An investigation of the local iron environment in iron phosphate glasses having different Fe(II) concentrations

The local environment around iron ions in iron phosphate glasses of starting batch composition 40Fe₂O₃-60P₂O₅ (mol%) melted at varying temperatures or under different melting atmospheres has been investigated using Fe-57 Mössbauer and X-ray absorption fine structure (XAFS) spectroscopies. Mössbauer spectra indicate that all of the glasses contain both Fe(II) and Fe(III) ions. The quadrupole splitting distribution fits of Mössbauer spectra show that Fe(II) ions occupy a single site whereas Fe(III) ions occupy two distinct sites in these glasses. When melted at higher temperatures or in reducing atmospheres, the Fe(II) fraction in the glass increases at the expense of Fe(III) ions at only one of the two sites they occupy. The pre-edge feature in the XAFS data suggests that the overall disorder in the near-neighbor environment of iron ions decreases with increasing Fe(II) fraction. The XAFS results also show that the average iron-oxygen coordination is in the 4-5 range indicating that iron ions have mixed tetrahedral-octahedral coordination.     

Structural features of hafnium iron phosphate glasses

Structural features and properties of a series of hafnium iron phosphate glasses have been investigated by Mössbauer spectroscopy and X-ray diffraction. Mössbauer spectra indicate that all of the glasses contain both Fe(II) and Fe(III) ions. The isomer shift values obtained from the Mössbauer fits show that both Fe(II) and Fe(III) ions are in octahedral or distorted octahedral coordination. The crystalline HfP₂O₇ phase was detected in all the samples by powder X-ray diffraction but this did not degrade the chemical durability of the glasses as the dissolution rates of the glasses are comparable to that of base iron phosphate glass.     

Molecular dynamics study of amorphous titanium silicate

The molecular dynamics computer simulation was used to study the local structure of titanium in silica glass. The empirical potential function used in previous simulations of other glasses performed in our lab was extended to study titanium in silica. The results indicate that at low concentrations (<10 mol% TiO₂), titanium is mainly tetrahedral in bulk silica. At higher TiO₂ concentrations, titanium coordination increases. Slower quench rates enhance these trends observed in the changes in titanium coordination. A cluster of overcoordinated species similar to rutile was found in TiO₂: 9SiO₂ and is discussed within the constraints of the MD experiment. Overall, the MD result are consistent with EXAFS data of bond lengths and coordination numbers of TiO₂ in titanium silicate glasses.     

Molecular dynamics simulation of La2O3-Na2O-SiO2 glasses. II. The clustering of La cations

Clustering of high-field strength rare-earth ions in silicate glasses has been experimentally observed for a wide range of concentrations. Clustering has also been observed by molecular dynamics (MD) computer simulations over a range 1-10 mol% in soda silicate glasses. Although there have been numerous experimental studies, atomic-level details of the mechanisms that lead to clustering remain unclear. Coupling experiment with MD simulations is essential to uncovering the factors that lead to clustering. In this work, MD computer simulations are used to verify that clustering found in previous MD simulations is not an artifact of the simulation method. This work also provides clues as to the mechanism of atomic-level clustering.     

An X-ray absorption spectroscopy study of the local environment of iron in degradable iron–phosphate glasses

Degradable iron–phosphate glasses with the composition of (CaO)_0.30–(Na₂O)_0.20?x–(Fe₂O₃)_x–(P₂O₅)_0.50, x = 0.01–0.05, were studied by Fe K-edge X-ray absorption spectroscopy (both near-edge, XANES, and extended, EXAFS). The addition of up to 5 mol% iron oxide is known to enhance the durability of the phosphate glass while maintaining biocompatibility. The results from the two techniques used here both show that iron is in the Fe(III) oxidation state and has octahedral coordination. This suggests that Fe is cross-linking the phosphate chains and therefore strengthening the network structure, resulting improved chemical durability of the glasses.     

Dielectric properties and structural features of barium-iron phosphate glasses

The dielectric constant of barium-iron phosphate glasses with the general composition (40−x)BaO · xFe₂O₃ · (60−x)P₂O₅ has been investigated at two fixed frequencies (100 kHz and 9.0 GHz). The dielectric constant measured using microwave technique, and the ratio O/P of these glasses increase with increasing Fe₂O₃ content. The structure and valence states of the iron ions in these glasses were investigated using Mössbauer spectroscopy, infrared spectroscopy and differential thermal analysis. Both Fe(II) and Fe(III) ions present in these glasses in octahedral coordination act as permanent dipoles, and the increase of the iron concentration increase these permanent dipoles, contributing to the dielectric constant.     

