Cations In Glasses Research Articles

Barium diffusion in mixed cation glasses

Diffusion of barium (Ba) in mixed cation glasses of the composition xNa2O x (3 - x)BaO.4SiO2 with x = 0.0, 0.1, 0.3 and 1.0 and 0.4K2O.2.6BaO.4SiO2 was investigated by means of the radiotracer diffusion technique below the respective glass transition temperatures. In accord with our previous results of calcium (Ca) diffusion in soda-lime silicate glasses (F. Natrup et al., Phys. Chem. Chem. Phys., 2005, 7, 2279), the mobility of alkaline-earth ions increases with the alkali content in all analyzed glass systems with no decrease in the diffusion activation enthalpy, but a raise in the pre-exponential factor. A distinct dependency of the activation enthalpy of alkaline-earth ions on the type and content of the alkali ions in the glass is observed. The results provide evidence for elastic and electrostatic contributions to cation diffusion in glasses and support the formation of dissimilar cation pairs, that were derived from nuclear magnetic resonance (NMR) investigations of soda-lime silicate glasses and glasses containing sodium and barium. Finally, a striking correlation between the pre-exponential factor of alkaline-earth ion diffusion in soda-lime and potassium barium glasses is found, the origin of which remains unsolved.

A rapid field-free electroosmotic micropump incorporating charged microchannel surfaces

A rapid field-free electroosmotic micropump (RFEP) was constructed incorporating separate anionic and cationic glass microchannel surface modifications and its performance was investigated. Two of the arms of the devised Y-shaped RFEP were coated with anionic and cationic polymers, respectively. Different charges localized on these interior surfaces produced electroosmotic flow in opposite directions when an electric field was applied along the two coated arms. The hydrodynamic pressure developed at the common junction of the three arms generated field-free flow, which responded rapidly and reversibly to the applied electric field. The flow rate of the devised RFEP was 262.4 nL min −1 when 1.0 kV cm −1 was applied in 10 mM phosphate buffer at pH 7.0. Field-free flow in the vertical channel was quickly and precisely controlled by an external voltage program, as previously demonstrated by field-free gated injection and microbead sorting experiments. Gated field-free injection was achieved by combining two RFEPs at a simple cross. The devised RFEP was found to produce rapid short hydrodynamic pulses, which allowed the sorting of microbeads at up to 120 objects per minute in a biocompatible manner.

Activation volumes and site relaxation in mixed alkali glasses

Evidence is presented for site relaxations occurring in mixed alkali (cation) glasses based on activation volumes, VA(σ)=−RT[dlnσ/dp]T, which are determined for sodium aluminoborate glasses of varying Na2O content, and for mixed alkali glasses where Na+ is partially replaced by Li+, K+ or Cs+ ions. In accordance with the ‘updated’ dynamic structure model, activation volumes are identified here with local expansions that accompany the opening up of C′ sites to admit incoming ions. ‘Anomalous’ increases in activation volume in mixed cation glasses correlate with the size of minority (guest) cations. This anomaly is interpreted in terms of a ‘leader follower’ mechanism that involves dynamic coupling between the faster (majority) and slower (minority) cations. Because of mismatch effects in mixed cation glasses this coupling requires the opening up of additional cation sites by the slower follower cations. The resulting disturbances in the glass network are responsible for many characteristic features of the mixed alkali effect, including the appearance of high temperature internal friction peaks and observed minima in glass transition temperatures and melt viscosities.

Structure and lattice dynamics of binary lead silicate glasses investigated by infrared spectroscopy

The polar lattices dynamics of seven binary lead silicate glasses have been studied by infrared spectroscopy. The analysis of the reflectivity spectra with a dielectric function model, based on a modified Gaussian profile, allows a quantitative evaluation of the presence of lead cations within different structural sites. From the role of the lead cations versus the degree of polymerization of the silicate network and the comparison with literature results, we may to give a scenario for explaining the observed structural evolution of the glass matrix and more particularly the drastic change occurring around 45% of lead content. Below this threshold, lead cations act only as modifiers of the silicate network. Above, the glass structure is deeply modified; a lead network involving around 10% of the lead content appears in glasses whose composition is just above the threshold and progressively grows at the expense of the silicate network with the increase of lead content. For high lead content, lead cations can act as modifiers of the silicate network or as network formers. Results also show that the analysis of far infrared measurements combined with the knowledge of the UV edge optical response is very promising to characterize the local disorder around cations in glasses.

