Effect of substituting CaO with BaO on the viscosity and structure of CaO‐BaO‐SiO2‐MgO‐Al2O3 slags

In this study, the effect of CaO and BaO substitution on the viscosity and structure of CaO‐BaO‐SiO₂‐MgO‐Al₂O₃ slags was investigated. The results showed that the viscosity increased with an increase in the BaO substitution concentration, which was correlated to an increase in the degree of polymerization (DOP) of the slag structural units as the activation energy increased from 207.9 to 263.8 kJ/mol for viscous flow. Deconvolution and area integration of the Raman spectrum of the slag revealed that the ratio of Q³/Q² (Qⁱ, i is the number of O⁰ in a [SiO₄]‐tetrahedral unit) increased and NBO/Si (nonbridging oxygen per unit silicon atom) decreased with higher BaO content. It was also observed from the ²⁷Al magic angles pinning nuclear magnetic resonance (²⁷Al MAS‐NMR) spectrum that the relative proportion of Al^IV increased, while that of Al^V decreased because of the decrease in the percentage of nonbridging oxygen (O^?), indicating the polymerization of the slag. O_1s X‐ray photoelectron spectroscopy (XPS) was also carried out to semi‐quantitatively analyze the various types of oxygen anions present in the slag. The XPS results correlated well with the results obtained from the analysis of the Raman and ²⁷Al MAS‐NMR spectra of the slags and its viscous behavior.     

The effect of the addition of alumina on the viscosity,surface tension,and foaming efficiency of 2.5(CaO/SiO2)-xAl2O3-yFeO-MgO melts

Viscosity and surface tension strongly influence the efficiency of slag foam in metallurgical processes. An excellent foaming slag preserves heat and lowers the cost of smelting in an electric furnace. In this study, we investigated the viscosity, surface tension, and foaming efficiency of a 2.5CaO/SiO₂-xAl₂O₃-yFeO-MgO slag. We also investigated the different valence oxygen ions by X-ray photoelectron spectroscopy (XPS). The results showed that with a gradual increase in the content of Al₂O₃, the viscosity initially increased and then decreased, and the changes in surface tension followed a similar pattern. The change in viscosity was caused by the increase in the degree of polymerization of the slag, which was determined by the competitive relationship between polymerization and the reduction in the stability of the overall network structure. Adding a small amount of Al₂O₃ to the slag slightly increased the number of Al–O–Al structures, whereas adding a large amount of the Al₂O₃ led to the formation of low-strength Al–O–Si structures, which reduced the stability of the network structure, thus reducing the viscosity. Because the surface tension is related to the concentration of non-bridging oxygens (NBOs), when the NBO content increased, the instability of the surface structure caused an increase in energy, thus increasing the surface tension. In addition, the CaO–SiO₂–5MgO-xAl₂O₃-yFeO five-element oxide in this study had the lowest surface tension at the same NBO concentration, which positively contributed to slag foaming. Finally, When the Al₂O₃ content in the system increased from 5.1 to 15.7 wt%, the foaming efficiency increased from 24.2 to 69.2 (minute‧centimeters), an increase of 286%.     

Structure–property relationship amphoteric oxide systems via phase stability and ionic structural analysis