Structure and lithium ion diffusion in lithium silicate glasses and at their interfaces with lithium lanthanum titanate crystals

Solid state lithium ion electrolytes are important to the development of next generation safer and high power density lithium ion batteries. Lithium containing glasses such as lithium silicate glasses have been widely studied due to their high ionic conductivity. Recently, lithium silicate glasses were introduced in polycrystalline lithium lanthanum titanate (LLT) ceramics as intergranular thin films between the crystalline grains to achieve higher lithium ion conductivities in these solid state electrolytes. In this work, we present investigations of the structure and diffusion behavior of lithium silicate glasses and their interfaces with LLT crystals using molecular dynamics simulations. The short and medium range structures of the lithium silicate glasses were characterized and the ceramic/glass interface models were obtained using MD simulations. Lithium ion diffusion behaviors in the glass and across the glass/ceramic interfaces, as well as the effect of atomic structure on diffusion behaviors, were investigated. It was found that there existed a minor segregation of lithium ions at the glass/crystal interface. The interface lithium ion diffusion energy barrier was found to be dominated by the glass phase.     

Sensitivity of structural results to initial configurations and quench algorithms of lead silicate glass

The sensitivity of resulting structures to starting configurations and quench algorithms were characterized using molecular dynamics (MD) simulations. The classical potential model introduced by Damodaran, Rao, and Rao (DRR) Phys. Chem. Glasses 31 (1990) 212 for lead silicate glass was used. Glasses were prepared using five distinct initial configurations and four glass forming algorithms. In previous MD work of bulk lead silicate glasses the ability of this potential model to provide good structural results were established by comparing to experimental results. Here the sensitivity of the results to the simulation methodology and the persistence of clustering with attention to details of molecular structure are determined.     

Voronoi polyhedra analysis of MD simulated silicate glasses

The relationship between the volume and the surface of Voronoi polyhedra (VP) appeared to be an effective tool for the characterization of the structure of MD simulated silicate glasses. This feature was demonstrated on the series of binary potassium-silicate glasses with the alkali content ranging from 5 to 20 mol%. Moreover, the temperature dependence of the slope of the above dependence, when expressed for various types of central atoms (Si, O and K in our case) was studied. Each type of central atom is characterised by its own iron line linearly relating the volume of VP to its properly powered surface, so that new geometrical constrains to the VP was found.     

Electronic relaxation in zinc iron phosphate glasses

The electrical and dielectric properties of 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses, melted at different temperatures were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe(II)/[Fe(II) + Fe(III)] ratio. With increasing Fe(II) ion content from 17% to 37% in these glasses, the dc conductivity increases. Procedure of scaling conductivity data measured at various temperatures into a single master curve is given. The conductivity of the present glasses is made of conduction and conduction-related polarization of the polaron hopping between Fe(II) and Fe(III), both governed by the same relaxation time, τ. The high frequency dispersion in electrical conductivity arises from the distribution in τ caused by the disordered glass structure. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions.     

Free energy profiles of Al and La cation distribution in silica and soda silicate glasses

The factors that control the distribution of Al³⁺ and La³⁺ cations in silica and soda silicate glasses is examined by using molecular dynamics (MD) simulations. In particular, the response of the glass network to the presence of metal oxide is probed using a liquid state theory that treats the glass network as a solvent and the metal cation as a solute. MD simulations are used to obtain the mean solvent-solute and solute-solute force. The trajectory used to determine the free energy is analyzed to determine the stable configurations of a cation pair. Details of determining the potential of mean force for an Al cation pair in silica and silicate glass is presented. A comparison of these results with those previously calculated for a La cation pair in the same glass systems is given. The results reveal that there are differences in how the network accommodates the two different size cations. It is found that for the potential used here, based on the Vessal potential, the network wraps itself around the larger La cation forming a solvent shell, whereas, the smaller Al cation is incorporated into the network backbone. In silica and soda silicate glasses, La ion pairs cluster to form La-O-La linkages. In contrast, the glasses favor a separated state of the Al ion pair.     