Spinel structure and liquidus temperature relationships in nuclear waste glass

The liquidus temperature of spinel (AB2O4) crystals frequently limits the waste loading of vitrified nuclear waste. In other studies, glass structure-spinel liquidus temperature relationships were discovered for the non-spinel forming cations in glass, but no such relationship was identified for the spinel-forming metal-ions. In this paper, coefficients from an empirical first order mixture model describing spinel liquidus temperature were correlated with the octahedral site preference energy (OSPE) of the spinel-forming metal ions. The OSPE was calculated using approximations of glass and spinel structure made by extrapolating from data on similar glasses and spinels. A literature review indicated that Cr(III), Ni(II), and Fe(II) are most likely to be in the octahedral sites in spinel, Mn(II) in the tetrahedral site, and that Fe(III) will be common in both sites. These assignments were used as starting assumptions to calculate the OPSE for each metal ion. A strong correlation between the OSPE and the liquidus temperature coefficients of spinel-forming metal ions was observed. Further correlations determined that the Crystal Field Stabilization Energy was the most important portion of the OSPE for these spinels. These results indicate that the thermodynamic properties of spinel could be predicted based on assumptions of spinel structure, underpinned by fundamental properties, when scaled empirically.

Chemistry versus phase constitution of yellow ancient tile glazes: A non-destructive insight through XAS

Using synchrotron radiation-based analytical (SRXRF) and spectroscopic (XAS) non-destructive techniques, a study was carried out on lead-rich, tin-opacified yellow glazes (silica-lime-alkali type glasses) decorating ancient tiles (17–19th century).These glasses have a rather complex chemistry, being currently assumed that the yellow pigment used for centuries – a pyrochlore-type double oxide of lead and antimony – prevails within the glaze, despite Sb(3+) being recognized as a network-forming cation in glasses. Minerals and synthetics with known crystal structure were used as model compounds to interpret X-ray absorption spectroscopy data at Sb K-edge and Pb L3-edge collected from ancient glazes. Theoretical modelling of Sb 1s XANES spectra was applied to demonstrate that antimony alone may be responsible for the yellow colour through a finely dispersed pyrochlore-type Sb-oxide, while lead remains hosted in the glassy matrix.

A Mechanistic Approach to Conductivity Relaxation in Ionic Glasses

Abstract A mechanism is proposed for conductivity relaxation in ionic glasses, based on the operation of a memory effect (as postulated in the dynamic structure model) that provides the necessary synergy for ‘pair-wise’ coupling of ionic motions in single cation glasses. It is suggested that this mechanism (which resembles in some respects the older ‘interstitialcy’ process) constitutes an essential component of the single and many-particle reaction route’ in Funke’s concept of mismatch and relaxation (CMR). This coupling is also present in mixed cation glasses, but the appearance of a site mismatch effect and the consequent disappearance of synergy leads to anomalous behaviours typical of the mixed cation effect, including recently reported increases in activation volume.

Non-random cation distribution in sodium?strontium?phosphate glasses

Vitreous compositions in the (0.55− x )Na 2 O: x SrO:0.45P 2 O 5 (0⩽ x ⩽0.55) system were characterized by differential scanning calorimetry and 31 P solid state NMR. High strontium containing glasses were found to be partly crystallized. In the pure glass samples a general increase in T g and a decrease in isotropic chemical shift with increasing x were observed. Two distinct linear ranges were observed in plots of these parameters against composition, with a transition point at x ≈0.20. This composition corresponds to the point at which all Na + ions associated with charge balance of the terminal Q 1 phosphate tetrahedra are substituted for Sr 2+ . In the mixed cation glasses, this suggests a non-random distribution of cations, with preferential location of Sr 2+ ions near the chain ends. Crystalline models have been used to discuss trends in the variation of chemical shift anisotropy and propose possible coordination environments for the metal cations in the glasses.

Non-random cation distribution in sodium–strontium–phosphate glasses

Vitreous compositions in the (0.55− x)Na 2O: xSrO:0.45P 2O 5 (0⩽ x⩽0.55) system were characterized by differential scanning calorimetry and 31P solid state NMR. High strontium containing glasses were found to be partly crystallized. In the pure glass samples a general increase in T g and a decrease in isotropic chemical shift with increasing x were observed. Two distinct linear ranges were observed in plots of these parameters against composition, with a transition point at x≈0.20. This composition corresponds to the point at which all Na + ions associated with charge balance of the terminal Q 1 phosphate tetrahedra are substituted for Sr 2+. In the mixed cation glasses, this suggests a non-random distribution of cations, with preferential location of Sr 2+ ions near the chain ends. Crystalline models have been used to discuss trends in the variation of chemical shift anisotropy and propose possible coordination environments for the metal cations in the glasses.

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.

What variable-pressure variable-temperature measurements are telling us about ion transport in glass.

