Oxygen Speciation Research Articles

Synergistic effects of ultraviolet radiation and corrosion on Zr alloys

Ultraviolet (UV) radiation and corrosion can be coupled in non-trivial ways and such coupling is of critical importance for the performance of materials in extreme environments. However, the confluence of these phenomena and their collective impact remains underexplored. This study bridges this knowledge gap by presenting a thorough investigation of Zr alloys exposed to corrosive high-temperature water under in-situ UV irradiation. We found that initial UV exposure significantly accelerates corrosion and induces distinct changes in the microstructure of ZrO2 oxides. Over time, we observed the formation of Fe3O4 particles on the ZrO2 surface due to photo-dissolution of zirconia and photo-deposition of iron oxide magnetite. The emergence of Fe3O4 particles could enhance the corrosion resistance of the Zr alloy substrate. Furthermore, our results indicate that UV irradiation modulates the density and distribution of nanopores within the oxide layer by influencing oxygen speciation and the dynamics of oxide growth. These insights advance our understanding of the complex interplay between UV irradiation and high-temperature water corrosion.

Impact of Lanthanide (Nd3+, Gd3+, and Yb3+) Ionic Field Strength on the Structure and Thermal Expansion of Phosphate Glasses.

Phosphate glasses containing Nd3+, Gd3+, and Yb3+ as lanthanide ions are attractive for applications in laser materials, phototherapy lamps, and solar spectral converters. The composition-structure-property relation in this type of glass system is thus of interest from fundamental and applied perspectives. In this work, the impact of the differing ionic radius of Nd3+, Gd3+, and Yb3+ and consequent field strength on the physical properties of phosphate glasses is investigated, focusing ultimately on thermal expansion effects. The glasses were made by melting with a fixed concentration of the lanthanide ions having 50P2O5-46BaO-4Ln2O3 nominal compositions (mol %) with Ln = Nd, Gd, and Yb. The investigation encompassed measurements by X-ray diffraction (XRD), optical spectroscopy, density, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and dilatometry. XRD supported the amorphous nature of the glasses, whereas absorption and photoluminescence spectra showed the optical features of the Nd3+, Gd3+, and Yb3+ ions in the glasses. Oxygen speciation by XPS indicated an increase in nonbridging oxygens for the larger radii Nd3+ and Gd3+ ions relative to the host, contrasting with Yb3+. Phosphorus XPS analysis further supported the hypothesis that the P 2p binding energies of the glasses increased with the cation field strength of the lanthanides. The Raman spectra were interpreted based on glass depolymerization effects and the impact of Ln3+ ions with high field strength. Particularly, the band position of the symmetric out-of-chain nonbridging oxygen stretch, νs(PO2-), shifted to higher frequencies correlating with the Ln3+ field strength. Dilatometry ultimately revealed a steady decrease in the coefficient of thermal expansion for the glasses, which correlated linearly with Ln3+ field strengths and thus indicated to sustain increased glass rigidities. The various analyses performed thus illuminated the structural foundation of the thermomechanical behavior of the glasses connected with changes in the Ln3+ field strengths.

Iodine dissolution mechanisms in high-pressure aluminoborosilicate glasses and their relationship to oxygen speciation

Incorporation of iodine into high-pressure vitrified glasses is a strong function of alkali content. The vitrification under high-pressure conditions could represent a potential solution for the immobilization of 129I radioisotopes.

Structure and fragility of normal and invert lanthanum borate glasses: Results from 11B and 17O NMR spectroscopy and calorimetry

The structure of select normal and invert (La2O3)x(B2O3)100-x glasses with 22 ≤ x ≤ 60, synthesized using conventional and containerless levitation melt-quenching, is studied using 11B and 17O NMR spectroscopy. The results indicate that the structure of normal glasses with 22 ≤ x ≤ 30 is characterized by a borate network of B-O-B bridging oxygen (BO) linkages between BO3 and BO4 units and non-bridging oxygen (NBO) with La acting as a charge balancing modifier cation. In contrast, the invert glasses with 55 ≤ x ≤ 60 consist of a network of La-O polyhedra connected via La-O-La linkages i.e. “free-oxide” ions and via La-NBO-B linkages with interspersed [BO3]3− units. This structural evolution in lanthanum borate glasses is consistent with the composition dependence of their glass transition temperature. The temperature dependent boron and oxygen speciation in these glasses are linked to their calorimetrically determined fragility index.

