO₃ tp species occurring in the medium range order structure.     

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.     

On the structure of phosphosilicate glasses

Vibrational spectra of phosphosilicate glasses with P₂O₅ concentrations up to 15 mol% are investigated by the methods of Raman spectroscopy and quantum-chemical modeling. We have found that the Raman band at 1320 cm⁻¹ characteristic for such glasses is not simple and may be decomposed into two components with frequencies at ≈1317 and ≈1330 cm⁻¹ caused in our opinion by single phosphorus centers (OPO₃ tetrahedra surrounded by SiO₄ ones) and by double phosphorus centers (pairs of OPO₃ tetrahedra bonded by a common oxygen atom). In the investigated phosphosilicate glasses manufactured by MCVD and SPCVD methods the ratio of concentrations of single and double centers varies from 1:5 to 1:2. A novel interpretation of the Raman bands distinct from the traditional one is suggested. The approach to the Raman spectra analysis developed in this article can be applied for control and optimization of manufacturing process of phosphosilicate and similar glasses as well as optical fibers.     

X-ray diffraction studies of the structure of LaA1Ga metallic glasses

The LaLa and LaA1 partial atomic distribution functions have been determined for glassy La_1?xA1_x from X-ray diffraction studies of isomorphous alloys of La_1?xA1_x, La_1?x(A1Ga), and La_{1 ?x}Ga_x for x = 0.20, 0.24 and 0.28. The atomic short range order of these La based metallic glasses is quite different from that of typical amorphous transition metal-metalloid alloys and from dense random packing models. A relatively short and well defined LaA1 nearest neighbor distance suggests some covalency in the bonds between unlike atoms and possible chemical ordering in the alloys.     

Influence of europium ions on structure and crystallization properties of bismuth borate glasses and glass ceramics

Glasses of the xEu₂O₃ · (100?x)[2Bi₂O₃ · B₂O₃] system with 0 ? x ? 25 mol% have been characterized by X-ray diffraction and FTIR spectroscopy measurements. Melting at 1100 °C and the rapid cooling at room temperature permitted us to obtain glass samples. In order to improve the local order and to develop crystalline phases, the glass samples were kept at 625 °C for 24 h. After heat treatment two crystalline phases were put into evidence. One of the crystalline phases was observed for the host glass matrix, the x = 0 mol% sample, and belongs to the cubic system. The second one was observed for the x = 25 mol% sample and was find to be orthorhombic with two unit cell parameters very close to each other. For the samples with 0 < x < 25 mol% there is a mixture of the two mentioned phases. FTIR spectroscopy data suggest that both Bi₂O₃ and B₂O₃ play the glass network former role while the europium ions play the network modifier role in the studied glasses.     

Spectroscopic and structural properties of Cr in silicate glasses: Cr does not probe the average glass structure

Cr³⁺-containing alkali, alkaline earth and mixed alkali-alkaline earth silicate glasses have been investigated using Cr K-edge extended x-ray absorption fine structure (EXAFS) and optical absorption spectroscopy. The local environment of Cr³⁺ appears similar in all glasses regarding EXAFS analysis, in particular for Cr-O distance (1.99 Å). By contrast, optical absorption spectra show variations of crystal field values and disorder effects with the nature of the modifier cation, revealing that some differences exist in the local surrounding of Cr³⁺ in glasses. In addition, in mixed alkali-alkaline earth glasses optical absorption parameters remain close to the values found in binary silicate glasses with the same alkali, which reveals a preference for alkalis in the surrounding of Cr³⁺. As a whole, these data show that Cr³⁺ is not probing the average glass structure and demonstrate its heterogeneous distribution at a local scale.     

Raman study of the structure of alkali germanosilicate glasses II. Lithium,sodium and potassium digermanosilicate glasses

Polarized and depolarized Raman spectra of lithium, sodium and potassium digermanosilicate glasses of the composition 33 A₂O 67[(1 ? y)SiO₂, y GeO₂] and powder Raman spectra of crystalline Li₂O · 2 GeO₂ and K₂O · 2 GeO₂ are given. From the Raman spectra it is concluded that the structure of the glasses mainly consists of networks of SiO₄ and GeO₄ tetrahedra with one non-bridging oxygen atom, probably mixed in a random way. The structure is comparable to that of crystalline disilicates and digermanates.     

Diffraction intensities and the structure of amorphous carbon

A model structure of amorphous carbon is investigated incorporating layered domains connected by means of a random network, the relative proportions of the two regions being a variable of the model. It is shown that for a certain relative proportion of the two regions there is good agreement between the predicted and experimentally observed electron and X-ray scattering intensities. The effects of covalent bonding are also studied and shown to be significant for small k.     

