Structure Of Silicate Glasses Research Articles

Nature of polymerization and properties of silicate melts and glasses at high pressure

The structures of sodium silicate and aluminosilicate glasses quenched from melts at high pressure (6–10 GPa) with varying degrees of polymerization (fractions of nonbridging oxygen) were explored using solid-state NMR [ 17O and 27Al triple-quantum magic-angle spinning (3QMAS) NMR]. The bond connectivity in melts among four and highly coordinated network polyhedra, such as [4]Al, [5,6]Al, [4]Si, and [5,6]Si, at high pressure is shown to be significantly different from that at ambient pressure. In particular, in the silicate and aluminosilicate melts, the proportion of nonbridging oxygen (NBO) generally decreases with increasing pressure, leading to the formation of new oxygen clusters that include 5- and 6-coordinated Si and Al in addition to 4-coordinated Al and Si, such as [4]Si-O- [5,6]Si, [4]Si-O- [5,6]Al and Na-O- [5,6]Si. While the fractions of [5,6]Al increase with pressure, the magnitude of this increase diminishes with increasing degrees of ambient-pressure polymerization under isobaric conditions. Incorporating the above structural information into models of melt properties reproduces the anomalous pressure-dependence of O 2− diffusivity and viscosity often observed in silicate melts.

Structure of Silicate Glasses and Melts at High Pressure: Quantum Chemical Calculations and Solid‐State NMR.

Despite their strong implications for magmatic processes in the earth's interior and pressure-induced structural changes in other amorphous, covalent oxide materials, little is known, beyond the coordination numbers of the framework cations, about the structures of silicate melts and glasses at high pressure. Here, we use multinuclear (27Al and 17O) solid-state NMR and quantum chemical calculations to study the structures of silicate glasses quenched from melts at pressures up to 10 GPa in a multianvil apparatus. The pressure-induced changes in atomic configurations in the silicate melts include the formation of highly coordinated framework units such as [5,6]Al and [5,6]Si and bridging oxygens linking these framework units and the presence of nonbridging oxygens coordinated by network-modifying cations and [5,6]Si. The proportion of high-pressure configurations such as [5,6]Al−O−[4]Si increases with pressure, but the fraction of nonbridging oxygen (Na−O−[4]Si) decreases with pressure, leading to an incre...

Structure of Silicate Glasses and Melts at High Pressure: Quantum Chemical Calculations and Solid-State NMR

Despite their strong implications for magmatic processes in the earth's interior and pressure-induced structural changes in other amorphous, covalent oxide materials, little is known, beyond the coordination numbers of the framework cations, about the structures of silicate melts and glasses at high pressure. Here, we use multinuclear (27Al and 17O) solid-state NMR and quantum chemical calculations to study the structures of silicate glasses quenched from melts at pressures up to 10 GPa in a multianvil apparatus. The pressure-induced changes in atomic configurations in the silicate melts include the formation of highly coordinated framework units such as [5,6]Al and [5,6]Si and bridging oxygens linking these framework units and the presence of nonbridging oxygens coordinated by network-modifying cations and [5,6]Si. The proportion of high-pressure configurations such as [5,6]Al−O−[4]Si increases with pressure, but the fraction of nonbridging oxygen (Na−O−[4]Si) decreases with pressure, leading to an incre...

Degree of Polymerization of Aluminosilicate Glasses and Melts

This paper presents the results of analyzing the data available in the literature on the structure and properties of silicate glasses and melts that contain Ti4+, Al3+, and Fe3+ cations in addition to alkali and alkaline-earth cations. It is established that the aforementioned multivalent cations in glasses and melts have a coordination number of four and play the role of network-formers. Aluminosilicate glasses and melts with the mole fraction ratio Al2O3/M 2(M′)O = 1 are of special interest. For these glasses, the structure is considered to be completely polymerized and, contrary to traditional concepts, their properties depend on the concentration ratio Al2O3/SiO2. Taking into account that the structure of aluminosilicate glasses involves unusual structural units (such as triclusters) and a certain number of nonbridging oxygen atoms, a formula is proposed for calculating the degree of polymerization. The proposed formula is used to calculate the degree of polymerization for a number of Na2O · Al2O3 · mSiO2 glasses and the CaO · Al2O3 · 2SiO2 glass. It is demonstrated that the calculated degrees of polymerization correlate with the experimentally measured viscosities of the relevant melts.