Predicting iodine solubility at high pressure in borosilicate nuclear waste glasses using optical basicity: an experimental study

Constraining the systematic evolution of iodine solubility in borosilicate glasses is required for the formulation of adequate glass matrices able to immobilize 129I representing a major troublesome radioisotopes produced by nuclear anthropic activities. We investigated experimentally the change in iodine solubility in a large series of borosilicate glasses synthesized under high-pressure conditions (1.5 GPa) and at 1350 °C. The nature of network modifying cation (Na, K, Rb, Ca, Ba and Sr) and the concentration of B2O3 (~ < 10 mol.% and ~ > 10 mol.%) have been tested. The XPS measurements showed that iodine speciation in glasses is mostly represented by iodide (I−) for a range of iodine solubility from 0.3 to 3.1 mol.% as determined by SEM EDS and LA-ICP-MS analyses. The iodine solubility is enhanced in glasses with lower B2O3 content and with a higher concentration in network modifying cation. Regardless of the cation nature: alkali or alkaline-earth; increasing the cation size appears to induce a decrease in iodine solubility. We used the optical basicity (ΛGlass) and iono-covalent parameter (ICPGlass) to express the large variety of investigated glass compositions; both relating to the electron donor capability of the glass. We show that iodine solubility is positively correlated with ΛGlass and accordingly negatively correlated with ICPGlass. It implies that more iodine will be dissolved in glass compositions having a stronger electron-donating capability. Evidence of the relationship between iodine solubility and oxygen network is shown by the iodine solubility positive trend with equilibrium constant of the oxygen speciation. Future work should take these parameters into consideration for modeling iodine solubility in borosilicate glasses providing additional information are collected such as the boron, aluminum and silicon speciation in glasses.

A new self‐consistent empirical potential model for multicomponent borate and borosilicate glasses

AbstractA new self‐consistent empirical potential model based on the BMP potential [Bertani et al., Phys. Rev. Mat. 5 (2021) 045602] has been developed for the simulation of multicomponent borate and borosilicate glasses. We exploited the Bayesian optimization approach to create a set of parameters for the B–O interaction, which depends on the glass composition, and in particular on the R = ([A2O]+[AEO])/[B2O3] (A = alkaline and AE alkaline‐earth ions) and K = [SiO2]/[B2O3] ratios. The obtained force field (FF) has been applied to several borate and borosilicate glass series containing, as modifier oxide, Na2O, Li2O, CaO, and MgO and tested on experimental data, such as the fraction of BO4 (N4), density, non‐bridging oxygen speciation, neutron diffraction spectra, 11B, 29Si, and 17O magic angle spinning nuclear magnetic resonance. A comparison with other interatomic potentials available in literature has also been performed. The results show that the FF reproduces well almost all the abovementioned properties, showing excellent agreement with experimental data in a wide range of compositions.

Study on oxygen species in the products of co-liquefaction of coal and petroleum residues

From the view of atom economy, it is important to know the form and apportion of oxygen in products, especially the apportion of oxygen in CO2 during direct coal liquefaction (DCL) and co-liquefaction of coal and petroleum residues (CCPR). Herein, co-liquefaction of Naomaohu lignite with Tahe petroleum residue (THPR) was conducted and the O distribution in liquefaction products including gas, n-hexane soluble (HS), asphaltene and pre-asphaltene (AP), and residue (Res) were analyzed by elemental analyzer, GC, GC-MS, FT-IR and APCI-MS. The results showed that CCPR did not change the oxygen speciation but the O distribution in products. O-containing radicals generated from coal pyrolysis would bind to the alkyl radicals from petroleum residue decomposition during CCPR, resulting in remarked increase in the content of O-containing alkanes. Compared with DCL, the apportion of organic oxygen (HS and AP) was increased by 25.94% during CCPR, and the inorganic oxygen (CO, CO2, H2O, and Res) was decreased by 7.10%, wherein, CO2 content in inorganic oxygen was reduced by 2.13%. This implies that the CCPR not only significantly enhances the utilization of organic oxygen in coal but also inhibits CO2 generation, which is vital for realizing the goal of carbon peak and neutrality.

