Lead and aluminum bonding in Pb-AI metaphosphate glasses

The bonding properties of cations in phosphate glasses determine many short- and medium-range structural features in the glass network, hence influencing bulk properties. In this work, Pb-Al-metaphosphate glasses (1 - x)Pb(PO(3))(2).xAI(PO(3))(3) with 0 < or = x < or = 1 were analyzed to determine the effect of the substitution of Pb by AI on the glass structure in the metaphosphate composition. The glass transition temperature and density were measured as a function of the Al concentration. The vibrational and structural properties were probed by Raman spectroscopy and nuclear magnetic resonance of (31)P, (27)AI, and (207)Pb. Aluminum incorporates homogeneously in the glass creating a stiffer and less packed network. The average coordination number for AI decreases from 5.9 to 5.0 as x increases from 0.1 to 1, indicating more covalent AI-O bonds. The coordination number of Pb in these glasses is greater than 8, showing an increasing ionic behavior for compositions richer in AI. A quantitative analysis of the phosphate speciation shows definite trends in the bonding of AIO(n) groups and phosphate tetrahedra. In glasses with x < 0.48, phosphate groups share preferentially only one nonbridging O corner with an AIO(n) coordination polyhedron. For x > 0.48 more than one nonbridging O can be linked to AIO(n) polyhedra. There is no corner sharing of O between AIO(n) and PbO(n) polyhedra nor between AIO(n) themselves throughout the compositional range. The PbO(n) coordination polyhedra show considerable nonbridging O sharing, with each O participating in the coordination sphere of at least two Pb. The bonding preferences determined for Al are consistent with the behavior observed in Na-AI and Ca-AI metaphosphates, indicating this may be a general behavior for ternary phosphate glasses.     

L’importance des phénomènes d’oxydo-réduction dans le verreImportance of oxidoreduction phenomena in glass.

Oxydo-reduction phenomenon concerns glass manufacturing as well as its properties. Various oxidation scales have been used by glassmakers, the most widely used being the ratio between Fe²⁺ and the total iron content. Comparisons are made with others, such as the partial pressure of oxygen or the redox potential. The influence of the chemical composition of the glass is also studied as well as that of the temperature, through the results obtained in Saint-Gobain Recherche and in other laboratories. In industrial glass manufacturing, the oxidation equilibrium is usually achieved during the fining process, an intense ‘bubbling’ being necessary to obtain a homogeneous glass. Thus, the oxidoreduction state of the glass does not depend on the atmosphere under which it was made, but depends on the mixture of gases generated during the fining process, which is described through the reactions generated by the two main fining systems, arsenic oxide or antimony oxide + sodium nitrate and sodium sulphate + a reducing agent. The former provides oxygen and the latter, a mixture of oxygen and sulphur dioxide. Consequently, a glass melted with arsenic oxide is more oxidised than a glass melted with sodium sulphate. The equilibrium constant used to interpret the latter system, mostly used for soda lime silica glasses fining, is given as a function of the temperature and some of the consequences are examined. Obviously, the coloration by iron oxide impurities is influenced by the manufacturing process. Some other colouring oxides are submitted to a change of oxidation state and thus to a modification of their colouring action. A quick review is done of the transition metal coloration, of the electron-charge-transfer absorption due to sulphides or selenides, and of aggregate formation inducing the well-known red ruby glass, for instance.