The effect of cooling rate on the structure of sodium silicate glass

The effect of cooling rate on the structure of Na 2Si 3O 7 glass was studied by Raman spectroscopy. The shape of Raman spectra of glasses varies with cooling rate. By fitting the high-frequency region of the Raman spectra with Gaussian curves, the abundance of SiO 4 tetrahedra with different number of non-bridging oxygens (Q n ) are deduced. The structural species Q 4 and Q 2 increase with cooling rate and Q 3 decrease with cooling rate. Providing the faster cooling rate leads higher temperature feature of melt to preserve in glass, the changes of structural species in Na 2Si 3O 7 glass with cooling rate are consistent with the in situ high-temperature Raman results. From this study the rate-dependent glass structure should be taken into consideration in the glass processing, even with ordinary cooling technique.

Relationships Between Gas-Phase Film Deposition, Properties and Structures of Thin SiO2 and BPSG Films

This paper is devoted to the analysis of relationships between chemical vapor deposition (CVD) features and silicon dioxide and silicate glass film properties and structures. Deposition processes have been quantitatively characterized using an empirical parameter the effective constant of the deposition rate, The observed CVD kinetic features are explained by using a simplified multiroute and multistep kinetic scheme of the silicon-based thin-film gas-phase deposition processes, and a classification of the studied processes. The basic physical and chemical properties of thin glass films are explained in terms of the glass structural disordering, which defines the oxide film density, wet etch rate, shrinkage, and intensity of film-moisture interaction. The structure disordering depends in reverse order with respect to It is shown that the borophosphosilicate glass (BPSG) film structure and properties are affected by the quantity of boron atoms with a threefold coordination with respect to the oxygen atoms. The boron coordination in BPSG film tends to change from threefold to fourfold coordination along with the increase of phosphorus to boron ratio in the film. The best BPSG structural ordering and the best film properties correspond to the boron concentrations defined by an empirical equation The respective glass film structure is supposed to consist of and tetrahedra. © 2003 The Electrochemical Society. All rights reserved.

NMR study of Q-speciation and connectivity in K 2O–SiO 2 glasses with high silica content

The structure of binary potassium silicate glasses with SiO 2 content ranging between 76.0 and 97.6 mol% has been studied with 29Si and 17O NMR spectroscopy. Besides the Q 3 species, two broadly different types of Q 4 species have been identified in the 29Si MAS NMR spectra of glasses with >83.5 mol% SiO 2. These species correspond to Q 4 connected to one or more Q 3 nearest neighbors and Q 4 with only Q 4 nearest neighbors, and are denoted as Q 4–3 and Q 4–4, respectively. The compositional variation of the concentration of Q 4–3 species implies a statistically random distribution of Q 3 and Q 4 species in these glasses. Such a silicate network with random connectivity between Q 3 and Q 4 species is shown to be consistent with the strongly non-linear compositional variation of the transport properties of binary alkali silicate glasses with high silica content.

Short-range and medium-range order in amorphous barium germanate

Amorphous barium germanate of composition BaGe 2.55O 6.10 was prepared by ultra-rapid quenching using a double-roller melt-spinning device. The short-range and medium-range order was investigated by high-energy synchrotron X-ray diffraction, resonant scattering at the Ba K-absorption edge and by electron diffraction. There are indications that 6-fold coordinated germanium exists in the structure of the present amorphous alkaline earth germanate, i.e. both GeO 4 tetrahedra and GeO 6 octahedra are supposed to be basic building units. The ratio of 6-fold coordinated Ge to 4-fold coordinated Ge is approximately 1:3. A structure model is proposed which favours the existence of barium chains and/or barium layers, similar to the structure of barium silicate glasses.

Voronoi polyhedra analysis of MD simulated silicate glasses

The relationship between the volume and the surface of Voronoi polyhedra (VP) appeared to be an effective tool for the characterization of the structure of MD simulated silicate glasses. This feature was demonstrated on the series of binary potassium-silicate glasses with the alkali content ranging from 5 to 20 mol%. Moreover, the temperature dependence of the slope of the above dependence, when expressed for various types of central atoms (Si, O and K in our case) was studied. Each type of central atom is characterised by its own iron line linearly relating the volume of VP to its properly powered surface, so that new geometrical constrains to the VP was found.