Nickel ion extracellular uptake by the phototrophic bacterium Rhodobacter sphaeroides: new insights from Langmuir modelling and X-ray photoelectron spectroscopic analysis

The presence of anionic functional groups on the surface of microbial cells makes the bacterial biomass very attractive as metal cation removal tool for environmental bioremediation. To go insight to the heavy metal sorption abilities of Gram-negative bacteria, we characterized the extracellular uptake of Ni2+ ions by the phototrophic bacterium Rhodobacter sphaeroides, whose ability to sequester toxic metal ions has been widely documented. The sorption process proved to be well described by the Langmuir model, suggesting that the cell envelope behaves as an ideal adsorbent surface, showing a layer of chemically equivalent binding sites with high affinity for Ni2+. X-ray photoelectron spectroscopic data reveal that, at binding site saturation, Ni2+ ions fully displace alkali metal ions from the bacterial surface, while they are unable to displace Ca2+ and Mg2+ from their tight binding sites. The analysis of carbon, nitrogen and oxygen speciation, as arisen from high resolution spectra fitting, highlights that the Ni2+/K+ cation exchange affects the surface distribution of main chemical components, with increased contribution by peptides and phosphatidylcholines. A role of Ni2+ ions in favoring the stacking and/or packing of lipopolysaccharide sugar chains by interaction with carboxylate groups is proposed and discussed.

Congruent dissolution of Al2O3 in a K-disilicate melt, with constraints on the reaction mechanism

High resolution O 1s X-ray Photoelectron Spectroscopy (XPS) was employed to determine oxygen speciation in 31 mol% K2O silicate glass containing 0.0, 1.0 and 3.0 mol% dissolved Al2O3. Highly resolved bridging oxygen (BO) and non-bridging oxygen (NBO) peaks were observed. The NBO peak intensity decreased and the BO increased with dissolution of Al2O3. The enhanced BO intensity is localized at two binding energies (BE), one characteristic of a Si-O-Si moiety (e.g., quartz, or v-SiO2), the other of a Si-O-Al moiety (e.g., kyanite, Al2SiO5). The dissolution reaction proceeds in two steps; NBO− of a Q3 species attacks and bonds to an under-coordinated surface Al atom of crystalline Al2O3. The reaction produces a Si-O-Al surface species thus converting the attacking NBO to a BO and altering the reactant Q3 species to a Q4 product species:[Q3]−melt + [AlO1.5]surface → [Q4]0melt + [AlO2]−meltThe second step involves reaction of [AlO2]−melt with K+converting it from a modifier cation to a charge compensator, resulting in the polymerization reaction:[K-Q3]0melt + [AlO2]−melt → [Q4]0 + [KAlO2]0melt + 1/2O2−The stoichiometric reaction is the sum of the two:[Q3]−melt + [K-Q3]0melt + [AlO1.5]surface → 2[Q4]0 + [KAlO2]0melt + 1/2O2−Al species detached from the solid surface initially must be bonded to some NBO. Within the melt, nucleophilic substitution attack of the Al-bearing species by Q3 species converts NBO to BO, resulting in [KAlO2]melt being bonded four BOs. This reaction may not go to completion in all melts, depending on dissolved Al content and melt composition.

Study on Oxygen Speciation in the Products of Direct Coal Liquefaction and Co-Liquefaction of Coal and Petroleum Residues

Study on Oxygen Speciation in the Products of Direct Coal Liquefaction and Co-Liquefaction of Coal and Petroleum Residues

The rise of electrochemical NAPXPS operated in the soft X-ray regime exemplified by the oxygen evolution reaction on IrOx electrocatalysts.

