Alkaline Earth Silicate Glasses Research Articles

Evidence for Mixed Mg Coordination Environments in Silicate Glasses: Results from 25Mg NMR Spectroscopy at 35.2 T.

The Mg-O coordination environment of silicate glasses of composition CaMgSi2O6, Na2MgSi3O8, and K2MgSi5O12 is probed using ultrahigh-field (35.2 T) 25Mg magic angle spinning nuclear magnetic resonance (MAS NMR) and triple-quantum MAS NMR spectroscopy. These spectra clearly reveal the coexistence of 4-fold- (MgIV) and 6-fold- (MgVI) coordinated Mg in all glasses. The MgIV/MgVI ratio implies an average Mg-O coordination number of ∼5 for CaMgSi2O6 glass, bringing NMR results for the first time in good agreement with those reported in previous studies based on diffraction and X-ray absorption spectroscopy, thus resolving a decade-long controversy regarding Mg coordination in alkaline-earth silicate glasses. The Mg-O coordination number decreases to ∼4.5 in the alkali-Mg silicate glasses, indicating that Mg competes effectively with the low field strength alkali cations for the nonbridging oxygen in the structure to attain tetrahedral coordination. This work illustrates the promise of ultrahigh-field NMR spectroscopy in structural studies involving nuclides with low gyromagnetic ratio.

Na environment, conductivity, and water leaching of sodium alkaline earth silicate glass

Na environment, conductivity, and water leaching of sodium alkaline earth silicate glass

Silicon-29 echo train coherence lifetimes and geminal [formula omitted]-couplings in network modified silicate glasses

The natural abundance 29Si echo-train coherence lifetimes in network-modified silicate glasses were examined under static and magic-angle spinning (MAS) conditions. The nuclear magnetic properties of modifier cations were found to play a major role in determining 29Si coherence lifetimes, leading to differences as large as three orders of magnitude. In compositions with abundant NMR active nuclei, such as alkali silicates, the 29Si coherence lifetimes are dominated by coherent dephasing due to residual heteronuclear dipolar couplings, whereas in compositions dilute in NMR active nuclei, such as alkaline earth silicates, the 29Si coherence lifetimes are dominated by incoherent dephasing due to paramagnetic impurities. Expressing the inverse of the coherence lifetime as a residual full width at half maximum (FWHM), we found that increasing rates of both MAS and a π-pulse train are effective in removing the residual 29Si heteronuclear broadenings, with a near-linear relationship between FWHM and MAS rotor period and π-pulse spacing. Based on these results, we conclude that accurate 29Si J coupling measurements will be the most challenging in lithium silicate glasses due to strong homonuclear dipolar couplings among 7Li nuclei, requiring MAS speeds up to 100 kHz, and be the least challenging in the alkaline earth silicate glasses. At a modest MAS speed of 14kHz, distributions of geminal J couplings across Si-O-Si linkages were measured in alkali and alkaline earth silicate glasses giving mean values of 4.2Hz and 5.1Hz in 0.4 CaO·0.6 SiO2 and 0.33 Ba2O·0.67 SiO2 glasses, respectively, and 5.2Hz and 5.3Hz in 0.33 Na2O·0.67 SiO2 and 0.33 K2O·0.67 SiO2 glasses, respectively. We also observe greater variance in the J distributions of alkaline earth silicate glasses consistent with greater structural disorder due to increased modifier cation potential, i.e., the charge-to-radius ratio, Z/r of the cation.

Lead silicate glass structure: New insights from diffraction and modeling of probable lone pair locations