Résumé

Les phénomènes d’oxydo-réduction concernent aussi bien l’élaboration du verre que ses propriétés. Plusieurs échelles d’oxydo-réduction ont été utilisées par les verriers, la plus courante étant le rapport entre Fe²⁺ et la teneur totale en fer. Nous faisons la comparaison entre les différents systèmes, comme la pression partielle d’oxygène. Dans l’élaboration des verres industriels, l’équilibre redox est réalisé habituellement pendant le processus d’affinage, un « bullage » intense étant indispensable à l’obtention d’un verre homogène. L’état redox du verre ne dépend pas de l’atmosphère sous laquelle il a été élaboré, mais du mélange de gaz généré durant le processus d’affinage. Les deux systèmes affinants principaux sont l’oxyde d’arsenic ou d’antimoine couplé au nitrate de sodium ou bien le sulfate de sodium couplé à un agent réducteur. Le premier génère de l’oxygène et le second un mélange d’oxygène et de dioxyde de soufre. En conséquence, un verre élaboré avec de l’oxyde d’arsenic est plus oxydé qu’un verre élaboré avec du sulfate de sodium. La coloration par l’impureté oxyde de fer est évidemment fortement influencée par le procédé. D’autres oxydes colorants subissent aussi des changements d’état d’oxydo-réduction et voient leur action colorante modifiée en fonction des conditions d’élaboration.     

Dynamic light scattering in mixed alkali metaphosphate glass forming liquids

We report the first ever photon correlation spectroscopy performed on single alkali and mixed alkali metaphosphate glasses at refractory temperatures above the glass transition. We find not only a significant decrease in the glass transition temperature but also a decrease in fragility for the mixed alkali composition as compared with the single akali glasses. We argue that structural relaxation in these polymeric oxide glasses is largely controlled by the cross linking cations and that the changes in fragility that we observed are a reflection of changes in the cooperativity of structural relaxation wrought by the substantial decrease in the ion mobility that accompanies the mixing of alkali ions.     

Densification of silica glass at ambient pressure

We show that densification of silica glass at ambient pressure as observed in irradiation experiments can be attributed to defect generation and subsequent structure relaxation. In our molecular dynamics simulations, defects are created by randomly removing atoms, by displacing atoms from their nominal positions in an otherwise intact glass, and by assigning certain atom excess kinetic energy (simulated ion implantation). The former forms vacancies; displacing atoms and ion implantation produce both vacancies and "interstitials." Appreciable densification is induced by these defects after equilibration of the defective glasses. The structural and vibrational properties of the densified glasses are characterized, displaying resembling features regardless of the means of densification. These results indicate that relaxation of high free-energy defects into metastable amorphous structures enriched in atomic coordination serves as a common mechanism for densification of silica glass at ambient pressure.     

Validation of Structural Grounds for Anomalous Molecular Mobility in Ionic Liquid Glasses

Ionic liquid (IL) glasses have recently drawn much interest as unusual media with unique physicochemical properties. In particular, anomalous suppression of molecular mobility in imidazolium IL glasses vs. increasing temperature was evidenced by pulse Electron Paramagnetic Resonance (EPR) spectroscopy. Although such behavior has been proven to originate from dynamics of alkyl chains of IL cations, the role of electron spin relaxation induced by surrounding protons still remains unclear. In this work we synthesized two deuterated imidazolium-based ILs to reduce electron–nuclear couplings between radical probe and alkyl chains of IL, and investigated molecular mobility in these glasses. The obtained trends were found closely similar for deuterated and protonated analogs, thus excluding the relaxation-induced artifacts and reliably demonstrating structural grounds of the observed anomalies in heterogeneous IL glasses.     

Local structure of magnesium in silicate glasses: a 25Mg 3QMAS NMR study

We have reported the 25Mg triple-quantum magic-angle spinning (3QMAS) NMR spectra of silicate glasses. The two-dimensional spectra suggest that the magnesium ions in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses are mainly in octahedral environments, although in Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses they form tetrahedral species. We discussed the coordination environments of Mg based on the field strength of competing Mg2+, Ca2+, Na+, K+, and Li+ cations, and convincingly demonstrated that there is a correlation between them. These results indicate that the two-dimensional NMR spectroscopy such as MQMAS technique is a very useful method to analyze the local environments of nonframework cations in noncrystalline solids.     