Variable-pressure, variable-temperature (VPVT) measurements of ionic conductivity in typical inorganic (sodium aluminoborate and lithium-sodium phosphate) glasses provide information concerning two independent activation parameters: the activation energy (E(A) = - R d ln sigma T/d(1/T)) and the activation volume (V(A) = - RT d ln sigma/dP). In single-cation glasses there is no simple link between E(A) and V(A). The strong increase in E(A) seen with decreasing alkali content is not paralleled by an increase in V(A). However, in mixed cation glasses, maxima are observed in both E(A) and V(A). To account for these observations, a new model is being developed where ion transport is limited by a shortage of available volume. In this model, the measured V(A) values represent the volumes of opened up 'target sites', the larger values observed in mixed-cation glasses providing evidence for the coupled motion of unlike ions. The extent of this pairwise coupling increases with increasing temperature.

Significance of activation volumes for cation transport in glassy electrolytes

A new model is described for cation transport in glass, in which below Tg a dynamic equilibrium exists between ‘open’ and ‘closed’ sites. Cations migrate by hopping into a relatively small number of open sites, which must be just the right size for them to occupy. This model accounts plausibly for the apparent constancy of activation volumes (−RTdlnσ/dP) in single cation glasses, since the activation volumes are identified with the volumes of the open sites. The effect of increasing the external pressure is to close off some of these sites, thus reducing the number of ions which are free to move. This new approach draws emphasis away from the nature of ‘occupied’ to ‘unoccupied’ sites in glass; in this sense it is consistent with the ‘dynamic structure model’ proposed by Bunde et al.

Site Preference and Si/B Mixing in Mixed‐Alkali Borosilicate Glasses: A High‐Resolution 11B and 17O NMR Study.

In a series of mono and mixed-alkali borosilicate glasses (∼0.7M2O·B2O3·2SiO2, M = Li, Na, K, (1/2Li2O + ∼1/2Na2O), (1/2Li2O + 1/2K2O), and (∼1/2Na2O + 1/2K2O)), we have applied triple-quantum magic-angle spinning (3QMAS) and 11B and 17O NMR to obtain high-resolution information about short-range structure and the connections among various network structural units, to study the effect of mixed cations on the glass structure, and to explore the distribution and mixing of these modifier cations. For a Li-containing glass, a lower fraction of [4]B, a lower fraction of Si−O−B, and a higher fraction of nonbridging oxygen than those found in Na- and K-containing glasses suggests that there is a significant heterogeneity in terms of Si/B mixing in the former. For Li−Na and Li−K mixed cation glasses, anomalously high fractions of nonbridging oxygen were also observed along with smaller fractions of [4]B and Si−O−B, again indicating significant chemical heterogeneity. For Na−K mixed cation glasses, the populations...

Site Preference and Si/B Mixing in Mixed-Alkali Borosilicate Glasses: A High-Resolution 11B and 17O NMR Study

In a series of mono and mixed-alkali borosilicate glasses (∼0.7M2O·B2O3·2SiO2, M = Li, Na, K, (1/2Li2O + ∼1/2Na2O), (1/2Li2O + 1/2K2O), and (∼1/2Na2O + 1/2K2O)), we have applied triple-quantum magic-angle spinning (3QMAS) and 11B and 17O NMR to obtain high-resolution information about short-range structure and the connections among various network structural units, to study the effect of mixed cations on the glass structure, and to explore the distribution and mixing of these modifier cations. For a Li-containing glass, a lower fraction of [4]B, a lower fraction of Si−O−B, and a higher fraction of nonbridging oxygen than those found in Na- and K-containing glasses suggests that there is a significant heterogeneity in terms of Si/B mixing in the former. For Li−Na and Li−K mixed cation glasses, anomalously high fractions of nonbridging oxygen were also observed along with smaller fractions of [4]B and Si−O−B, again indicating significant chemical heterogeneity. For Na−K mixed cation glasses, the populations...

The concept of matrix-mediated coupling: a new interpretation of mixed-cationeffects in glass

The concept of matrix-mediated coupling assumes that coupling betweenmovements of unlike cations in glass occurs in order to dissipate mechanicalstresses created by small cations entering sites which are too large (e.g. Li+ions entering empty sodium, N̄ā,sites) and large cations entering sites which are too small (e.g. Na+ions entering empty lithium, L̄ī,sites). A central feature of the mixed-alkali effect is therefore the intervention ofstructural interactions that minimize these stresses; these processes are essentiallyisochoric in nature. This concept helps in the understanding of several importantphenomena in addition to the high pressure effects discussed previously, includingthe growth/suppression effects observed in mechanical loss spectra; the strongeffect of foreign cations on the conductivity in the dilute foreign alkali region; therole of divalent cations in glass and the ‘anomalous’ behaviour of ion-exchangedglasses.