The effects of basicity and amphoteric oxides (Al₂O₃ and Fe_tO) on the structure–property relationships of CaO–SiO₂–(Al₂O₃ and Fe_tO) and CaO–SiO₂–Al₂O₃–Fe_tO slags were investigated to determine the constitutional effects on the structure of high-temperature ionic melts. The proportion of Qⁿ species, which is determined by Raman spectroscopy, and the viscosity measured by the rotating cylinder method are both correlated and shown together with the slag structure index (NBO/T) concept. The NBO/T of CaO-SiO₂ binary slags showed a linear relationship with basicity (CaO/SiO₂), including an inflection point at CaO/SiO₂ = 1.0 resulting from the stability and Qⁿ-dominant unit of the melt, which changes close to the wollastonite (CaSiO₃) congruent point. This inflection point changes with the increasing amphoteric oxide content (Al₂O₃ and Fe_tO) because of the change in the dominant polymeric unit (Si⁴⁺–O–Si⁴⁺→M⁴⁺–O–Si⁴⁺; M: Al and Fe), in accordance with the equilibrated primary phases. As the Al₂O₃ content increased, the viscosity and activation energy of slags both drastically increased owing to the change in the flow unit (Si–O–Si, Al–O–Si, and Al–O–Al). In contrast, as Fe_tO increased, the viscosity and activation energy (E_η) of slags decreased because of the change in the flow unit (Si–O–Si, Fe–O–Si, and Fe–O–Fe). Ultimately, the flow unit (T–O–T; T = Si, Al, and Fe) and activation energy of the slags were found to be closely related to the solid primary phase on the phase diagram, and the physical-property–structure relationship was determined from the phase stability.     

Influence of Al2O3 on the electrical conductivity and structure of calcium-silicate-based melts

Electrical conductivity and structure of the CaO-SiO₂-based mold flux melts with various Al₂O₃ contents were investigated. The results show that the electrical conductivity increases with the addition of Al₂O₃ from 2 wt% to 4 wt%, but decreases with the further increase of Al₂O₃ from 4 wt% to 8 wt%. Correspondingly, the apparent activation energy reduces firstly from 55.12 ± 1.20 kJ mol to 41.09± 0.38 kJ mol, and then increases from 41.09 ± 0.38 kJ mol to 98.99 ± 1.42 kJ mol. The structure analyses suggest that complex structural units, such as Si-O-Al, Al-O⁰, Si-O-Si and Q³(Si), reduce first, but increase with the further addition of Al₂O₃. Conversely, these simple structural units, such as Al-O^-, Q⁰(Si), Q¹(Si) and Q²(Si) vary in the opposite way with the change of Al₂O₃ content. From the variations of electrical conductivity, activation energy and structural units, it can be found that when Al₂O₃ works as network breaker to simplify the melt structure, the energy barrier for transportation of conducting ions/ionic reduce, which results in the increase of electrical conductivity; while when Al₂O₃ becomes into network former, the conductivity increases, correspondingly.     

Prediction of coal slag foaming under gasification conditions by thermodynamic equilibrium calculations

In slagging gasifiers, slag foaming can cause serious operational problems, so there is a need for investigation into the conditions causing slag foaming. Viscosity experiments were carried out examining viscosity, extent of swelling and Fe formation. Although extensive swelling was not observed, FeO reduction was observed under an N₂/CO gas atmosphere, but not under CO₂/CO. In order to predict FeO reduction conditions in the gasifier, a model for an adiabatic equilibrium gasifier was developed. The gas composition, the amount of gas to slag, and P_o2 were calculated for a slurry-feed gasifier, and the results of the calculation were used to predict the reduction of FeO in slag by using FactSage. Under typical gasification conditions for Denisovsky coal, the predicted P _o2 in the gasifier was not low enough to cause FeO reduction. The FactSage simulation for the viscometer conditions predicted no FeO reduction under a CO/CO₂ atmosphere, but did predict Fe formation under CO/N₂ conditions. At a 20% CO concentration, FeO reduction starts at temperatures above 1,600°C. Since the slag has a low viscosity at 1,600°C, the oxygen bubble may have escaped as it formed. Therefore, slag foaming, caused by FeO reduction in the slag, can only occur when the right conditions of viscosity and oxygen partial pressure are met. This paper was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Searching for correlations between vibrational spectral features and structural parameters of silicate glass network