Photoelectron spectroscopy offers detailed information about the electronic structure and chemical composition of surfaces, owing to the short distance that the photoelectrons can escape from a dense medium. Unfortunately, photoelectron based spectroscopies are not directly compatible with the liquids required to investigate electrochemical processes, especially in the soft X-ray regime. To overcome this issue, different approaches based on photoelectron spectroscopy have been developed in our group over the last few years. The performance and the degree of information provided by these approaches are compared with those of the well established bulk sensitive spectroscopic approach of total fluorescence yield detection, where the surface information gained from this approach is enhanced using samples with large surface to bulk ratios. The operation of these approaches is exemplified and compared using the oxygen evolution reaction on IrOx catalysts. We found that all the approaches, if properly applied, provide similar information about surface oxygen speciation. However, using resonant photoemission spectroscopy, we were able to prove that speciation is more involved and complex than previously thought during the oxygen evolution reaction on IrOx based electrocatalysts. We found that the electrified solid-liquid interface is composed of different oxygen species, where the terminal oxygen atoms on iridium are the active species, yielding the formation of peroxo species and, finally, dioxygen as the reaction product. Thus, the oxygen-oxygen bond formation is dominated by peroxo species formation along the reaction pathway. Furthermore, the methodologies discussed here open up opportunities to investigate electrified solid-liquid interfaces in a multitude of electrochemical processes with unprecedented speciation capabilities, which are not accessible by one-dimensional X-ray spectroscopies.

Reaction paths of methane activation and oxidation of surface intermediates over NiO on Ceria-Zirconia catalysts studied by In-situ FTIR spectroscopy

Methane activation over NiO/Ce0.82Zr0.18O2 catalysts synthesized by combustion synthesis is studied by in-situ IR spectroscopy and correlated to the properties and oxygen speciation of the catalyst. Through XRD, H2-TPR, and pyridine adsorption followed by FTIR spectroscopy, the reducibility and Lewis acidity of the catalysts are assessed. In-situ FTIR spectroscopy is used to monitor the methane activation on catalyst surfaces. Complex IR features of methane-derived surface species are observed and are attributed to the formation of surface alkyl/alkoxy, aldehyde, formate/carbonate, and aromatic species. A data analysis algorithm is developed to evaluate the evolution of different surface species over time. The formation of formate and carbonate species is driven by adsorbed surface oxygen, while less reactive oxygen species associated with NiO allow for the production of aromatics and alkoxy intermediates. By tuning the reducibility and Lewis acidity of the catalyst, the selectivity to alkoxy intermediates can be improved.

Infrared spectroscopy quantification of functional carbon groups in kerogens and coals: A calibration procedure

The determination of the abundances of the CHx, C = O and aromatic groups in chondritic Insoluble Organic Matter (IOM) and coals by Infrared (IR) spectroscopy is a challenging issue due to insufficient knowledge on the absorption cross-sections and their sensitivity to the molecular environment. Here, we report a calibration approach based on a 13C synthetic model material whose composition was unambiguously determined by Direct-Pulse/Magic Angle Spinning Nuclear Magnetic Resonance (DP/MAS NMR). Ratios of the cross-sections of the CHx, C = O and aromatic groups have been determined, and the method has been applied to IOM samples extracted from four chondrites as Orgueil (CI), Murchison (CM), Tagish Lake (C2-ungrouped) and EET 92042 (CR2), and to a series of coals. The estimate of the aliphatic to aromatic carbon ratio (nCHx/nAro) in IOM samples from Orgueil, Murchison and Tagish Lake chondrites is in good agreement with Single-Pulse/NMR estimates earlier published, and is lower by a factor of 1.3 in the case of the CR chondrite EET 92042 (but the error bars overlap). In contrast, the aliphatic to carbonyl ratio (nCHx/nC=O) is overestimated for the four chondrites. These discrepancies are likely due to the control of the absorption cross-section of the C = O and C = C bonds by the local molecular environment. Regarding coals, the use of published NMR analyses has brought to light that the integrated cross-section ratio ACHx/AAro varies with the vitrinite reflectance over an order of magnitude. Here as well, the local oxygen speciation plays a critical control in AAro, which decreases with increasing the vitrinite reflectance. We provide an analytical law that links ACHx/AAro and vitrinite reflectance, which will allow the determination of nCHx/nAro for any coal sample, provided its vitrinite reflectance is known.