AbstractStructures of binary PbO‐SiO2 glasses have been studied in detail over the compositional range 35 to 80 mol% PbO using high‐resolution neutron diffraction, high‐energy X‐ray diffraction, static 207Pb NMR, and structural modeling. The changes in the local environment of Pb(II) are subtle; it has a low coordination to oxygen (∼3 to 4) plus a stereochemically active electron lone pair and, thus, behaves as a glass network forming (or intermediate) cation over the entire composition range. This conclusion contradicts previous reports that Pb(II) is a network modifier at low concentrations, and is supported by an analysis of lead and alkaline earth silicate glass molar volumes. The Pb‐O peak bond length shortens by 0.04 Å with increasing PbO content, indicating stronger, more covalent bonding, and consistent with an increase in the number of short (≤ 2.70 Å) Pb‐O bonds, from 3.3 to 3.6. This is accompanied by increased axial symmetry of the Pb(II) sites, and is interpreted as a gradual transition toward square pyramidal [PbO4] sites such as those found in crystalline PbO polymorphs. An attendant decrease in the periodicity associated with the first sharp diffraction peak (FSDP) toward that of β‐PbO, accompanied by increases in the correlation lengths associated with the plumbite network (FSDP) and silicate anions (neutron prepeak), provides evidence of increased intermediate‐range order and has implications for the glass forming limit imposed by crystallization. Pb(II) electron lone pairs occupy the natural voids within the silicate network at low PbO contents, while at high PbO contents they aggregate to create voids that form part of the plumbite network, analogous to the open channels in Pb11Si3O17 and the layered structures of α‐ and β‐PbO. Si‐O and Pb‐O bond lengths have been correlated with 29Si and 207Pb NMR chemical shifts, respectively. This is the first time that such correlations have been demonstrated for glasses and attests to the accuracy with which pulsed neutron total scattering can measure average bond lengths.

New interaction potentials for alkaline earth silicate and borate glasses

New interaction potentials were developed for molecular dynamics simulations to study the role of Mg and Ca in modifying the structure and properties of alkaline earth silicates and borates. Competition between the depolymerization of the silica network and the formation of new bonds between oxygen atoms and modifiers leads to the enhancement of the elastic moduli with increasing modifier content in alkaline earth silicate glasses. Compared with calcium silicate, the higher elastic moduli of magnesium silicate result from a higher connectivity of the overall glass network due to the incorporation of fourfold coordinated magnesium and a more rigid connection between oxygen atoms and modifiers. In contrast to the silicates, the effect of modifier on the elastic moduli of alkaline earth borates is dominated by the formation of fourfold coordinated boron (N4). Calcium borate with higher N4 shows a more rigid network structure and higher elastic moduli.

Pentacoordinated and hexacoordinated silicon cations in a potassium silicate glass: Effects of pressure and temperature

Silicon cations with five or six oxygen first neighbors (SiO5 and SiO6, pentacoordinated and hexacoordinated) are known primarily from alkali and alkaline earth silicate glasses melted at pressures up to about 12 GPa, where their formation is thought to be an important part of the densification of the liquid. These species, especially SiO5 groups, have also been proposed as high-energy intermediate structures in the bond-swapping that controls network cation and anion diffusion, and hence viscous transport, in high temperature, silica-rich melts, as well as in low-temperature liquid-state silicate reactions, and have thus been the subject of numerous recent theoretical studies. In spite of this interest, the amount of experimental data on controls of pressure, temperature, and composition on concentrations of penta- and hexacoordinated silicon is very limited. Here we report new results from 29Si MAS NMR on high pressure glasses with the model composition K2Si4O9, which show that because of transient pressure drop during quench, previous studies probably significantly underestimated SiO5 and SiO6 concentrations. In addition, we refine the earlier results on SiO5 even in 1 bar glasses, and demonstrate a strong increase in its concentration at higher fictive temperature. A positive enthalpy for the reaction forming this species is consistent with its role as an energetically important transition complex in melt dynamics.

Refractory ceramic fibers: Fiber characteristics, potential health effects and clinical observations

Refractory ceramic fibers (RCFs) are amorphous fibers that belong to a class of materials termed synthetic vitreous fibers (SVFs), also called man-made mineral fibers (MMMFs), which includes alkaline earth silicate wool, glass wool, rock (stone) wool, slag wool, and special-purpose glass fibers. RCFs are more durable and biopersistent than several other SVFs, although very much less biopersistent than either amosite or crocidolite asbestos. Chronic inhalation studies indicated that rats and hamsters exposed to RCF fibers developed fibrosis and tumors. Epidemiological studies at the University of Cincinnati funded by the Industry indicated that exposed workers; (1) exhibited symptoms (e.g., dyspnea) similar to those reported in other dust-exposed populations, (2) developed statistically, but not clinically, significant deficits in certain measures of pulmonary function in a cross sectional study, but no excessive decline in a longitudinal study, and (3) a dose related increase in pleural plaques, but no interstitial fibrosis. The 2003 mortality study indicated no incremental lung cancer and no cases of mesothelioma. RCF producers developed a comprehensive industry wide product stewardship program (PSP) beginning in the late 1980s. In conjunction with the PSP, there has been a progressive decrease in the TWA concentration of fibers by manufacturers and end-users. The research program has successfully produced more soluble fibers and undertaken efforts to develop larger diameter fibers. The results of the ongoing epidemiology studies confirm that occupational exposure to RCF is associated with the development of pleural plaques and minor decrements in lung function, but no interstitial fibrosis or incremental lung cancer.

