Number Of Non-bridging Oxygens Research Articles

Investigation of coordination state of Zr 4+ ions in the sol—gel-derived ZrO 2SiO 2 glasses by EXAFS

The coordination state Zr 4+ ions in sol—gel-derived ZrO 2SiO 2 glasses of different ZrO 2 contents was examined using EXAFS. Two types of ZrO bonds were found in the first coordination shell. In the gels, Zr 4+ ions were coordinated by approximately four oxygens. One of the four oxygens was located at a distance of about 2.08 Å from Zr 4+ ion, forming a bridging ZrO bond. The others were at about 2.63 Å, and are attributed to non-bridging ZrO bonds (probably ZrOH). As a temperature was increased the coordination number of Zr 4+ ions with respect to oxygens increased to about 5 owing to the increase of bridging oxygens. The number of bridging oxygens around a Zr 4+ ion was increased from unity to three or four, and the number of non-bridging oxygens was decreased, at a smaller rate than that of the increase of bridging oxygens.

Structural differences and phase separation in alkali silicate glasses

Reasonable and significant structural differences between potassium silicate glass and sodium silicate glass can be revealed by molecular dynamics computer simulations.The differences in their immiscibility behavior reflect these structural differences. The simulation results are consistent with the experimental observations of many researchers that the variation in the number of nonbridging oxygens per silicon decreases in the order Li>Na>K. The distribution of the silicon species with various numbers of nonbridging oxygens is a reflection of the degree of homogeneity in the distribution of the modifier cation and nonbridging oxygen clusters. Nanoscale inhomogeneity seems to be an inherent nature of alkali silicate glasses.

Structural environments of incompatible elements in silicate glass/melt systems: I. Zirconium at trace levels

The structural environments of trace levels (2̃000 ppm) of Zr 4+ in several silicate glasses were examined as a function of melt composition and polymerization using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Glass compositions investigated were albite (NaAlSi 3O 8: AB) and a peralkaline composition (Na 3.3AlSi 7O 17: PR)- Zirconium was added to the oxide-carbonate mix prior to melting in the form of ZrO 2 (baddeleyite). A second set of Zr-silicate glasses containing 2000 ppm Zr and 1.0 to 2.4 wt% halogens (F as NaF and Cl as NaCl) was also synthesized. These included the Zr-AB and Zr-PR base-glass compositions as well as Zr-sodium trisilicate composition (Na2Si 3O 7: TS). In all glasses studied, Zr is mainly 6-coordinated by oxygen atoms ( d[Zr-O] $ ̃ 2.07 ± 0.01 A ̊ ). In the most polymerized glass (AB), a small but significant amount of Zr was also found to occur in 8-coordinated sites ( d[Zr-O] $ ̃ 2.22 A ̊ ). No clear evidence for F or Cl complexes of Zr was observed in any of the halogen-containing glasses. The regularity of the Zr site increases in the series AB < TS $ ̃ PR. We attribute this change to an increase in the number of non-bridging oxygens in the first-coordination sphere of Zr related to the depolymerizing effects of halogens and/or sodium. Minor but significant interactions of Zr with the tetrahedral network were observed ( d[Zr-{Si, Al}] $ ̃ 3.65–3.71 A ̊ ± 0.03 A ̊ ), which are consistent with Zr-O-{Si, Al} angles close to 160–170°, as in catapleiite (Na 2ZrSi 3O 9 · 2H 2O). Intermediaterange order, as reflected by the presence and number of second-neighbor {Si, Al} around Zr, increases significantly with increasing melt polymerization. The local environment around Zr is more strongly influenced by bonding requirements than by the network topology of the melt. Stabilization of zirconium in 6-coordinated sites in relatively depolymerized melts should act to decrease the crystal-melt partition coefficients of Zr and may explain the normally incompatible character of Zr during magmatic differentiation. The presence of Zr in sites of higher coordination (ZrO 8) in highly polymerized melts could be a precursor to the crystallization of zircon from such melts and thus may explain why Zr becomes a more compatible element, especially in the latest stages of magmatic differentiation.