Local structure and chemical durability of FeOOH-fixed sodium silicate glass prepared from water glass

Abstract A 12M HCl solution of iron oxyhydroxide (a-FeOOH: goethite) was mixed with water glass (18Na2O.36SiO2 .46H2O) at room temperature. The mixture (sol) changed into a dry gel when dried at 25 °C for 120 hours in air. Glass-ceramic and glass samples were prepared when the dry gel was heated for 1-3 hours in an electric furnace at 800 and 900 °C, respectively. The 57Fe Mossbauer spectrum of the dry gel is composed of a magnetic hyperfine structure owing to the formation of g-FeOOH (lepidocrocite). By contrast, the 57Fe Mossbauer spectrum of glass-ceramic and glasses is composed of paramagnetic Fe(III) with distorted tetrahedral symmetry. This proves that Fe(III) atoms occupy network-forming Si(IV) sites in the FeOOH-fixed sodium silicate glass. A leaching test of the silicate glass in the acid rain simulant composed of HNO3 (pH 3.5) and H2SO4 (pH 3.5) revealed high chemical durability, indicating that Fe(III) is firmly fixed in the glass matrix.

Structural inhomogeneity in glasses from the system Li2O3–Al2O3–SiO2 revealed by IR spectroscopy

MIR spectra of lithium silicates and aluminosilicates were measured and compared with the simulated spectra. Simulation was carried out by mathematical summation of appropriate glass spectra taken in the right proportion. It was shown that the spectra obtained in this way are very similar to the measured spectra. It confirms phase inhomogeneity of studied glasses with short or even medium range of order. Structures of lithium silicate and aluminosilicate glasses consist of structural elements resulting from its composition and position in the field of primary crystallisation. The structure of these elements is very similar to their crystalline analogues with the same chemical composition, but they are much more disordered and do not have translation symmetry.

Silicate-phosphate interactions in silicate glasses and melts: I. A multinuclear (27Al,29Si,31P) MAS NMR and ab initio chemical shielding (31P) study of phosphorous speciation in silicate glasses

An investigation of the structure of sodium aluminum silicate glasses [composition (x)Na2O, (1-x)Al2O3, 0.9SiO2 perturbed through the addition of 2 mol.% P2O5] was performed using a combination of 27Al, 29Si, and 31P solid state NMR spectroscopy and ab initio chemical shielding calculations (GIAO method). Speciation within these glasses was estimated using a physically consistent method for the deconvolution of inhomogeneously broadened 31P MAS-NMR spectra by employing a combination of spinning sideband analysis with the results of ab initio shielding calculations. A minimum of 13 different phosphorus-bearing species were estimated to contribute to the spectral complexity in these glasses over the compositional range investigated. The majority of these species are oxygen-bridged to the silicate lattice. These are described as (Al,Si)Qpn species, where n corresponds to the number of bridging oxygens (n = 1–4) and where there is variation in the Al/Si ratio of the tetrahedral nearest neighbors. In addition to the species bridging to the silicate lattice, nonlattice bridged Na-P type species, including ortho-, pyro-, and tripolyphosphates, are observed or deduced based on spectral behavior. A sharp crossover in the predominance of Na-P species relative to (Al,Si) Qpn dominant species occurs at low Al2O3 content. Within the group of nonbridging (Na-P) species, increased bulk aluminum content shifts the predominance from orthophosphate toward tripolyphosphate. Within the group of bridging [(Al,Si) Qpn] species, increasing bulk aluminum content shifts the predominance toward a greater number of oxygen bridges to the silicate lattice. To highlight the perturbative effect of 2 mol.% P2O5 addition on glass structure, 29Si MAS-NMR of P-bearing and P-absent glasses were compared. The speciation estimated from the combined 31P MAS-NMR and ab initio shielding calculations is consistent with 29Si MAS-NMR spectroscopic data. Analysis of 27Al MAS-NMR spectra for P-bearing and P-absent glasses reveals extensive Al-O-P interactions evident by systematic shifts in peak maxima to lower frequencies (greater shielding). Increases in the central transition peak half widths in the P-bearing samples reflect enhanced second-order quadrupolar interactions, also consistent with an abundance of (Al,Si) Qpn species. Analysis of the 27Al inner satellite transition sidebands reveals only a minor amount of VIAl. It is concluded that the addition of even a small amount of P2O5 (2 mol.%) results in a large perturbation of the structure of silicate glasses and should manifest significant effects on the physical properties of melts derived from them.