Metastable equilibrium density of hydroxyl-free synthetic vitreous silica

It is shown that the volume behaviour of hydroxyl-free vitreous silica (type IV silica glass) is similar to that of the so-called natural vitreous silica (fused quartz, type I and II silica glasses) with low hydroxyl content. Thus, in view of that, a distinction may be made between two groups, types I, II and IV on the one hand and type III, the hydroxyl-rich silica glasses, on the other hand, although a distinction may also be made between the group of “natural” (type I and II) and “synthetic” silica glasses (type III and IV) in view of production method and starting material (quartz or SiCl₄). It is further shown that the type III silica glasses give an example for a change from the anomalous volume behaviour of “pure” silica glasses (thermal expansion, high-temperature minimum of volume) to the common behaviour of silicate glasses.     

O K-edge XANES studies of alkali germanophosphate glasses

Alkali germanophosphate glasses exhibit a ‘germanate anomaly’, which is a maximum or minimum in their density profiles with the addition of alkali oxide. Several structural mechanisms have previously been recognized to affect the density behavior of these glasses: (1) depolymerization of the PO₄ tetrahedral network; (2) conversion of 4- to 3-membered GeO₄ rings and (3) depolymerization of the GeO₄ network through the formation of non-bridging oxygens. A coordination change from 4- to 5-fold Ge has been detected in alkali germanate glasses, which also exhibit the germanate anomaly. In this study, oxygen K-edge XANES of alkali germanophosphate glasses is used to investigate the coordination change of Ge with increasing alkali content and at various Ge to P ratios. The dominant peak at ～538 eV in the spectra undergoes a shift to lower energy of ～0.4 eV, indicating the formation of 5-fold Ge similar to previous studies on alkali germanate glasses. However, the formation of 5-fold Ge does not correlate with the density anomaly compositions of these glasses suggesting that the appearance of higher-coordinated Ge species is not controlling the anomaly behavior.     

X-ray absorption studies of the local strontium environments in borosilicate waste glasses

X-ray absorption spectra were collected and analyzed to characterize the strontium environments in borosilicate glass formulations developed for the immobilization of nuclear wastes. Sr can become a major constituent in some radioactive wastes because of the use of Sr-compounds in waste pretreatment processes. Data are presented for 13 borosilicate glasses that are used to simulate the anticipated waste glass compositions, where SrO concentrations range from approximately 2 to 30 wt%. The XANES data are statistically identical for all glasses investigated, and indicate divalent strontium. The EXAFS data for both crystalline standards and glasses appear to provide information only about the inner four to six coordinating oxygens that surround Sr. The data and analyses for all glasses show that the inner coordination environments around Sr have average Sr-O distances near 2.53 Å, which are statistically invariant with respect to glass composition and to various synthesis conditions. Sr-O distances determined for the glasses are within the shorter end of the range of individual Sr-O distances in the SrCO₃ and haradaite (SrVSi₂O₇) crystalline standards.     

Determination of sulfur environments in borosilicate waste glasses using X-ray absorption near-edge spectroscopy

Sulfur can be the waste-loading limiting constituent for vitrification of sulfur-bearing radioactive wastes due to low solubility in silicate melts. Methods to improve sulfur loading would benefit from improved understanding of the structural aspects of sulfur incorporation in borosilicate and other glasses. To this end, sulfur XANES spectra were collected for eight crystalline standards and twenty-four glasses, including borosilicate, phosphate, and borate compositions. Spectra for the standards show a systematic energy shift of the sulfur K-edge from 2469 to 2482 eV, as sulfur valence increases from 2− (in sulfides) to 6+ (in sulfates). Most crucible glasses investigated have simple edges near 2482 eV that indicate sulfur in the form of sulfate only. Other glasses, some synthesized under reducing conditions, have complicated edges, indicating sulfate, sulfite, and more reduced species that may include S, S-S doublets, or short polysulfide chains. Sulfide species (S²⁻) were not dominant in any of the samples over the range of redox conditions investigated. These results indicate that sulfur incorporation is considerably more complex than would be suggested by the conventional interpretation of the redox-dependence of sulfur solubility, which considers only sulfate and sulfide species. Raman data indicate that several of the glasses investigated are not homogeneous with regard to all sulfur species.     