Time-resolved microscopy of fs-laser-induced heat flows in glasses

Time-resolved phase-contrast microscopy is employed to visualize spatio-temporal thermal transients induced by tight focusing of a single Ti:sapphire fs-laser pulse into a solid dielectric sample. This method relies on the coupling of the refractive index change and the sample temperature through the thermo-optic coefficient dn/dT. The thermal transients are studied on a timescale ranging from 10 ns up to 0.1 ms after laser excitation. Beyond providing direct insights into the laser–matter interaction, analyzing the results obtained also enables quantifying the local thermal diffusivity of the sample on a micrometer scale. Studies conducted in different solid dielectrics, namely amorphous fused silica (a-SiO2), a commercial borosilicate glass (BO33, Schott), and a custom alkaline earth silicate glass (NaSi66), illustrate the applicability of this approach to the investigation of various glassy materials.

Size-dependent photoluminescence of PbS QDs embedded in silicate glasses

PbS quantum dots (QDs) with a mean radii of 1.27 nm to 7.32 nm were precipitated in alkaline-earth silicate glasses enriched in sulfur. A transmission electron microscope image and absorption spectra evidenced the narrow size dispersion of PbS QDs and discrete energy levels of PbS QDs. Upon above-band-gap excitation, symmetry of the photoluminescence from PbS QDs was strongly dependent on their size. Photoluminescence from small PbS QDs showed a long-wavelength tail and vice versa for large PbS QDs. The size dependence of full width at half maximum, PL energy and Stokes shift on the size of PbS QDs suggested that the electron/hole trap states of PbS QDs and the defects states on the interface between PbS QDs and glass matrix have great effect on the photoluminescence properties of PbS QDs.

Electron densities over Si and O atoms of tetrahedra and their impact on Raman stretching frequencies and Si-NBO force constants

Systematics of SiO Raman stretching frequencies in the range 800–1200cm−1, and Si 2p and O 1s XPS binding energies (BE) are examined for a large number of alkali and alkaline earth silicate glasses, melts, and crystals. Several relationships in the Raman and X-ray Photoelectron spectral (XPS) data lead to new insights concerning charge delocalization over Si tetrahedra and to recognition of controls on both Raman stretching frequencies, and Si 2p and O 1s XPS chemical shifts. The Raman stretching frequencies of the five Q species and the Si 2p and O 1s XPS chemical shifts, vary linearly with the alkali and alkaline-earth content of glasses over the range 0–50mol% counter oxide (M2O or MO). The relationships demonstrate that the frequencies and chemical shifts are controlled by the same property, the valence band electron densities on the atoms of silicate tetrahedra. Specifically, where M-O ion interactions are strongly ionic, the number of NBO's attached to the central Si atom most affects the electron density of Si and O in tetrahedra, and is the major control on Raman symmetric stretching frequencies. Also, the Raman stretching frequency of individual Q species are effectively independent of the mass of the alkali counter cation (MNa, K, Rb, Cs).Addition of alkali oxides to vitreous SiO2 produces Si 2p and O 1s XPS chemical shifts. The magnitude of the chemical shifts increase with counter oxide abundance through transfer of valence ‘s’ electrons from alkali atoms to NBO of tetrahedra. The negative charge acquired by the NBO is redistributed (delocalized) over all atoms of the tetrahedron via SiO bonds thereby increasing valence band electron densities over the constituent Si and O atoms. The greater the number of NBOs associated with a tetrahedron the greater the enhancement of electron density on all atoms of the tetrahedron, thus explaining the Si 2p and O 1s XPS chemical shifts. With respect to Raman spectra, the increased valence electron density causes the central Si atom to become appreciably less positive, which diminishes the strength of Coulombic interactions between Si and O atoms, and weakens SiO force constants. In turn, the weakened force constants result in decreases to SiO stretching frequencies. The weakening of force constants and decrease in stretching frequencies occur in a stepwise manner and relate directly to the number of NBO attached to the tetrahedra. The SiO force constant for the Q4 species is much larger than the force constant for the Q0 species, causing their stretching frequencies to decrease (stepwise) from ~1200cm−1 to ~850cm−1. The mathematical relationships among alkali oxide compositions, Raman stretching frequencies and XPS chemical shifts are quantified.