Network entropy and connectivity: the underlying factors determining compositional trends in the glass-transition temperature

We present in this article a theoretical approach describing the viscosity increase in an overmelt liquid as the result of an augmentation of covalent bonds in the glass network being created, by means of an agglomeration of well-defined structural entities. Thus, one can study the characteristic temperature of this agglomeration process, identified as the glass-transition temperature, as a function of the modifier rate in a binary system. Result analysis and comparison with experiment points out that connectedness (and the subjacent entropy of the glass network) control mainly the variation of the glass-transition temperature with the modifier rate.     

Blends of metal acetates and polyurethanes containing pyridine groups II. SAXS and EXAFS studies

Polyurethanes containing pendant pyridine units were blended with various metal acetates and studied by small-angle x-ray scattering (SAXS) and extended x-ray absorption fine structure spectroscopy (EXAFS) to better understand the microscopic effect of blending on these materials. An earlier investigation found a dramatic enhancement in mechanical properties after blending, which suggests at least two pyridine units were coordinating to a single cation. This coordination would enable the cation to act as a cross-linking site, which could then cause the observed changes in mechanical properties. To determine the effect of complexation on the microphase-separated domain structure, small-angle x-ray scattering patterns were collected. Neutralization with a metal acetate increased the scattered intensity, which can be explained by an increase in electron density contrast but may also have been due to an improvement in phase separation. The distance between lamellar domains was basically unaffected by the addition of metal acetate, with the exception of nickel acetate. In this instance the distance decreased, which was caused by an improvement of packing inside the hard segments. EXAFS at the nickel and zinc edges indicated that the same qualitative changes occurred in the local environments around both cations after blending versus the unblended acetates. The magnitude of the first shell peak in the radial structure function (RSF) increased significantly upon blending, a result that is difficult to rationalize. The higher shell peaks exhibited significant changes in position and magnitude upon blending, which indicates substantial local rearrangement around the metal cation These fundamental changes in the EXAFS spectra may have been due to complexation between the cation and the pyridine group, but the results were not conclusive. © 1994 John Wiley & Sons, Inc.     

Clustering of rare earths in GeAs sulfide glass

Clustering of rare-earth dopants in GeAs sulfide glasses was studied by fluorescence spectroscopy of Pr-doped glasses and by EPR measurements of Gd-doped samples. The linewidth of the g  2 resonance of Gd³⁺, as well as the relative intensity of emission from the ¹D₂ level of Pr³⁺, was used as a relative measure of rare-earth clustering. Rare earths were found to have low solubility in uncodoped GeAs sulfide glasses, which also displayed poor fluorescence efficiency due to severe clustering. Codoping such glasses with Ga greatly enhanced rare-earth solubility and dispersal, particularly for Ga:rare earth ratios ≥ 10:1, as evidenced by the narrower EPR resonances and more intense luminescence of Gd- and Pr-doped glasses, respectively. In, P and Sn were also observed to ‘decluster’ rare earths, although less efficiently than Ga, whereas codoping with I was found to have no effect on clustering. These phenomena are explained by a structural model in which (1) rare-earth dopants and codopants are spatially associated and (2) rare-earth dispersal is accomplished by a statistical distribution of codopants in tetrahedral network sites.     

Double bandpass filter and dual band gap study for NiO doped into P2O5–ZnO–Na2O glassy system

Abstract

Glass compositions 42?P₂O₅–40ZnO–(18-x)Na₂O: xNiO in which x varies from 0 to 6?mol.% with step of 1?mol.%, are prepared by using the conventional melt annealing method. The effect of nickel ions in the investigated glass system appears in the combined structural and optical properties. Some structural and optical parameters such as molar volume, interionic distance, optical band gap, and Urbach energy are deduced. The variations in the spectroscopic data, which is affected by the deviations in composition and coordination state of nickel ions, are studied in relation to the structural evolution. The analysis of FTIR of the proposed structure shows that the local network structure is based mainly on Q ¹ and Q ² tetrahedron units connected by P–O–P linkages. Also, the optical properties show that the nickel ions occupy both tetrahedral and octahedral sites in the glass network.     