Structural investigation of amorphous FeZr,CoZr and NiZr alloys with low zirconium concentration

A structural investigation has been made of alloy glasses a low concentration of zirconium: compositions M_100?xZr_x with M = Fe, Co and Ni, and x = 9 at.% using X-ray diffraction. The characteristic second peak splitting in the radial distribution function is found for all samples presently investigated. The partial radial distribution functions of amorphous FeZr and CoZr alloys were derived from the measured total distribution function data by applying the concentration method with the anomalous scattering technique. The amplitude in the oscillation of the radial distribution function for M-M and M-Zr pairs is more enhanced in comparison with that of glasses with high Zr content, x = 40–45 at.%. The estimated coordination number of nearest neighbor Fe atoms (11.6 ± 0.5) for iron in the Fe₈₄Zr₁₆ glass is larger than that (10.7 ± 0.5) found in the Fe₈₃B₁₇ glass. This is consistent with the measurements of magnetic properties of these glasses with low zirconium concentration. The possible structural features of intertransition metal alloy glasses with low zirconium concentration is also discussed based on the present experimental data.     

Tetrahedral site of Fe(III) and Cu(II) in renierite

Renierite from Congo is used in the present investigations. It contains only two transition elements iron and copper. EPR results indicate the presence of Fe(III) and Cu(II) with g values 2.03 and 2.40 respectively. Optical absorption spectrum shows several energies in UV‐Vis and NIR region. These energies indicate both Fe(III) and Cu(II) in tetrahedral site. IR spectrum confirms the presence of sulphate and hydroxyl groups in the mineral. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Europium environment and clustering in europium doped silica and sodium silicate glasses

The local environment and distribution of rare earth ions are important to the optical properties of rare earth doped oxide glasses. In this paper, we report studies of the structures of europium doped (around 1 mol% Eu₂O₃) silica and sodium silicate glasses using molecular dynamics simulations. By using effective partial charge potentials, systems with over 24,000 atoms were modeled in order to obtain better statistics of rare earth ion distribution. The simulated glass structures were validated by comparing the calculated neutron and X-ray structure factors with experimental data. It was found that europium ions have higher coordination number (5.9 versus 4.8) and more symmetric environments in sodium silicate glasses than in the silica glass. Rare earth ion clustering has been characterized in detail and it was found that the clustering probability of europium ions in sodium silicate glass is consistently less than that predicted from a random distribution, while the probability of clustering in pure silica glass is higher than that of random distribution at the 1 mol% doping level.     

Chemistry of cultural glasses: the early medieval glasses of Monselice's hill (Padova, Italy)

To investigate the mechanisms of deterioration of historical glasses, under natural evolution, some early medieval glasses from the archaeological site of the Monselice's hill have been analysed. By an archaeological approach, developed at the Dipartimento di Scienze dell'Antichità, University of Padova, the glasses were dated between the VI and the beginning of the VII century and they were ascribed to the same artist or school. By a geological approach, developed at the Dipartimento di Mineralogia e Petrologia, University of Padova, it was found that some pieces of glasses, from the same archaeological site, were made of silica, rich in sodium and calcium, with iron and manganese. The composition was analogous the one of glasses produced during Roman empire, using `natron' (Na<sub>2</sub>CO<sub>3</sub>·NaHCO<sub>3</sub>·2H<sub>2</sub>O) as melting agent and glasses produced during medieval age, in the Mediterranean basin, using plant ash like `Salsola Kali' as melting agent. It was also found that there was a surface layer, with a special lamellar structure, easy to remove. The surface layer was found poor in alkali and alkaline-earth elements. By surface and microscopic analyses (optical microscopy, SEM-EDS, microRaman, XPS, SIMS and Mössbauer) it has been found that all the samples have a composition rich in silica, sodium and calcium except one that, unexpectedly, was rich in potassium and poorer in sodium. This sample, as composition, seems just like medieval glasses produced north of the Alps, using plant ash like ferns as melting agent. In all the samples the surface layers have less alkaline elements and the depletion goes to ten μm of depth. The extreme consequence of this depletion is the formation, in some samples, of an alteration layer, easy to remove, that the XPS analyses tell us it is made of very hydrated silica. The surface layers show a little accumulation of calcium. The calcium ion is also present in some birefringent crystal aggregates immersed in the glass that, in some samples, are around one mm large. These aggregates have a circular shape, with a nucleation centre in them. By microRaman spectroscopy it was found that the crystal aggregates are made of vateritic and calcitic calcium carbonate. By Mössbauer spectroscopy it was found that in the flat yellow coloured glasses, richer in iron, the Fe(III) species predominates. Instead in the pale green ones, poorer in iron, the Fe(II) prevails.