The structural environments of trace elements in dry and hydrous silicate glasses; a manganese and strontium K-edge X-ray absorption spectroscopic study

The structural environments of Mn2+ and Sr2+ at concentrations of 0.2–0.8 wt% in dry and hydrous silicate glasses have been studied using X-ray absorption spectroscopy (EXAFS and XANES). The environment of Mn in hydrous silica glasses containing 4.5 and 6.0 wt% H2O appears to be close to an undistorted octahedral site, whereas Mn in other compositions, both dry and hydrous, occupies either a distribution of octahedral and tetrahedral sites, or distorted, non-centrosymmetric sites. The fraction of non-centrosymmetric sites (including tetrahedral) or the average degree of distortion of the sites decreases with both increasing dissolved water concentration and number of non-bridging oxygens in the glass. For Sr the changes as a function of polymerisation and water concentration are less clear, but it appears that the average Sr-O distances are shorter than in crystalline silicates and decrease further with decreasing polymerisation. The structural data obtained in this study are not readily integrated with previous partitioning data for Mn, suggesting that changes in the first-shell coordination geometry of Mn do not play a direct role in controlling the melt compositional dependence of trace element partition coefficients. It is tentatively suggested that distortions of the framework resulting from the incorporation of Mn (i.e. a second shell effect) control the partitioning behaviour.

The effect of adsorbed lead on the chemical durability of nuclear waste glasses

The chemical durability of two types of nuclear waste glasses was measured by the MCC-1 method, with and without the presence of lead in the leachant. This presence has a significant effect on the chemical durability of the sample with the initially bad durability (T-60) and a minor effect on the other glass (C-411), initially more durable. Surface analysis using X-ray Photoelectron Spectroscopy (XPS) was performed on both samples. The O 1s spectra yield a correlation between the number of non-bridging oxygens (NBOs) on the surface and the chemical durability, the latter decreasing with increasing the numbers of NBOs. These NBOs are created in the rate limiting step of the silica-water reaction by breaking the structural Si-O bonds. A lead ion can react with an NBO and inhibit the reaction that destroys the structural component in the glass. This effect is dominant in the less durable T-60 but is only minor for C-411, where most of the surface is composed of Al 2O 3 which is non soluble, giving the glass its initial good durability.

Modeling of the Viscosity of Glasses Used in the Immobilization of High-Level Liquid Nuclear Waste

Compositional and structural considerations are used to understand viscous behavior of multicomponent borosilicate glasses used in the immobilization of high-level nuclear waste. The presence of alkali and alkaline earth oxides in these systems can be considered as diluents to a highly viscous silica matrix. The extent of dilution is characterized by the presence of singly bonded or nonbridging oxygens. For the analysis, the Arrhenius equation is combined with the number of nonbridging oxygens to develop a predictive temperature-dependent model for the viscosity of these systems based on composition.

The structure of sodium trisilicate glass via molecular dynamics employing three-body potentials

Molecular dynamics simulations (MD) employing multibody potentials were used to simulate sodium trisilicate glass (Na2O·3SiO2). A multibody term has been added to a pair potential in order to incorporate the bond directionality which is expected for the partially covalent silicate structure. The structure of the glass was analyzed and results were compared to those found using two-body potentials and molecular statics, as well as to experimental results found using x-ray diffraction, XPS, NMR, and EXAFS. Current results compared favorably to experiment and showed improvement over results obtained using two-body potentials. Nearest neighbor distances and coordination numbers agreed well with published data. Although two-body potentials normally show overcoordinated silicon (&gt;4.3) and broad O–Si–O tetrahedral angle distributions, in this study all silicon exhibited tetrahedral coordination (4.0) and the O–Si–O bond angle distribution was markedly sharpened. The number of nonbridging oxygens was shown to be nearly equal to the number of sodium ions present, and a reasonable distribution of Qc species was found. The overall structure closely resembled the modified network structure of glass proposed experimentally, with silicon tetrahedra remaining intact and sodium ions breaking up the network through the creation of nonbridging oxygens.