Local structures of MD-modeled vitreous silica and sodium silicate glasses

Structures of vitreous silica and sodium silicate glasses have been simulated using different potential models and compared with the neutron diffraction data, with good agreement. The environments of O, Si and Na have been thoroughly examined and compared with those in both crystalline structures and experimental glasses. Considerable similarities in local structures exist between crystalline and simulated glass structures in terms of bond length, coordination number and bond angle distribution (BAD). No significant environment changes were found, except for the coordination number (CN) variation due to composition changes. Further analysis suggests that a weaker interaction between Na and non-bridging oxygen (NBO) exists in the modeled glass structures, perhaps due to use of the same charge for both the bridging and non-bridging oxygens. On the other hand, the existence of fivefold silicon sites in sodium silicate glasses implies that these glasses more resemble high-pressure structures. In addition, the observation of neighboring Na around the fivefold silicons suggests that these fivefold silicons are transition state complexes. Examination of 2- and 3-rings reveals that these smallest rings are closely associated with the fivefold silicons, and are only intermediate states where oxygens transfer from one silicon site to another.

Molecular Dynamics Simulations of Silicate Glasses and Melts

The application of molecular dynamics to the modeling of glasses is reviewed. Several examples are used to illustrate the value of these computer simulations in advancing our understanding of the structure of silicate glasses. The mixed alkali effect is explained in terms of structural variations in going from single alkali to mixed alkali glasses that lead to distinct environments for each type of alkali ion and also result in the smaller alkali ion becoming more tightly bound to the network. Some recent results on the structure of silicate melts are presented, and the relationship between the structures of glass and melt are discussed.

Structure and dynamics of silicate glasses and melts

A one-day meeting was held in the Department of Earth Sciences, University of Cambridge, on the theme of the ‘Structure and Dynamics of Silicate Glasses and Melts’. The meeting was sponsored by the Mineral Physics group of the Mineralogical Society, and was attended by ∼80 people. The objective of the meeting was to bring together experts from the glass community with members of the mineralogical community who are interested in glasses and melts. The scope of the talks considered structural details over length scales from nearest-neighbour coordinations up to 1–2 nm, and dynamics in both the slow relaxational and fast vibrational ranges. The talks also covered both experimental (particularly neutron scattering and NMR) and computer simulation methods.The meeting was organized into three groups of themes, namely structure and dynamics in glasses, structure and dynamics in melts, and vibrational dynamics in glasses.

Relationship between the local structure and spontaneous emission probability of Er3+ in silicate, borate, and phosphate glasses

Extended X-ray absorption fine structure (EXAFS) measurements have been performed on Er31 in silicate, borate, and phosphate glasses in order to investigate the local structure surrounding the Er31.Er31 ions coordinate to non-bridging oxygen ion sites, where alkali or alkaline earth ions terminate the network structure of silicate glasses. In borate glasses, the local structure surrounding Er31 ions is altered by the structural change ofthe borate anion. Er31 ions coordinate to non-bridging oxygen ion sites and BO4 structural units in the cases with and without the formation of non-bridging oxygen, respectively. The former is similar to the case in silicate glasses. Er31 ions selectively coordinate to the PyO site regardless of the glass composition variation. A correlation was observed between the spontaneous emission probability for 4I13/2 → 4I15/2 transition of Er3+ and the average Er–O distance calculated by EXAFS analysis. It shows the maximum value near 2.32 Å, and we conclude that the overlapping radial integral of the 4f and 5d orbitals of Er3+ would be the largest at the optimum Er31– O22 distance 2.3 Å.

Adsorption of CO2 and Ar on glass surfaces. Computer simulation and experimental study

Isotherms of adsorption of CO2 and Ar are simulated by the grand canonical Monte Carlo on four model surfaces of amorphous silica. The surfaces designated A through D differ progressively in their degree of annealing, A being an unannealed, nonequilibrium surface and D being the most extensively annealed. The gas–gas interaction potentials for both gases were taken from the literature and the gas–solid interactions were modeled by applying Lorentz–Berthelot combining rules to the gas–gas potentials plus the TTAM representation of the atom–atom interactions in the solid. The simulated isotherms of Ar on surfaces A and D are close to each other and to the experimental isotherm for nonporous silica. In contrast, the simulated isotherms and isosteric heats of adsorption of CO2 on these surfaces differ considerably from each other. This leads to the conclusion that argon adsorption is not sensitive to the changes in surface structure that occur during annealing, but CO2 is. Since these gases differ considerably in their polarity, these results indicate that the observed differences in adsorption behavior are due in large part to the annealing-induced changes in the electrostatic part of the CO2–SiO2 interaction. The isotherm of CO2 on D was made to be very close to an experimental isotherm on dehydroxylated nonporous silica by diminishing the electrostatic part of the CO2–SiO2 interaction by 30% from that in the original model of the potential. Isotherms of adsorption of CO2 on multicomponent glass fibers measured at 194.5 K are reported here. The sub-monolayer experimental isotherms on glass lie higher than an isotherm on hydroxylated silica which in turn lies higher than that for dehydroxylated silica. The sensitivity of the physical adsorption of CO2 to the chemical nature and the structure of the SiO2 surface indicates that comparisons of experiment with simulations of the isotherms of polar or quadrupolar molecules like CO2 on such model surfaces can be a useful probe of surface structure of silica and silicate glasses.