PACKING OF IONS IN SILICATE GLASSES

Any silicate glass can be regarded as the relaxation of SiO_2 glass and the volumeis composed of two parts. One is the topological volume of O~(2-), which is equal to thevolume per oxygen of SiO_2 glass, the other is the topological volumes of cations. Thetopological volume of a monovalent cation equals the volume shared by each sphere indense haphazard packing of equal spheres of the cation's size. The charges of bivalentcations cause a decrease of volume, the decrement being the packing volume of Ca~(2+). Anequation is reached for the calculation of the volume per oxygen of silicate glasses. Alot of calculations show that the equation is accurate, so accurate that the densities canbe calculated. The study reveals some characters of the glass structure and clearly ex-presses the contributions of various cations to the volumes of glasses.     

Impact of network topology on cationic diffusion and hardness of borate glass surfaces

The connection between bulk glass properties and network topology is now well established. However, there has been little attention paid to the impact of network topology on the surface properties of glass. In this work, we report the impact of the network topology on both the transport properties (such as cationic inward diffusion) and the mechanical properties (such as hardness) of borate glasses with modified surfaces. We choose soda lime borate systems as the object of this study because of their interesting topological features, e.g., boron anomaly. An inward diffusion mechanism is employed to modify the glass surface compositions and hence the surface topology. We show that accurate quantitative predictions of the hardness of the modified surfaces can be made using topological constraint theory with temperature-dependent constraints. Experimental results reveal that Ca(2+) diffusion is most intense in glasses with lowest BO(4) fraction, whereas Na(+) diffusion is only significant when nonbridging oxygens start to form. These phenomena are interpreted in terms of the atomic packing and the local electrostatic environments of the cations.     

Glass Transition Behavior in Se-Rich Ge-Se Alloys

Differential scanning calorimetry (DSC) was utilized to study the behavior of six Ge-Se glasses containing 0, 5, 10, 15, 17.5 and 20 at.% Ge during the glass transition. These alloys readily form glasses and can be prepared by quenching in air. Moreover, their behavior depends greatly on the composition. This work reveals that two additional properties must be considered: the variation in the glass transition temperature and different structural relaxation. The quantity used to quantify the relaxation was the enthalpy relaxation as this measures the heat lost by the glass during annealing. Given the complexity of the relaxation process, the experimental results were analysed by means of the empirical Kohlrausch-Williams-Watts relaxation model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preferential binding of fluorine to aluminum in high peralkaline aluminosilicate glasses

For two series of fluoride-containing aluminosilicate glasses of high peralkaline type, we apply 27Al, 19F, 29Si, and 23Na NMR spectroscopy to understand the structural changes introduced by the addition of alkali fluorides. Adding fluoride in concentrations above the solubility limit causes crystallization of different phases in sodium and potassium glasses despite identical composition. However, the NMR spectra reveal that the structural evolution of the precrystallized states is similar in both series. In particular, fluorine coordinates exclusively to alkaline cations and aluminum. No indication of direct bonding with silicon was found from 19F --> 29Si cross-polarization experiments. In contrast to other glass systems, double resonance experiments in these peralkaline systems show that halide addition produces at most a minor fraction of tetrahedral aluminum containing fluorine in its coordination sphere. Instead, the fluorine addition prior to crystallization converts up to about 20% of the initial tetrahedral aluminum (1 mol % in absolute units) to 5- and 6-fold coordinated aluminum. A minor portion of five-coordinated aluminum groups is considered as the intermediate to the growing fraction of octahedral aluminum in the silicate matrix. The initialization of the crystallization process is correlated with the saturation of the silicate matrix by octahedral aluminum clusters segregating out under further doping by fluoride. It is suggested that the formation of the nonframework Al-F bonds is responsible for structural relaxation, reflected by the reduction of the glass transition temperature.     

