Silicate Network Research Articles

The Influence of Cu2+ and Mn2+ Ions on the Structure and Crystallization of Diopside–Calcium Pyrophosphate Bioglasses

The influence of two divalent cations, Cu2+ and Mn2+, on the structure, sintering, and crystallization of glasses and glass‐ceramics in the diopside–calcium pyrophosphate system (90 wt.% diopside (CaMgSi2O6), 10 wt.% calcium pyrophosphate (Ca2P2O7)) was investigated. Glasses with 1, 3, and 5 wt% MnO or CuO additives were prepared by melt‐quenching and characterized by XRD, 29Si and 31P NMR, DTA, and FTIR This revealed that the silicate network is predominantly coordinated in Q2 (Si) units for all glasses, while phosphorus tends to inhabit an orthophosphate (Q0) environment. All glasses had a high rate of bioactivity after immersion in simulated body fluid. A slight depolymerization was observed in the doped glasses leading to lower Tg values in comparison with the parent glass. All glass‐ceramics exhibited the formation of diopside as the primary crystalline phase after sintering at 850°C/1 h. In comparison with the parent glass, the doped glasses featured significantly larger processing windows (∆T = Tc–Tg), ensuring good sinterability. Further, with increasing doping levels, the glasses exhibited a gradual decrease in Tp and ∆T, suggesting an increased tendency toward devitrification. All Cu‐ and Mn‐containing glasses exhibited the formation of hydroxyapatite, making them good candidates for biomedical applications and tissue engineering.

Structure and mechanical properties of compressed sodium aluminosilicate glasses: Role of non-bridging oxygens

Clarifying the effect of pressure on the structure of aluminosilicate glasses is important for understanding the densification mechanism of these materials under pressure and the corresponding changes in macroscopic properties. In this study, we examine changes in density, network structure, indentation hardness, and crack resistance of sodium aluminosilicate glasses with varying Al/Si ratio and thus non-bridging oxygen (NBO) content before and after 1GPa isostatic compression at elevated temperature. With increasing NBO content, the silicate network depolymerizes, resulting in higher atomic packing density, lower hardness, and higher crack resistance. The ability of the glasses to densify under isostatic compression is higher in the high-NBO glasses, and these glasses also exhibit more pronounced pressure-induced changes in mechanical properties. The 27Al NMR data show a surprising presence of five-fold aluminum in the as-made high-NBO glasses, with additional formation upon compression. Our study therefore provides new insights into the complicated relationship between Al coordination and NBO content in aluminosilicate glasses and how it affects their densification behavior.

The effect of composition on structural, thermal, redox and bioactive properties of Ce-containing glasses

The effect of phosphate on the ability of Ce-containing bioactive glasses to inhibit oxidative stress was studied on compositions based on Hench (46.2%SiO224.3%Na2O26.9%CaO2.6P2O5, mol%) and Kokubo (50.0%SiO225.0%Na2O25.0%CaO) glasses. In particular, the reduction of catalase mimetic activity of Ce-containing glasses due to the presence: i) of P2O5 in the glass compositions, and ii) of phosphate groups in the solution employed for catalase mimetic activity tests was explained and rationalized by combining SEM, XPS, XRD, DTA, FT-IR and UV–vis experiments with Molecular Dynamics simulations.The results suggest that the Ce ions play a different structural role in the two series of glasses. In particular, in phosphate free glasses Ce is coordinated by non-bridging oxygens (NBOs) originated from the disruption of the silicate network, whereas in phosphate containing glasses the NBOs around Ce ions belong to orthophosphate groups. The latter groups stabilize the Ce3+ species subtracting them from the interconversion process between Ce3+ and Ce4+, which is of fundamental importance for the exhibition of the catalase mimetic activity.

Structural and dynamical properties of supercritical H2O–SiO2 fluids studied by ab initio molecular dynamics

In this study we report the structure of supercritical H2O–SiO2 fluid composed of 50mol% H2O and 50mol% SiO2 at 3000K and 2400K, investigated by means of ab initio molecular dynamics of models comprising 192 and 96 atoms. The density is set constant to 1.88g/cm3, which yields a pressure of 4.3GPa at 3000K and 3.6GPa at 2400K. Throughout the trajectories, water molecules are formed and dissociated via the network modifying reaction 2 SiOH=SiOSi+H2O. The calculation of the reaction constant K=[OH-]2/[H2O][O2-] is carried out on the basis of the experimentally relevant Qn species notation and agrees well with an extrapolation of experimental data to 3000K. After quench from 3000K to 2400K, the degree of polymerization of the silicate network in the 192-atom models increases noticeably within several tens of picoseconds, accompanied by release of molecular H2O. An unexpected opposite trend is observed in smaller 96-atom models, due to a finite size effect, as several uncorrelated models of 192 and 96 atoms indicate. The temperature-dependent slowing down of the H2O–silica interaction dynamics is described on the basis of the bond autocorrelation function.