Rings in the structure of silicate glasses

In this work infrared absorption spectroscopy was applied to investigate the mid-range order in silicate and aluminosilicate glasses. Ring systems containing various rings can exist in the structure of such glasses. Our previous studies on cyclosilicates have allowed us to determine positions of the bands characteristic of various membered rings [M. Sitarz, W. Mozgawa, M. Handke, J. Mol. Struct. 404 (1997) 193; M. Handke, M. Sitarz, W. Mozgawa, J. Mol. Struct. 450 (1998) 229]. Amorphous and crystalline layered silicates (Li2Si2O5, Na2Si2O5 and K2Si2O5) and framework ones (SiO2 and K[AlSiO4]) were investigated. Analysis of the obtained IR spectra and their mathematical decomposition allow us to identify the bands characteristic of rings systems, both for crystalline and amorphous samples. Appearance of the bands characteristic of pseudolattice vibrations (740–600cm−1 connected with rings) in the IR spectra of the amorphous samples is an evidence of the existence of over-tetrahedral order.

Isotopic fractionation accompanying helium diffusion in basaltic glass

The relative rates of 3He and 4He diffusion in basaltic glass set limits on the extent to which diffusive loss can alter initial magmatic 3He/4He ratios. Typically a value of 1.15 for the isotopic diffusivity ratio, D3He/D4He, has been assumed, because this corresponds to the inverse square-root of the masses ratio, (m4/m3)1/2. Measurements of the isotopic compositions of sequential releases of He from mid-ocean-ridge and seamount basalt glasses heated in-vacuo reveal this to be an overestimate. The observed isotopic diffusivity ratio ranges from 1.06 at room temperature to 1.10 at 500°C. Assuming Arrhenius temperature dependence and extrapolating suggests that insignificant He isotopic fractionation will occur at seafloor temperatures (D3He/D4He=1.02±0.03), with more pronounced fractionation at magmatic temperatures (e.g. 1.12±0.02 at 1100°C). These low values mean that significantly larger He losses are required to alter initial 3He/4He ratios than was previously assumed, and these larger losses require much longer loss times. For example, lowering an initial 3He/4He ratio by 10% requires 65% gas loss for D3He/D4He equal to 1.15, but 80% loss for D3He/D4He of 1.08, which requires twice as long to occur.Consideration of solid-state diffusion theory, and comparison to cation and other noble gas diffusivities, suggests that the low values and positive temperature dependence of the D3He/D4He ratio result from the relatively widely spaced vibrational energy levels of the He atoms in the silicate glass structure. Temperature dependent diffusive He isotopic fractionation is likely in other geologic materials.

The structure of sodium iron silicate glass – a multi-technique approach

Sodium iron silicate glasses of formula 0.3Na 2O· xFe 2O 3·(0.7 − x)SiO 2 (0.0 ⩽ x ⩽ 0.20) have been prepared by melt-quenching and their structures have been studied using X-ray photoelectron spectroscopy (XPS), neutron diffraction and Mössbauer spectroscopy. XPS of surfaces fractured in ultra-high vacuum showed the presence of both Fe 2+ and Fe 3+ in the glasses but the proportion of the smaller oxidation state is <20% for larger x. The O 1s spectrum can be fitted by bridging and non-bridging oxygen contributions but properties show that this description is inadequate and there are contributions from bridging oxygens, Si–O–Si, from non-bridging oxygens, Si–O –Na + and also directionally bonded Si–O–Fe. The neutron diffraction and Mössbauer data indicate that iron, irrespective of oxidation state, is 4-coordinated by oxygen. The Fe–O bond length decreases and the O–O bond length increases with increasing Fe 2O 3 content. Mössbauer spectroscopy confirms the predominance of the Fe 3+ state in these glasses and indicates smaller Fe 2+ concentrations than does XPS, possibly due to restructuring at the glass fracture surfaces examined in XPS. The effective Debye temperatures for Fe 2+ and Fe 3+ are 258 and 312 K, respectively, a consequence of the different M–O bond strengths of the two oxidation states.