Infrared (IR) and Raman spectroscopic features of silicate glasses are often interpreted based on the analogy with those of smaller molecules, molecular clusters, or crystalline counterparts; this study tests the accuracy and validity of these widely cited peak assignment schemes by comparing vibrational spectral features with bond parameters of the glass network created by molecular dynamics (MD) simulations. A series of sodium silicate glasses with compositions of [Na₂O]_x[Al₂O₃]₂[SiO₂]_98−x with x = 7, 12, 17, and 22 were synthesized and analyzed with IR and Raman. A silica glass substrate and a crystalline quartz were also analyzed for comparison. Glass structures with the same compositions were generated with MD simulations using three types of potentials: fixed partial charge pairwise (Teter), partial diffuse charge potential (MGFF), and bond order-based charge transfer potential (ReaxFF). The comparison of simulated and experimental IR spectra showed that, among these three potentials tested, ReaxFF reproduces the concentration dependence of spectral features closest to the experimentally observed trend. Thus, the bond length and angle distributions as well as Si–Qⁿ species and ring size distributions of silica and sodium silicate glasses were obtained from ReaxFF-MD simulations and further compared with the peak assignment or deconvolution schemes—which have been widely used since 1970s and 1980s—(a) correlation between the IR peak position in the Si–O stretch region (1050-1120 cm⁻¹) and the Si–O–Si bond angle; (b) deconvolution of the Raman bands in the Si–O stretch region with the Qⁿ speciation; and (c) assignment of the Raman bands in the 420-600 cm⁻¹ region to the bending modes of (SiO)_n rings with different sizes (typically, n = 3-6). The comparisons showed that none of these widely used methods is congruent with the bond parameters or structures of silicate glass networks produced via ReaxFF-MD simulations. This finding invokes that the adequacy of these spectral interpretation methods must be questioned. Alternative interpretations are proposed, which are to be tested independently in future studies.     

Structural and Textural Modifications of Ternary Phosphate Glasses by Thermal Treatment

Phosphate-based glasses of composition xNa₂O−(45+(10−x))CaO−45P₂O₅ with different Na₂O, CaO (x = 1, 5, 10, 15, and 20 mol%), and invariable P₂O₅ (45 mol%) contents were prepared using the rapid melt quench technique. The obtained thermal data from differential thermal analysis revealed a decline in glass transition (T_g) and crystallization (T_c) temperatures of glasses against the compositional changes. The inclusion of Na₂O at the cost of CaO in the glass network led to a reduction in its thermal stability. The thermal treatment carried out on glasses helped to derive their glass-ceramic counterparts. The amorphous and crystalline features of samples were characterized using X-ray diffraction patterns. The crystalline species that emerged out of the calcium phosphate phases confirmed the dominance of Q¹ and Q² structural distributions in the investigated glass-ceramics. The obtained scanning electron micrographs and atomic force microscopic images confirmed the surface crystallization and textural modification of the samples after thermal treatment. The N₂-adsorption–desorption studies explored the reduction of porous structures due to thermal treatment on the melt-driven glass surface. The measured elastic moduli and Vicker's hardness values of the glasses showed an increase after thermal treatment, which were reduced against the inclusion of alkali content in both glass and glass-ceramics.     

Modification of interface chemistry and slag structure by transition element Cr

A comparative study on CaO–MgO–Al₂O₃–SiO₂ slag and CaO–MgO–Al₂O₃–SiO₂–Cr₂O₃ slag was conducted to investigate the distribution of the elements at the gas-slag interface. The effect of redox states of chromium on the distribution of sulfur and oxygen at the interface was revealed by gas-slag equilibrium method using X-ray photoelectron spectroscopy at 1873K. From the analysis of the S2p core-level spectra, the negative divalent sulfur(S^2?) was detected at the interface in the Cr-bearing slag, which directly proved that sulfur exists in the form of S^2? in the slag for the first time. However, the S^2? peak is very weak at the interface of Cr-free slag. The reason for the difference between the two slags may be due to chromium changing the interface structure. According to the O1s and Cr2p core-level spectra, non-bridged oxygen(O^?) increased, while bridged oxygen(O⁰) decreased with the etching depth deepened. The increase of NBO/BO and Cr²⁺/Cr³⁺ elucidates that Cr³⁺ can modify the structure of the slag as basicity substance, but its effect is weaker than that of Cr²⁺. Meanwhile, due to the affinity of sulfur and chromium, the addition of chromium may also lead to the enhancement of the S^2? peak at the gas-slag interface. Gradient change of elements at the interface proved the existence of the boundary layer.     