Thermal Poling of Soda‐Lime Silica Glass with Nonblocking Electrodes—Part 1: Effects of Sodium Ion Migration and Water Ingress on Glass Surface Structure

It is generally well known that not only the sodium itself, but also the non‐bridging oxygen (NBO) sites associated with sodium ions are largely responsible for the surface reactivity of soda‐lime‐silica (SLS) glass. Thermal poling can modify the distribution of sodium in the subsurface region. In this work, a commercial SLS float glass was thermally poled using nonblocking electrodes in air. The Na+−depleted anode surface and the Na+−gradient cathode surface were characterized using a variety of methods to find the compositional, structural and morphological effects of thermal poling. Of particular significance is the use of nondestructive vibrational spectroscopy methods, which can lead to new and improved understanding of water interactions with sodium and its sites in the glass. It was found that during thermal poling, the Na+−depleted glass network on the anode side undergoes condensation reactions of NBO sites accompanied by the increase in concentrations of silanol (SiOH) groups and molecular water species. In contrast, silanol and water species do not increase and the silicate network change is negligible in the Na+−gradient cathode side. Vibrational sum frequency generation (SFG) spectroscopy analysis revealed the difference in distributions of hydrous species in the Na+−depleted and Na+−gradient surfaces. The structural information of the thermally‐poled surfaces provides critical insights needed to understand the mechanical and mechanochemical properties of the Na+−concentration modified SLS glass surfaces reported in the Part 2 companion paper.

Bioactive oxynitride glasses: Synthesis, structure and properties

The low mechanical strength of bioactive glasses restricts their use to non load-bearing applications. One way to increase strength of glasses is to introduce nitrogen into the silicate network. Oxynitride bioactive glasses in the SiO2–CaO–Na2O–Si3N4 system were successfully prepared. Structural characterization using 29Si MAS-NMR shows that, as nitrogen is substituted for oxygen, formation of [SiO3N] tetrahedra and Q4 units occurs with extra bridging anions at the expense of Q3 units. Glass transition temperature, hardness, Young’s modulus and fracture resistance increase with nitrogen content because of the extra cross-linking of the glass network.CaF2 was substituted for CaO in these glasses which results in decreases in glass transition temperature. Thus, glasses can be melted at lower temperatures while addition of fluorine allows higher solubility of nitrogen and has no effect on the significant increases in mechanical properties induced by nitrogen substitution.Initial assessment of bioactivity suggests that varying the N content may allow tailoring of the level of bioactivity of these glasses.

Controlling the ion release from mixed alkali bioactive glasses by varying modifier ionic radii and molar volume

Partially substituting one alkali oxide for another reduces the crystallisation tendency and improves the processing of bioactive glasses. Here, we investigate how we can use alkali ions of varying ionic radii to control glass degradation and ion release from Bioglass 45S5. Partially replacing sodium by lithium reduced ion release in static and dynamic dissolution studies in Tris buffer, while ion release increased with increasing potassium for sodium substitution. While the mixed alkali effect is known to reduce ion release from conventional silicate glasses (compared to compositions containing one alkali oxide only), in the glasses studied here ion release was controlled by the packing of the silicate network, described by glass molar volume and oxygen density. Incorporating an alkali ion of smaller ionic radius (Li for Na or Na for K) resulted in a more compact network of higher oxygen density, which reduced ion release. On the other hand, an alkali ion of larger ionic radius (K for Na or Na for Li) expanded the silicate network, allowing for faster ion release. This can be explained by water molecules penetrating an expanded silicate network more easily than a more compact one, thereby directly influencing the ion exchange between modifier ions and protons from the dissolution medium. This shows that the use of modifier ions of varying ionic radii allows for tailoring bioactive glass ion release and degradation while maintaining silicate network polymerisation and network connectivity. And, indeed, recent literature suggests that this concept can be extended to other modifiers besides alkali metal ions, making it possible to design bioactive glasses of tailored solubility.