A comparative study of the effectiveness of empirical potentials for molecular dynamics simulations of borosilicate glasses

Despite their practical importance, atomistic modeling of B2O3 containing glasses have been challenging due to the lack of reliable empirical potentials. Fortunately, a few recent developments have shown promises to simulate these glasses where the boron coordination has complex and non-linear dependence on glass composition. This work aims to provide an evaluation of the effectiveness of three recently developed potentials by a systematic study of a series (~20) of sodium borosilicate glasses with constant K (ratio of [SiO2]/[B2O3]) =2 and varying R (ratio of [Na2O]/[B2O3] ranging from 0.1 to 4) values and several sodium boroaluminosilicate glasses. A comparative assessment was established on the basis of the short- and medium-range structure features, such as boron N4 values, total correlation functions, bond angle distribution, oxygen speciation, and mechanical properties using experimental or well-established models as criteria. This work provides insights on the choices of empirical potentials for MD simulations of borosilicate glasses and gives directions of future potential development and refining.

Effects of Al:Si and (Al + Na):Si ratios on the properties of the international simple glass, part II: Structure

AbstractHigh‐alumina containing high‐level waste (HLW) will be vitrified at the Waste Treatment Plant at the Hanford Site. The resulting glasses, high in alumina, will have distinct composition‐structure‐property (C‐S‐P) relationships compared to previously studied HLW glasses. These C‐S‐P relationships determine the processability and product durability of glasses and therefore must be understood. The main purpose of this study is to understand the detailed structural changes caused by Al:Si and (Al + Na):Si substitutions in a simplified nuclear waste model glass (ISG, international simple glass) by combining experimental structural characterizations and molecular dynamics (MD) simulations. The structures of these two series of glasses were characterized by neutron total scattering and 27Al, 23Na, 29Si, and 11B solid‐state nuclear magnetic resonance (NMR) spectroscopy. Additionally, MD simulations were used to generate atomistic structural models of the borosilicate glasses and simulation results were validated by the experimental structural data. Short‐range (eg, bond distance, coordination number, etc) and medium‐range (eg, oxygen speciation, network connectivity, polyhedral linkages) structural features of the borosilicate glasses were systematically investigated as a function of the degree of substitution. The results show that bond distance and coordination number of the cation‐oxygen pairs are relatively insensitive to Al:Si and (Al + Na):Si substitutions with the exception of the B‐O pair. Additionally, the Al:Si substitution results in an increase in tri‐bridging oxygen species, whereas (Al + Na):Si substitution creates nonbridging oxygen species. Charge compensator preferences were found for Si‐[NBO] (Na+), [3]B‐[NBO] (Na+), [4]B (mostly Ca2+), [4]Al (nearly equally split Na+ and Ca2+), and [6]Zr (mostly Ca2+). The network former‐BO‐network former linkages preferences were also tabulated; Si‐O‐Al and Al‐O‐Al were preferred at the expense of lower Si‐O‐[3]B and [3]B‐O‐[3]B linkages. These results provide insights on the structural origins of property changes such as glass‐transition temperature caused by the substitutions, providing a basis for future improvements of theoretical and computer simulation models.