Ab initio molecular dynamics study of 45S5 bioactive silicate glass

Bioglass 45S5, the prototype of bioactive melt-quenched silicate glasses, was modeled by means of Car-Parrinello molecular dynamics (CPMD) simulations. Although long-range structural properties cannot be modeled by using this ab initio approach, the accuracy of CPMD simulations is exploited here to provide insight into the short-range structure and to analyze vibrational and electronic properties of this biomaterial. Detailed structural analysis in the short-range scale provided insight into the local environment of modifier Na and Ca ions: a possible key role of these cations in organizing the glass network by connecting different chains and fragments into specific, rather flexible geometries was proposed. The individual contributions of different species to the vibrational density of states were separated and discussed, allowing the identification of specific features in the vibrational spectrum, such as those related to phosphate groups. The components of the electronic density of states were also analyzed, enabling us to identify correlations between the electronic structure and the structural properties, such as the different bonding character of Si-O bonds involving bridging or nonbridging oxygen atoms.     

Perspective: Supercooled liquids and glasses

Supercooled liquids and glasses are important for current and developing technologies. Here we provide perspective on recent progress in this field. The interpretation of supercooled liquid and glass properties in terms of the potential energy landscape is discussed. We explore the connections between amorphous structure, high frequency motions, molecular motion, structural relaxation, stability against crystallization, and material properties. Recent developments that may lead to new materials or new applications of existing materials are described.     

LiCl熔盐急冷形成非晶固体的分子动力学计算机模拟研究

卤化物玻璃目前已成为引人注目的光纤新材料,用分子动力学方法研究液态急冷形成非晶态的过程,对于卤化物玻璃的形成过程研究也应是有用的。鉴于碱金属卤化物是最简单的熔盐,其动态结构亦很清楚。用分子动力学方法研究其急冷以形成玻     

Small-angle X-ray scattering characterization of inorganic glasses

Small-angle X-ray scattering is a powerful tool in glass science. The present paper reviews the characterization of inorganic glasses by means of this technique, with specific concern for four areas; (A) the correlations between structure and properties; (B) the middle-range order; (C) the kinetics of phase separation; and (D) the techniques of glass preparation. SAXS studies on various glass systems, e.g. vitreous silica, phosphate glasses, colour glasses, photochromic glasses, and glass-ceramic-forming systems, are described in terms of this approach.     

Photoluminescent coordination polymer bulk glasses and laser-induced crystallization

We synthesized luminescent coordination polymer glasses composed of d¹⁰ metal cyanides and triphenylphosphine through melt-quenching and mechanical milling protocols. Synchrotron X-ray total scattering measurements and solid-state NMR revealed their one-dimensional chain structures and high structural dynamics. Thermodynamic and photoluminescence properties were tunable by the combination of heterometallic ions (Ag⁺, Au⁺, and Cu⁺) in the structures. The glasses are moldable and thermally stable, and over centimeter-sized glass monoliths were fabricated by the hot-press technique. They showed high transparency over 80% from the visible to near-infrared region and strong green emission at room temperature. Furthermore, the glass-to-crystal transformation was demonstrated by laser irradiation through the photothermal effect of the glasses.

Over centimeter-sized luminescent coordination polymer glasses were fabricated. They showed high transparency (over 80%) and strong green emission at room temperature. The glass-to-crystal transformation by laser irradiation was demonstrated.     

TMC-95A–D and analogues: Chemistry and biology

The proteasome regulates diverse intracellular processes, including cell-cycle progression, cell adhesion and migration, apoptosis and antigen presentation: selective inhibitors of the proteasome, therefore, have great therapeutic potential for the treatment of cancer. TMC-95A–D are unique natural products and represent a new class of noncovalent, reversible, and selective proteasome inhibitors with exceptionally strong bioactivity profiles and interesting structural properties. Significant recent advances in the syntheses of these natural products have led to intense interest in the development of related compounds as potential anticancer agents: the chemistry and biology of these natural products and analogues will be described in this review article.