Optical, FTIR and DC Conductivity of Soda Lime Silicate Glass Containing Cement Dust and Transition Metal Ions

Four soda lime silicate glass samples of composition (70 % SiO2+ 20 % Na2O+ 10 % CaO mol %) were prepared after adding 5 wt% cement dust to each sample mixture besides 0.1 wt% of one transition metal (TM) oxide of Fe, Co or Cu. The four samples were melted by a conventional melt-annealing technique at 1400 ∘C for 2.5 h. Density, UV/VIS, FTIR and DC conductivity measurements were performed for each glass. Experimental results indicate that there are only slight differences in the density values. The optical spectra reveal that the TM free sample and the sample containing iron ions have the same spectral features while the samples containing copper or cobalt exhibit distinct characteristic absorption bands due to each TM ion. FTIR spectra reveal characteristic vibrational bands due to stretching and bending modes of the silicate network. DC conductivity data show variations in the values of the studied samples according to the type of TM ions added. All the experimental results were correlated with each other in accordance with the current views on the constitution of the studied glasses.

Melting characteristics during the vitrification of MSW incinerator fly ash by swirling melting treatment

Pilot tests of fly ash were investigated in a 0.2 MW swirling melting furnace at 1250–1400 °C. The properties of raw fly ash and melted slag were investigated in terms of their chemical and mineralogical composition, particle size distribution, thermal analysis and microstructure, heavy metals distribution and leaching toxicity. Experimental results indicated that over 85 % particle sizes of raw ash sample were below 149 μm. At 1350 °C the main identified compounds were the silicates and aluminates (CaAl2Si2O8, Ca3Si2O7, CaSiO3, and Ca(Al,Si)2O4). The main compositions of melted slag at 1400 °C contained large amounts of dark glassy materials was completely the formation of more amorphous structure (Si–O), and heavy metals were immobilized in the silicate network matrix. The heavy metals Cr and Ni were shown a high solidification rate and reached about over 90 and 83 % in the slag. The achieved immobilization efficiency as judged by the US EPA test was 38–45 % for Co, Mn, Cu, and Pb. It can be seen that the solidification rates of As, Zn, and Hg remain about 20, 23 and 2 %, respectively. TCLP of heavy metals of vitrified slag met the regulatory thresholds of US EPA and China Standard.

The molecular structure of melts along the carbonatite–kimberlite–basalt compositional joint: CO2 and polymerisation

Transitional melts, intermediate in composition between silicate and carbonate melts, form by low degree partial melting of mantle peridotite and might be the most abundant type of melt in the asthenosphere. Their role in the transport of volatile elements and in metasomatic processes at the planetary scale might be significant yet they have remained largely unstudied. Their molecular structure has remained elusive in part because these melts are difficult to quench to glass. Here we use FTIR, Raman, 13C and 29Si NMR spectroscopy together with First Principle Molecular Dynamic (FPMD) simulations to investigate the molecular structure of transitional melts and in particular to assess the effect of CO2 on their structure. We found that carbon in these glasses forms free ionic carbonate groups attracting cations away from their usual ‘depolymerising’ role in breaking up the covalent silicate network. Solution of CO2 in these melts strongly modifies their structure resulting in a significant polymerisation of the aluminosilicate network with a decrease in NBO/Si of about 0.2 for every 5 mol% CO2 dissolved.This polymerisation effect is expected to influence the physical and transport properties of transitional melts. An increase in viscosity is expected with increasing CO2 content, potentially leading to melt ponding at certain levels in the mantle such as at the lithosphere–asthenosphere boundary. Conversely an ascending and degassing transitional melt such as a kimberlite would become increasingly fluid during ascent hence potentially accelerate. Carbon-rich transitional melts are effectively composed of two sub-networks: a carbonate and a silicate one leading to peculiar physical and transport properties.

Structural characterization of heavy metal SiO2–Bi2O3 glasses and glass–ceramics

29Si MAS NMR spectroscopic data obtained for heavy metal SiO2–Bi2O3 glasses and partially crystallized samples of similar composition were used to investigate the changes induced in their local structure as the ratio between Bi3+ and Si4+ cations decreases from 18:1 to 6:7. Progressive increase of SiO2/Bi2O3 ratio contributes to the structural relaxation of vitreous network and enhances the glass stability. In glass samples with high silica content the increasing number of Si–O–Si bridges proves the three-dimensional polymerization of silicate network. The shape of NMR signals for glass–ceramic samples depends on the crystalline phases formed for different SiO2/Bi2O3 ratios. The crystalline phases developed by heat treatment of vitreous matrices were identified by X-ray diffraction as Bi12SiO20, Bi5.6Si0.5O9.4 and Bi2SiO5. Scanning electron microscopy analysis was used to examine the morphology of the crystals developed in the glassy silica rich matrices.