Study of the Structure of FeOx-CaO-SiO2-MgO and FeOx-CaO-SiO2-MgO-Cu2O-PdO Slags Relevant to Urban Ores Processing through Cu Smelting

Ferrous-calcium-silicate (commonly known as FCS) slags are used in the valuable metal recycling from urban ores through both primary and secondary copper smelting processes. In the present study, the structure of selected FCS-MgO (FCSM) and FCS-MgO-Cu2O-PdO (FCSM-Cu2O-PdO) slags, relevant to the processes, were investigated using Fourier-transform infrared (FTIR) spectrometry. Deconvolution of the FTIR spectra was carried out to calculate the relative abundance of different silicate structural units (Qn), the overall degree of polymerization (DOP) of the slags and the oxygen speciation in the FCS slags. It was observed that, for the slag investigated, the relative intensity of both the high-frequency band ≈ 1100 cm−1 (Q3) and low-frequency band ≈ 850 cm−1 (Q0) were affected by Fe/SiO2 ratio, basicity, temperature (T) and oxygen partial pressure (pO2). The DOP and the average number of bridging oxygen (BO) were found to decrease with increasing both Fe/SiO2 ratio and basicity. Improved semi-empirical equations were developed to relate the DOP of the slags with chemistry, process parameters and partitioning ratio (i.e., the ratio of the amount of element in the slag phase to metal phase, also known as distribution ratio) of Pd and Ge. Possible reactions, expressed as reactions between metal cations and silicate species, as a way to evaluate thermodynamic properties, are presented herein.

Subducting serpentinites release reduced, not oxidized, aqueous fluids

The observation that primitive arc magmas are more oxidized than mid-ocean-ridge basalts has led to the paradigm that slab-derived fluids carry SO2 and CO2 that metasomatize and oxidize the sub-arc mantle wedge. We combine petrography and thermodynamic modelling to quantify the oxygen fugacity (fO2) and speciation of the fluids generated by serpentinite dehydration during subduction. Silicate-magnetite assemblages maintain fO2 conditions similar to the quartz-fayalite-magnetite (QFM) buffer at fore-arc conditions. Sulphides are stable under such conditions and aqueous fluids contain minor S. At sub-arc depth, dehydration occurs under more reducing conditions producing aqueous fluids carrying H2S. This finding brings into question current models in which serpentinite-derived fluids are the cause of oxidized arc magmatism and has major implications for the global volatile cycle, as well as for redox processes controlling subduction zone geodynamics.

Oxygen species stabilized on the cobalt spinel nano-octahedra at various reaction conditions and their role in catalytic CO and CH4 oxidation, N2O decomposition and oxygen isotopic exchange

Periodic spin unrestricted DFT+U calculations joined with atomistic thermodynamics and model catalytic experiments (TPD-O2, 18O2/16O4 exchange, N2O decomposition, CO and CH4 oxidation) were used to study the structure, stability and reactivity of various surface oxygen species and oxygen vacancies, produced under different thermodynamic conditions on the (1 1 1) surface exposed by the cobalt spinel nanooctahedra. We thoroughly analyzed the stability of differently oxygen covered terminations starting from ΘO = 0.86 nm−2 (isolated monoatomic species) up to ΘO = 5.21 nm−2 (full oxygen monolayer). The constructed 3-dimensional thermodynamic (γ, T, pO2) surface redox state diagram revealed that in typical catalytic pressures of O2 (pO2/p° ∼ 0.01– 1) three principal states of the spinel surface, including a region associated with the presence of μ-superoxo CoO3c–O2–CoT3c and metal-oxo CoO3c-O species (T < 450– 500 °C), a bare surface region (450– 500 °C to 550–600 °C), and a region were surface oxygen vacancies appear (T > 600 °C) may be distinguished. This diagram was used for quantitative assignment of the experimental TPD-O2 desorption peaks, and also, for interpretation of the role that various adoxygen and lattice oxygen species play in the investigated model catalytic reactions. It was shown that CO is primarily oxidized by the suprafacial CoO3c-O2-CoT3c diatomic oxygen and/or cobalt-oxo CoO3c-O species, whereas in low temperature activation of CH4 only the CoO3c-O adducts are involved. With the increasing temperature, the suprafacial methane oxidation route gradually changes into the interfacial Mars van Krevelen scheme, and the later pathway is accelerated above 600 °C, due to the onset of oxygen vacancy formation (in line with 18O2/Co316O4 isotopic exchange experiments). In the case of N2O decomposition the reaction occurs preferably on a bare surface, and the favorable thermodynamics provides a steady driving force for efficient removal of the recombined adoxygen intermediates in the form of gaseous O2. The phase diagram of the alterable surface oxygen states on the (1 1 1) termination was compared to the corresponding diagram for the (1 0 0) facet, to explain the nature and differences in catalytic redox behavior of the cobalt spinel nano-octahedra and nano-cubes. This allows also for prediction of oxygen speciation anisotropy on cubo-octahedral nanocrystals at various temperatures.