Modifier cation effects on 29Si nuclear shielding anisotropies in silicate glasses

We have examined variations in the 29Si nuclear shielding tensor parameters of SiO4 tetrahedra in a series of seven alkali and alkaline earth silicate glass compositions, Cs2O·4.81 SiO2, Rb2O·3.96 SiO2, Rb2O·2.25 SiO2, K2O·4.48 SiO2, Na2O·4.74 SiO2, BaO·2.64 SiO2, and SrO·2.36 SiO2, using natural abundance 29Si two-dimensional magic-angle flipping (MAF) experiments. Our analyses of these 2D spectra reveal a linear dependence of the 29Si nuclear shielding anisotropy of Q(3) sites on the Si–non-bridging oxygen bond length, which in turn depends on the cation potential and coordination of modifier cations to the non-bridging oxygen. We also demonstrate how a combination of Cu2+ as a paramagnetic dopant combined with echo train acquisition can reduce the total experiment time of 29Si 2D NMR measurements by two orders of magnitude, enabling higher throughput 2D NMR studies of glass structure.

Direct and indirect evidence for free oxygen (O2−) in MO-silicate glasses and melts (M = Mg, Ca, Pb)

Oxygen 1s XPS spectra of a Pb-silicate glass containing 76.6 mol% PbO provide the first accurate, direct measurement of free oxide ion (O2−) in these glasses. O2− constitutes 35 (±3) mol% of total oxygen, with NBO and BO constituting, respectively, 52 (±3) and 13 (±3) mol%. All 29Si NMR and O 1s XPS results for Pb-silicate glasses indicate mol% levels of O2− containing more than ~30 mol% PbO. The O2− abundances are consistent with equilibrium thermodynamic considerations where K ~ 12 for the mass action equation involving NBO, BO, and O2−. Raman and 17O NMR spectra of two CaMg-silicate glasses indicate ~10 (±4) mol% O2− in CaMgSiO4 glass and ~18 (±4) mol% O2− in a Ca0.36Mg0.36Si0.28O1.28 glass. Oxygen species abundances are calculated using experimental results from 13 separate 29Si NMR, 17O NMR, and Raman measurements of Mg-, Ca-, and CaMg-silicate glasses. All reveal mol% levels of O2− with ~1 to 2.6 mol% in metasilicate glass and ~5 to 10 mol% in orthosilicate glass. Recent Raman experimental results also indicate O2− in CaMg-silicate glasses at levels ranging from about 1 to 10 mol%. In all there are 23 separate 29Si NMR, 17O NMR, and Raman measurements indicating mol% levels of O2− in alkaline earth silicate glasses. Eight recent MD simulations of Mg, Ca, and CaMg-silicate glasses include 21 separate simulations over a wide compositional range. All indicate mol% levels of O2− in the glasses demonstrating that the MD simulations and experimental results on these systems are in accord. There are two fundamentally important implications of these studies. First, free oxygen (O2−) is an essential constituent of Pb, Mg, Ca, and CaMg binary silicate melts and glasses. It is not an “accidental” product associated with glass or melt defects. It is instead, a thermodynamically important constituent of these binary melts (and glasses). Second, where melts are equilibrated, the mass action equation relating BO, NBO, and O2− must hold across the entire Ca, Mg, CaMg, and Pb binary systems, thereby requiring the activities and mole fractions of all three species to be defined and finite in the melts. Free oxygen, however, may be too low to be detected in highly siliceous glasses using conventional spectroscopic techniques.

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.