High thermal neutron flux effects on structural and macroscopic properties of alkali-borosilicate glasses used as neutron guide substrate

The behaviour of four alkali-borosilicate glasses under homogeneous thermal neutron irradiation has been studied. These materials are used for the manufacturing of neutron guides which are installed in most facilities as devices to transport neutrons from intense sources such as nuclear reactors or spallation sources up to scientific instruments. Several experimental techniques such as Raman, NMR, SANS and STEM have been employed in order to understand the rather different macroscopic behaviour under irradiation of materials that belong to a same glass family. The results have shown that the remarkable glass shrinking observed for neutron doses below 0.5·1018n/cm2 critically depends upon the presence of domains where silicate and borate network do not mix.

Multidiffusion mechanisms for noble gases (He, Ne, Ar) in silicate glasses and melts in the transition temperature domain: Implications for glass polymerization

Noble gases are ideal probes to study the structure of silicate glasses and melts as the modifications of the silicate network induced by the incorporation of noble gases are negligible. In addition, there are systematic variations in noble gas atomic radii and several noble gas isotopes with which the influence of the network itself on diffusion may be investigated. Noble gases are therefore ideally suited to constrain the time scales of magma degassing and cooling. In order to document noble gas diffusion behavior in silicate glass, we measured the diffusivities of three noble gases (4He, 20Ne and 40Ar) and the isotopic diffusivities of two Ar isotopes (36Ar and 40Ar) in two synthetic basaltic glasses (G1 and G2; 20Ne and 36Ar were only measured in sample G1). These new diffusion results are used to re-interpret time scales of the acquisition of fractionated atmospheric noble gas signatures in pumices.The noble gas bearing glasses were synthesized by exposing the liquids to high noble gas partial pressures at high temperature and pressure (1750–1770K and 1.2GPa) in a piston-cylinder apparatus. Diffusivities were measured by step heating the glasses between 423 and 1198K and measuring the fraction of gas released at each temperature step by noble gas mass spectrometry. In addition we measured the viscosity of G1 between 996 and 1072K in order to determine the precise glass transition temperature and to estimate network relaxation time scales. The results indicate that, to a first order, that the smaller the size of the diffusing atom, the greater its diffusivity at a given temperature: D(He)>D(Ne)>D(Ar) at constant T. Significantly, the diffusivities of the noble gases in the glasses investigated do not display simple Arrhenian behavior: there are well-defined departures from Arrhenian behavior which occur at lower temperatures for He than for Ne or Ar. We propose that the non-Arrhenian behavior of noble gases can be explained by structural modifications of the silicate network itself as the glass transition temperature is approached: as the available free volume (available site for diffusive jumps) is modified, noble gas diffusion is no longer solely temperature-activated but also becomes sensitive to the kinetics of network rearrangements. The non-Arrhenian behavior of noble gas diffusion close to Tg is well described by a modified Vogel–Tammann–Fulcher (VTF) equation:Da2=A1a2∗exp-B1R(T-T2)-CRTwhere D is the diffusion coefficient, a the diffusion domain size (taken to be the size of the sample), A1 and C are respectively equivalent to the pre-exponential factor and to the activation energy (Ea in Jmol−1) of the classical Arrhenius equation, B1 can be interpreted as a “pseudo-activation energy” that reflects the influence of the silicate network relaxation, T2 is the temperature where the diffusion regime switches from Arrhenian to non-Arrhenian, and R is the gas constant (=8.314JK−1mol−1).Finally, our step heating diffusion experiments suggest that at T close to Tg, noble gas isotopes may suffer kinetic fractionation at a degree larger than that predicted by Graham’s law. In the case of 40Ar and 36Ar, the traditional assumption based on Graham’s law is that the ratio D40Ar/D36Ar should be equal to 0.95 (the square root of the ratio of the mass of 36Ar over the mass of 40Ar). In our experiment with glass G1, D40Ar/D36Ar rapidly decreased with decreasing temperature, from near unity (0.98±0.14) at T>1040K to 0.76 when close to Tg (T=1003K). Replicate experiments are needed to confirm the strong kinetic fractionation of heavy noble gases close to the transition temperature.