Effect of synthesis method on the structure and properties of AgPO3-based glasses

The relation between ionic conductivity, glass transition temperature and structure has been investigated in silver-metaphosphate, AgPO₃, glasses synthesized under varying conditions. The key finding of this work concerns the AgPO₃ doping with up to 3 mol.% Al₂O₃ when melting is done in alumina crucibles, a practice used widely for the synthesis of fast ion conducting glasses. Vibrational spectroscopy showed that Al₂O₃-doping increases the phosphate network connectivity by P-O-Al-O-P cross-links. This effect causes a drastic enhancement in ionic conductivity and glass transition temperature relative to Al₂O₃-free AgPO₃ glasses obtained by melting in Pt crucibles.     

On the effect of glass composition in the dissolution of glasses by water

The dissolution rate in aqueous solution has been measured for three families of glasses of technological interest (borosilicates, aluminosilicates and lead-silicates). Borosilicates have been altered at a slightly basic pH (8–9), whereas the aluminosilicates and the lead-silicate glasses have been corroded in acidic solutions. In these conditions, silica is always less soluble than all other glass components. In each glass family, the effect of the silica content on the glass dissolution rate has been studied. The three series display a similar behavior as a function of the glass composition, namely a sharp increase of the dissolution rate with the content in soluble oxides. These experimental results have been explained in the framework of a Monte-Carlo simulation of glass dissolution. The simulations show that the fast extraction of the soluble species as compared to that of silica generates a porous surface layer, which becomes thicker and thicker when the percentage of soluble oxide increases in the glass. This increases the interface area between the glass and the solution, and consequently the dissolution rate in the same proportion. The numerical simulations reproduce quite well the experimental variations of the dissolution rates with glass composition.     

The peculiarities of fluoride glass structure. Spectroscopic study

Glass systems based on ZrF₄, InF₃, GaF₃ with added BiF₃, SnF₂, BaF₂, SrF₂, PbF₂ were obtained to analyze changes of the structure and properties of these glasses with the addition of various components. The glass structures were investigated by means of vibration spectroscopy. IFS EQUINOX 55S, T64000 and DFS-24 spectrometers were used for measurements of infrared and Raman spectra. It was shown that BiF₃ and SnF₂ are the second glassformers as ZrF₄, InF₃ and GaF₃. The structures of glassforming polyhedra were determined for each glass type. The structural peculiarities and the length scale of the medium range order were discussed and estimated by the frequency of the ‘boson’ peak in all low frequency Raman spectra of the glasses under study. The medium range order in InF₃-BiF₃-BaF₂ glasses built from InF₆ polyhedra that are not bound directly to BiF_n polyhedra and Ba-modifier essentially determines the size of the medium range order of the glasses. Nanometer scale separation of mono-component fractions was not observed in glasses of the GaF₃-SnF₂ system.     

Study of the dynamic structure factor of strong glasses

The dynamic structure factors (DSFs) of several strong glasses (SF6, SF10, BK7, SUPRASIL) measured by Brillouin light scattering spectroscopy are reported. Spectra have been collected, at and above room temperature, at two scattering angles, θ=90° and θ=180° corresponding to exchanged wavevector q values ranging from 0.0256 to 0.0448 nm⁻¹. In particular we find that the isotropic spectral lineshapes are in all cases well described by the simple hydrodynamic theory of an amorphous solid. The width of the Brillouin peaks are found to be consistent with the predicted q² dependence at both investigated temperatures. This damping is however found to account only partially for the strong asymmetry of the Brillouin line clearly visible on a logarithmic intensity scale. As a matter of fact there is an excess intensity in the very low frequency plateau underlying the central component. The height of this plateau and hence the entire lineshape is well reproduced if a relaxation process is taken into account in the hydrodynamic equations. Owing to the intense elastic scattering we are able to determine unambiguously only the ratio between amplitude and characteristic time of this process which quantifies the sound dispersion to be of the order of a few percent in all samples. The temperature dependence of the parameters indicates that this relaxation cannot be attributed to thermally activated relaxation phenomena. These general findings favorably compare with molecular dynamics simulation results on similar systems.     

Investigation of the network structure of niobium borate glasses

Some structure parameters of niobium borate glasses have been determined with the analytical data of X-ray diffraction and IR spectra. On the basis of experimental results, the model of the Nb₂O₅---B₂O₃---K₂O (NBK) glass network was inferred to be framed by circular structural units which are composed of six-membered and four-membered groups of NbO₆, BO₄ and BO₃ polyhedra in the glasses.