Structure of Strontium Aluminosilicate Glasses from Molecular Dynamics Simulation, Neutron Diffraction, and Nuclear Magnetic Resonance Studies.

The structure of strontium glasses with the composition (SiO2)1-2 x(Al2O3) x(SrO) x ( R = [SrO]/[Al2O3] = 1) and (SiO2)1-4 x(Al2O3) x(SrO)3 x ( R = 3) has been explored experimentally over both short- and intermediate-length scales using neutron diffraction, 27Al and 29Si nuclear magnetic resonance, and classical molecular dynamics simulations in model systems containing around 10 000 atoms. We aim at understanding the structural role of aluminum and strontium as a function of the chemical composition of these glasses. The short- and medium-range structure such as aluminum coordination, bond angle distribution, Q( n) distribution, and oxygen speciation have been systematically studied. Two potential forms of the repulsive short-range interactions have been investigated, namely, the Buckingham and Morse forms. The comparison of these forms allows us to derive general trends independent of the particular choice of the potential form. In both cases, it is found that aluminum ions are mainly fourfold coordinated and mix with the silicon network favoring the Al/Si mixing in terms of Al-O-Si linkages. For the R = 1 glass series, despite the full charge compensation ([SrO] = [Al2O3]), a small fraction of fivefold aluminum is observed both experimentally and in MD simulations, whereas the concentration of sixfold aluminum is negligible. MD shows that the fivefold aluminum units AlO5 preferentially adopt a small ring configuration and link to tricoordinated oxygen atoms whose population increases with the aluminum content and are mainly found in OAl3 and OAl2Si configurations. The modeled Sr speciation mainly involves SrO7 and SrO8 polyhedra, giving a range of average Sr2+ coordination numbers between 7 and 8 slightly dependent on the short-range repulsive potential form. A detailed statistical analysis of T-O-T' (T, T' = Al,Si), accounting for the population of the various oxygen speciations, reveals that both potentials predict a nearly identical Al/Si mixing.

Effects of surface initial condition on aqueous corrosion of glass—A study with a model nuclear waste glass

AbstractBeing a nonequilibrium material, the structure of glass varies with the sample history. Thus, the initial surface condition of a glass can vary with the preparation condition and have a large impact on its reactivity. This paper shows that the aqueous corrosion behavior of international simple glass (ISG) varies depending on the initial surface state. The ISG glass samples were prepared as‐polished‐only and polished‐then‐annealed and they were immersed in aqueous solution saturated with soluble SiO2 at 30°C (modeling a mild condition) and at 90°C (modeling a severe condition). Molecular dynamics simulations were performed to obtain coordination numbers of network formers of ISG to assist oxygen speciation calculations. The surface structures of as‐prepared and corroded ISG samples were analyzed using various imaging and spectroscopic techniques. Among these analyses, only the oxygen speciation with x‐ray photoelectron spectroscopy showed discernable differences between two uncorroded surfaces with different preparation histories; all other methods could not differentiate the surface preparation history before aqueous corrosion. Such minor difference in chemical structures was found to have a profound impact on corrosion behaviors in the mild condition. In the harsh condition, the surface history dependence was not as drastic as the corrosion in the mild condition. The analysis results of the corroded surfaces suggested that the thickness and structure of the alteration layer formed on ISG in aqueous corrosion can vary with the initial surface state.