Archeological slag from Glinet: An example of silicate glass altered in an anoxic iron-rich environment

Vitreous slag wastes produced by a blast furnace working during the 16th century at the Glinet ironworks site (Normandy, France) are studied as analogs for vitrified nuclear waste. They display a specific interface between iron corrosion products and the glass matrix, which can contribute to understanding the influence of iron on glass alteration mechanisms and kinetics under anoxic conditions. Characterization of the archeological artifacts, especially of the interfacial area between the glass and corrosion products inside cracks, using micro and nanobeam techniques (micro-Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, scanning transmission X-ray microscopy) suggests that four main mechanisms have taken place: iron carbonate precipitation, glass hydration and interdiffusion, silicate network hydrolysis, and precipitation of iron silicates. Extrapolation using the results from short-term experiments and comparison with alteration thickness measurements carried out on archeological artifacts reveal that the presence of iron has a detrimental effect on glass durability. This can be attributed to the precipitation of iron silicates, consuming silicon and sustaining glass hydrolysis.

On topological indices of poly oxide, poly silicate, DOX, and DSL networks

Topological indices are numerical parameters of a graph that characterize its topology and are usually graph invariant. In a QSAR/QSPR study, physicochemical properties and topological indices such as Randić, atom–bond connectivity (ABC), and geometric–arithmetic (GA) indices are used to predict the bioactivity of chemical compounds. Graph theory has found a considerable use in this area of research. In this paper, we study different interconnection networks and derive analytical closed results of the general Randić index (Rα(G)) for α = 1, [Formula: see text], –1, [Formula: see text] only, for dominating oxide network (DOX), dominating silicate network (DSL), and regular triangulene oxide network (RTOX). All of the studied interconnection networks in this paper are motivated by the molecular structure of a chemical compound, SiO4. We also compute the general first Zagreb, ABC, GA, ABC4, and GA5 indices and give closed formulae of these indices for these interconnection networks.

Q-Speciation and Network Structure Evolution in Invert Calcium Silicate Glasses.

Binary silicate glasses in the system CaO-SiO2 are synthesized over an extended composition range (42 mol % ≤ CaO ≤ 61 mol %), using container-less aerodynamic levitation techniques and CO2-laser heating. The compositional evolution of Q speciation in these glasses is quantified using (29)Si and (17)O magic angle spinning nuclear magnetic resonance spectroscopy. The results indicate progressive depolymerization of the silicate network upon addition of CaO and significant deviation of the Q speciation from the binary model. The equilibrium constants for the various Q species disproportionation reactions for these glasses are found to be similar to (much smaller than) those characteristic of Li (Mg)-silicate glasses, consistent with the corresponding trends in the field strengths of these modifier cations. Increasing CaO concentration results in an increase in the packing density and structural rigidity of these glasses and consequently in their glass transition temperature Tg. This apparent role reversal of conventional network-modifying cations in invert alkaline-earth silicate glasses are compared and contrasted with that in their alkali silicate counterparts.

Structure and ionic diffusion of alkaline-earth ions in mixed cation glasses A2O–2MO–4SiO2 with molecular dynamics simulations

A series of mixed cation silicate glasses of the composition A2O–2MO–4SiO2, with A=Li, Na, K and M=Ca, Sr, Ba has been investigated by means of molecular dynamics simulations in order to understand the effect of the nature of the cations on the mobility of the alkaline-earth ions within the glass network. The size of the alkaline-earth cation was found to affect the inter-atomic distances, the coordination number distributions and the bond angle distributions, whereas the medium-range order was almost unaffected by the type of the cation. All the alkaline-earth cations contribute to lower vibrational frequencies but it is observed that there is a shift to smaller frequencies and the vibrational density of states distribution gets narrower as the size of the alkaline-earth increases. The results from our modeling for the ionic diffusion of the alkaline-earth cations are in a qualitative agreement with the experimental observations in that there is a distinct correlation between the activation energy for diffusion of alkaline earth-ions and the cation radius ratio. An asymmetrical linear behavior in the diffusion activation energy with increasing size difference is observed. The results can be described on the basis of a theoretical model that relates the diffusion activation energy to the electrostatic interactions of the cations with the oxygens and the elastic deformation of the silicate network.