A Novel Alkali and Alkaline Earth Silicate Glass for Marine Applications

Aims: To test the corrosion properties of a novel alkali and alkaline earth silicate glass. Place and Duration of Study: Department of Metallurgical and Materials Engineering, between 1999-2001. Methodology: We included a glass which contains 75% mole lithium disilicate (Li2O.2SiO2-LS2) and 25% mole barium disilicate (BaO.2SiO2-BS2) phases. 1 mole% of P2O5 was used as nucleating agent. The scanning electron microscope (JEOL JSM-330) was used to observe the glass microstructures. X-ray diffraction (XRD) was carried out on the samples using a diffractometer (Philips PW1820) employing Cu Kα. In the chemical durability tests, the corrosive environment was boiling solutions of 0.625N HCl and 5% wt. NaOH along with boiling water. Another reference composition was also used, which included equal moles of LS2 and BS2, for the corrosion tests. Results: When A and B glass were compared, the basic and boiling water corrosion value of B glass in terms of weight loss, (3.12; 0.38) is higher than that of A glass (4.2; 0.70). Slight differences were observed in the corrosion properties in basic and boiling water environment when A and B glass were compared. However, the weight loss of A glass in acidic environment, was 3.55% wt whereas that of B glass was determined to be 0.4% wt. Conclusion: The chemical durability of A and B glasses against acidic environment was comparable to that of soda-lime glass. It was concluded that this novel composition is a candidate for the substitution of soda-lime glass in marine applications. Original Research Article Ertug; BJAST, 7(3): 333-341, 2015; Article no.BJAST.2015.150 334

Nonlinear absorption and refraction of binary and ternary alkaline and alkaline earth silicate glasses

Nonlinear optical properties such as the nonlinear refractive index and nonlinear absorption are characterized by z-scan measurements for a series of silicate glasses upon irradiation with laser pulses of 130 fs duration and 800 nm center wavelength. The stoichiometry of the silicate glasses is varied systematically to reveal the influence of the glass composition on the nonlinear optical properties. Additionally, the thermal properties such as glass–transformation temperature and thermal expansion coefficient are obtained from dilatometric measurements. It is found that the nonlinear refractive index is mainly related to the silica matrix. The nonlinear absorption is increased with the addition of network–forming ions.

Glass Sealants and Their Interaction Study with Interconnect Materials

In order to check the feasibility of various glasses as sealing materials, it is necessary to understand the crystallization kinetics, thermal and structural properties of glasses and their interactions with other components of SOFCs. In the present study, different alkaline earth silicate glasses have been studied for chemical interaction with different interconnects (Crofer 22APU, ODS and steel). These heat treated diffusion couple processes were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) in order to understand the mechanism of interface formations and their growth. No chromium rich zone could be seen along the interface after 750 h at 850°C heat treated diffusion couple indicating absence of any adverse reaction at the interface in these samples.

Boson peak of alkali and alkaline earth silicate glasses: Influence of the nature and size of the network-modifying cation

The influence of the size of the alkaline earth cation on the boson peak of binary metasilicate glasses, MSiO(3) (M = Mg, Ca, Sr, Ba), has been investigated from vibrational densities of states determined by inversion of low-temperature heat capacities. As given both by C(p)/T(3) and g(ω)/ω(2), the intensity of the boson peak undergoes a 7-fold increase from Mg to Ba, whereas its temperature and frequency correlatively decrease from 18 to 10 K and from 100 to 20 cm(-1), respectively. The boson peak results from a combination of librations of SiO(4) tetrahedra and localized vibrations of network-modifying cations with non-bridging oxygens whose contribution increases markedly with the ionic radius of the alkaline earth. As a function of ionic radii, the intensity for Sr and Ba varies in the same way as previously found for alkali metasilicate glasses. The localized vibrations involving alkali and heavy alkaline earth cations appear to be insensitive to the overall glass structure. Although the new data are coherent with an almost linear relationship between the temperature of the boson peak and transverse sound velocity, pure SiO(2) and SiO(2)-rich glasses make marked exceptions to this trend because of the weak transverse character of SiO(4) librations. Finally, the universality of the calorimetric boson peak is again borne out because all data for silicate glasses collapse on the same master curve when plotted in a reduced form (C(P)∕/T(3))/(C(P)/T(3))(b) vs. T/T(b).