Application of a New Three-Body Potential to Vitreous Silica and Sodium Silicate Glasses

AbstractAn empirical three-body potential, suitable for molecular dynamics (MD) simulations, has been developed to model the natural covalency of the Si-O bond in vitreous silica and silicate glass systems. Through the addition of a small directional-dependent three-body term to a previously used modified ionic pair interaction, a narrow distribution of tetrahedral angles and a low concentration of defects were obtained, in good agreement with experiment. The structure of bulk silica resulting from the MD technique also contained a larger average ring size, no edge-sharing tetrahedra, and a calculated static structure factor in good agreement with neutron diffraction results. The simulated sodium silicate glass was also largely improved over previous simulations using pair interactions alone. All silicon atoms were found to be exactly four coordinated while the number of non-bridging oxygen nearly equaled the number of sodium ions present with a reasonable distribution of Qi species.

X-ray photoelectron spectroscopy of a cerium-doped lanthanum aluminosilicate glass

Core-level XPS spectra of a Ce-doped La aluminosilicate glass are reported. A deconvolution of the o 1s peak into two subpeaks corresponding to bridging and non-bridging oxygens indicates that 66% of the Al ions occupy tetrahedral sites and that the number of non-bridging oxygens in a (Si, Al)O 4 4− tetrahedron is 1.20 which is consistent with the value obtained from the Si 2p chemical shift. The 3d core levels of Ce and La show main and satellite peaks. The broadening of the Ce and La peaks in the glass compared with the spectra of the corresponding sesquioxides is attributed to the random nature of the glassy host.

Structure Transition in Alkali Silicate Glasses

The logarithm of measured high‐temperature fluidities is related to the square of the calculated probability of finding a nonbridging oxygen in a randomly chosen SO4 group. This is interpreted in terms of a flow mechanism (gliding) dependent on juxtaposition of two such oxygens. A theory of the glass transformation is developed. On cooling, the coordination number of nonbridging oxygens increases until, ideally, at T, each touches three oxygens of an external Si04 group (stabilization). Below this temperature, thermal expansion/contraction contains no coordination change component. These changes constitute a second‐order transition. Observed volume data are in agreement with hypothesis. On this view, stabilization creates vacant tetrahedral environments adjacent to existing ones. Migration of glass‐forming ions to these empty sites can unlock the structure and permit the operation of a new set of flow processes. The coordination requirements for stabilization severely restrict the types of local arrangement possible at high silica contents, and therefore reduce the configurational entropy and induce unmixing. The overall transformation is composite in character. Rather extensive bond rearrangement and unmixing (when present) are features of first‐order kind.

Gamma‐Ray Induced Coloring of Some Phosphate Glasses

The changes in gamma‐ray induced optical absorption in phosphate glasses resulting from changes in composition, conditions during melting, and additions of small amounts of some oxides are discussed. A resolution of the induced spectra showed that the observed absorption is due to the superposition of three bands at 2.3, 2.9, and 5.5 e.v. (540, 425, and 225 mü) and to a fourth band whose absorption peak is beyond 6 e.v. The ultraviolet induced absorption increases, whereas the visible absorption decreases in glasses melted under reducing conditions as opposed to those melted under normal conditions in air and on replacement of K+ by Na+ or Li+. A similar effect is produced on replacement of Ba++ by Pb++ and on the addition of T12O to a CaO‐P2O5 glass or the addition of As2O3 to a CaOP2O5, GeO2 glass. Additions of large amounts of GeO2 are accompanied by a decrease in the number of nonbridging oxygens and a decrease in the visible induced absorption. Replacement of Ca ++ by Ba++ ions showed an over‐all decrease in the induced absorption. The addition of small amounts of the oxides of germanium, titanium, iron, thallium, niobium, and arsenic showed an appreciable effect in inhibiting the visible induced coloration.