Structural and Optical Correlation of Gamma-Irradiated 3d Transition Metals-Doped Lithium Disilicate Glasses

Undoped lithium disilicate glasses together with glasses of the same nominal composition containing 0.2 % of one of the 3d transition metal oxides (TMO) were prepared. Combined optical and FT infrared absorption spectral analysis were carried out for the prepared glasses before and after gamma irradiation with a dose of 8 Mrad. The optical spectrum of undoped glass reveals strong UV absorption which is related to the presence of unavoidable trace iron impurities (mainly Fe3+ ions). UV/Visible absorption spectra of 3d TM ions show additional characteristic absorption bands due to each respective TM ion. The TM ions mostly exhibit the higher valance or tetrahedral coordination states. Gamma irradiation of the undoped glass produces some variations in the optical spectra. The UV spectra reveal a slight increase in intensity while the visible spectra exhibit an induced defect at about 420 nm due to a silicon oxygen hole center or a nonbridging oxygen hole center. Some TM ions are assumed to share or compete for generated electrons and positive holes during the irradiation process. Infrared absorption spectra of the glasses reveal repetitive characteristic vibrational bands due to bending, symmetric stretching and antisymmetric stretching of the Si-O-Si or O-Si-O modes in the silicate network. The introduction of TM ions within the doping level (0.2 %) causes no effect on the IR spectra. The minor effects of irradiation on the IR spectra are related to some changes in the bond angles and/or bond lengths of some structural building units.

Na/Ca Intermixing around Silicate and Phosphate Groups in Bioactive Phosphosilicate Glasses Revealed by Heteronuclear Solid-State NMR and Molecular Dynamics Simulations.

We characterize the intermixing of network-modifying Na(+)/Ca(2+) ions around the silicate (QSi(n)) and phosphate (QP(n)) tetrahedra in a series of 16 Na2O–CaO–SiO2–P2O5 glasses, whose P content and silicate network connectivity were varied independently. The set includes both bioactive and bioinactive compositions and also encompasses two soda-lime-silicate members devoid of P, as well as two CaO–SiO2 glasses and one Na2O–SiO2–P2O5 glass. The various Si/P↔Na/Ca contacts were probed by molecular dynamics (MD) simulations together with heteronuclear magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) experimentation utilizing (23)Na{(31)P} and (23)Na{(29)Si} REDOR, as well as (31)P{ (23)Na} and (29)Si{(23)Na} REAPDOR. We introduce an approach for quantifying the extent of Na(+)/Ca(2+) ordering around a given QP(n) or QSi(n) group, encoded by the preference factor 0⩽ PM ⩽ 1 conveying the relative weights of a random cation intermixing (PM = 0) and complete preference/ordering (PM = 1) for one of the species M, which represents either Na(+) or Ca(2+). The MD-derived preference factors reveal phosphate and silicate species surrounded by Na(+)/Ca(2+) ions intermixed nearly randomly (PM ≲ 0.15), except for the QSi(4) and QSi(1) groups, which manifest more significant cation ordering with preference for Na+ and Ca2+, respectively. The overall weak preferences are essentially independent of the Si and P contents of the glass, whereas PM primarily correlates with the total amount of network modifiers: as the latter is increased, the Na/Ca distribution around the {QP(0), QSi(1), QSi(2)} groups with preference for Ca2(+ )tend to randomize (i.e., PCa decreases), while the PNa-values grow slightly for the {QP(1), QSi(3), QSi(4)} species already preferring coordination of Na. The set of experimental preference factors {PCa} for the orthophosphate (QP(0)) groups extracted from (31)P{(23)Na} REAPDOR NMR-derived M2(P–Na) dipolar second moments agrees well with the MD-generated counterparts. Our results on the Na/Ca intermixing in soda-lime-silicate glasses are discussed in relation to previous reports, highlighting the dependence of the conclusion on the approach to data evaluation.

In Situ High-Temperature Probing of the Local Order of a Silicate Glass and Melt during Structural Relaxation

Direct observation of structural relaxation at molecular scale in network glasses near the glass transition and in the melt is very challenging. Distribution of structural units forming the glass is commonly believed to depend uniquely on composition and temperature. In this paper, we evidence the dynamical structural changes of the silicate network upon structural relaxation by monitoring the temperature and time evolution of its vibrational signature. Just above Tg, the silicate network presents an unexpected time dependence of its relaxation process characterized by two subsequent regimes leading to a disordered equilibrium state. When annealed at 925 K, the network evolves rapidly toward silica and sodium silicate crystalline phases. Evolution models to describe the temperature time dependences of the Qn distributions are reported and discussed. A general picture of the hierarchical character of structural relaxation in network glasses is drawn.