Structural investigations of magnesium silicate glasses by 29Si 2D Magic-Angle Flipping NMR

The two-dimensional Magic Angle Flipping Nuclear Magnetic Resonance (2D MAF NMR) experiment on 29Si nuclei is used to determine the distribution of Q(n) sites in two 29Si-enriched magnesium silicate glasses with compositions 2MgO·SiO2 and MgO·SiO2. A significant degree of polymerization is observed in the 2MgO·SiO2 glass, supporting previous studies using Raman and 29Si NMR spectroscopy. Relative abundances of 0.629±0.001 for Q(0) and 0.371±0.001 for Q(1) were obtained from spectral fits of the 2D MAF spectrum of the 2MgO·SiO2 glass. Mole fractions for the free oxygen anion and each Q(n)-species were calculated and used in a thermodynamic model of Q(n) disproportionation to calculate an equilibrium constant of k0=0.04±0.02 in 2MgO·SiO2. In the MgO·SiO2 glass relative abundance of 0.014±0.001 for Q(0), 0.191±0.003 for Q(1), 0.530±0.004 for Q(2), 0.252±0.003 for Q(3), and 0.014±0.001 for Q(4) were measured. The mole fractions for the free oxygen anion and each Q(n)-species in MgO·SiO2 were used to calculate corresponding disproportionation equilibrium constants of k1=0.19±0.02, k2=0.174±0.009, and k3=0.11±0.01. A comparison of k3 values from previous MAF studies of various alkali and alkaline earth silicate glasses indicate an exponential increase in k3 with the increasing modifying cation potential. Using the van't Hoff relation, we show that differences in both thermal history and modifier cation potential contribute to this spread in k3 values. Nuclear shielding tensor anisotropy, ζ, and asymmetry, η, values of ζ=0.0ppm and η=0.0 for Q(0) and ζ=33.0±0.1ppm, and η=0.4±0.1 for Q(1) in 2MgO·SiO2 glass were determined from its 2D MAF spectrum. These values were used in obtaining the remaining values of ζ=−36.0±0.5ppm and η=0.99±0.01 for Q(2), and ζ=−27.5±0.5ppm and η=0.45±0.11 for Q(3), ζ=0.0ppm and η=0.0 for Q(4) in the MgO·SiO2 glass from its 2D MAF spectrum. The magnitude of ζ values observed are lower than those reported in previous MAF studies of alkali and alkaline earth silicate glasses containing different modifier cations, consistent with previously reported trends in ζ versus modifying cation potential.

IR and Raman spectroscopic studies of sol–gel derived alkaline-earth silicate glasses

IR and Raman spectroscopies have been utilized to study the structure and vibrational modes of sol–gel-derived binary silicate glasses. The present study is motivated by the immense geological significance and focuses on the MO–SiO 2 (M = Ca, Mg) binary systems in an effort to unveil the role of the CaO and MgO modifiers when incorporated to the 3D silica structure. Glasses in the composition range x =0, 0·1, 0·2, 0·3 and 0·4 prepared by the sol–gel method were compared with the corresponding glasses formed by appropriate mixing of SiO 2 and MO powders through melting and fast cooling. The vibrational spectra of the sol–gel-derived glasses have revealed considerable changes in relative intensities as a function of the MO mole fraction. These changes signify structural modifications on the silica network. The population of the Q 3 species was found to increase for both modified silicate systems. The rate of increase is more pronounced in the CaO–SiO 2 glasses. The extent of network depolymerization in the porous glass is higher at the same content of alkaline earth oxide compared to the bulk glass. The results are indicative of a more ‘defective’ nature of the sol–gel glasses compared to the corresponding melt-quenched ones.

Spectroscopic and structural properties of Cr 3+ in silicate glasses: Cr 3+ does not probe the average glass structure

Cr 3+-containing alkali, alkaline earth and mixed alkali–alkaline earth silicate glasses have been investigated using Cr K-edge extended x-ray absorption fine structure (EXAFS) and optical absorption spectroscopy. The local environment of Cr 3+ appears similar in all glasses regarding EXAFS analysis, in particular for Cr–O distance (1.99 Å). By contrast, optical absorption spectra show variations of crystal field values and disorder effects with the nature of the modifier cation, revealing that some differences exist in the local surrounding of Cr 3+ in glasses. In addition, in mixed alkali–alkaline earth glasses optical absorption parameters remain close to the values found in binary silicate glasses with the same alkali, which reveals a preference for alkalis in the surrounding of Cr 3+. As a whole, these data show that Cr 3+ is not probing the average glass structure and demonstrate its heterogeneous distribution at a local scale.