Leachability of zinc ions from ternary phosphate glasses

With a view to using glasses as a source of zinc, which is one of the micronutrients required by plants, a systematic study on leachability of zinc ions from ternary phosphate glasses having molar composition 60ZnO · 5X · 35P2O5 (where X = Li2O, Na2O, K2O, CaO, B2O3 and SiO2) was carried out. Leaching of zinc ions has been studied as a function of pH and the results show that leaching increases with increasing pH of the solutions. The concentration of zinc ions in the leachate decreases with increasing bond strength between non-bridging oxygen ions and the cations in glasses. The leaching of zinc ions increases with increase in leaching time and decreases with increase in particle size of glasses. P2O5 was also leached along with zinc ions. The leaching characteristics of P2O5 from the glasses are also reported.

Ion dynamics in mixed alkali glasses

To study the ion dynamics in an amorphous mixed alkali system, frequency-dependent conductivities of 0.3[xLi2O·(1 − x)Na2O]·0.7B2O3 glasses have been measured by impedance spectroscopy in a wide temperature range. The conductivities show a transition from their dc values into a dispersive regime where they increase continuously with frequency, tending towards a linear frequency dependence at sufficiently low temperatures. In addition to the “classical” mixed alkali effect, i.e. the occurrence of a minimum in the dc conductivity, we also observe the following new mixed alkali effect. In contrast to conductivity spectra of single cation glasses which follow the time-temperature superposition principle, featuring a temperature-invariant shape, the shapes of the conductivity spectra of the mixed alkali glasses studied here are found to change with temperature. To explain the effect, we suggest differently activated mobilities of the two different ionic species. The spectra are discussed in the framework of the concept of mismatch and relaxation (CMR).

Mixed cation effect in chalcogenide glassesRb2S−Ag2S−GeS2

Chalcogenide glasses with composition 0.5@(12x)Rb2S-xAg2S#-0.5GeS2 ,0 <x<1, were prepared and their ion transport properties and glass transition temperatures were found to show clear manifestations of the mixed cation effect. These Rb-Ag thiogermanate glasses were shown to be homogeneous in the mesoscopic domain by small-angle x-ray scattering. Extended x-ray-absorption fine structure indicated that each mobile cation forms its own specific environment that is not influenced by the presence of the dissimilar cation. On the other hand, Raman and infrared spectra showed nonlinear structural changes of the glassy matrix upon cation mixing. The maximum deviation from structural additivity was observed for compositions giving the extreme values for conductivity and Tg , and was found to result from cation-triggered rearrangements of the local germanate polyhedra toward a more homogeneous glass structure. These results support the recently proposed CIRON model that focuses on cation-induced relaxations of the network in mixed cation glasses.

Short- and medium-range structural order around cations in glasses: a multidisciplinary approach

The structural environment of cations at a short- and medium-range scale may be investigated either by spectroscopic methods or by radiation diffraction giving either a description of the geometry and symmetry of the cationic site, including the nature of the chemical bond, or a chemically resolved radial distribution function. Cations exhibit several original structural properties in oxide glasses. Short-range order is characterized by unusual coordination numbers, such as five-coordinated sites or tetrahedral sites which are in a network forming position, with the relative proportion of these sites depending on glass composition. Oxide glasses can also exhibit elements with unusual oxidation states, such as pentavalent uranium. The determination of the sites occupied by the elements in their different oxidation states allows to rationalize the chemical dependence of redox equilibria, which is the way to predict Fe behavior in magmatic silicate systems. Several experimental data lend support that cations are located in domains extending up to more than 8 Å radius, in which cationic polyhedra may be linked together either by edges or by corners. In low alkali borate glasses, transition elements such as Co, Ni, Zn exist in peculiar highly ordered domains corresponding to the presence of rigid borate units. Strong differences are observed between modifying and charge compensating cations, either concerning site geometry or medium-range organization. The use of numerical models for experimental data inversion allows to rationalize the structural behavior of the various glass components.

Cation ordering at fluoride sites in silicate glasses: a high-resolution 19F NMR study

High-resolution 19F NMR, with sample spinning rates to 25 kHz, can provide quantitative, new information about the structure of fluoride sites in F-doped silicate glasses. Here we present results for sodium-, potassium-, calcium-, lanthanum- and mixed cation silicate glasses, containing 1–2 wt% F. In the end member compositions, most fluoride is in sites that are structurally similar to the corresponding simple metal fluoride, although a few percent of Si–F bonds can be detected in the sodium silicates. In the mixed cation glasses, partially resolved peaks are observed for series of distinct mixed cation coordination environments for fluoride. Fluoride ions seem to be preferentially bonded to higher field-strength modifier cations, but considerable disorder is present for all cation pairs (Na–K, Na–Ca, Na–La) studied.