The evolution of the mold flux melt structure during the process of fluorine replacement by B2O3

This study presented the melt structure evolution of mold flux during the substitution of fluorine by B₂O₃, and a computational model for the degree of polymerization (DOP) for borosilicate structure was developed. The results showed that the reduction of fluorine content would promote the replacement of F in [SiF₆]-octahedral unit by the dissociative free oxygen ions (O²⁻), and release F⁻ ions into the melt to compensate the reduction of F⁻ ions. With the 2 mass% addition of B₂O₃, the original Si–O–Si bond would be disrupted, and connect with [BO₃]-trihedral to form boroxol ring structure containing [BO₂O⁻]-trihedral and [BO₃]-trihedral structural units. Then, the Si–O–B bond that [BO₃]-trihedral links [SiO₄]-tetrahedral in boroxol ring was destroyed with the further addition of B₂O₃, and then the [BO₃]-trihedral could link with the dissociative Q¹(Si) and Q⁰(Si) structural units to transform into [BO₄]-tetrahedral and form a borosilicate long chain. Finally, with 6 mass% addition of B₂O₃, the borosilicate chain would combine with simple borate and borosilicate structures, and a complex borosilicate structure containing boroxol ring with certain symmetry was formed ultimately. Besides, the calculated result of DOP suggested that the DOP of the melt structure improved during the process of fluorine replacement by B₂O₃.     

Understanding the adsorption behavior of oxygen on the 3C–SiC(1 1 0) surface: A first-principles study

The adsorption behavior of atomic oxygen and molecular O₂ on the 3C–SiC(1 1 0) surface is investigated by first-principles calculations. The atomic O prefers to be adsorbed at the C top site (C–O) with adsorption energy of −1.95 eV after zero-point energy correction, followed by the C–O–Si bridge site, Si–O–Si bridge site, and the Si top site (Si–O) with adsorption energies of −1.46, −1.36, and −1.13 eV, respectively. The molecular O₂ separately trapped by the second nearest neighboring C and Si atoms (C–O–O–Si, M4 type) is the most stable configuration with the adsorption energy of −2.46 eV, which is followed by the Si–O–O–Si (M5 type), C–O–O–Si (M3 type), O–Si–O (M2 type), and Si–O=O (M1 type) configurations with the adsorption energies of −2.24, −1.87, −1.07, and −0.75 eV, respectively. All these molecular O₂ adsorption configurations exhibit high tendency to dissociate with the dissociation barriers range of 0.09–0.19 eV. The adsorbed atomic O seems to be easily trapped at the C–O site due to the extremely low diffusion barrier. In addition, the infrared spectra of all the atomic O and molecular O₂ adsorption configurations are predicted and compared with available experimental observations.     

Phase equilibria of CaO–SiO2–CaF2–P2O5–Ce2O3 system and formation mechanism of britholite

Recently, the sustainable utilization of REE-bearing slag for the recovery and application of rare-earth elements (REEs) has attracted considerable attention. However, a limited amount of thermodynamic data and crystal information for REE systems has been reported, which greatly limits the utilization of REE-bearing slag. In this study, the isothermal phase diagram of the CaO–SiO₂–CaF₂(30 wt%)-P₂O₅(10 wt%)-Ce₂O₃ system was constructed to provide phase equilibria data for the REEs in REE-bearing slag. The formation mechanism of britholite (Ca_5-xCe_x[(Si,P)O₄]₃F) in the quinary system was found: it evolved from Ca₅(PO₄)₃F, when a Ce³⁺ replaced a Ca²⁺, there would be a SiO₄^4? instead of a PO₄^3?. The phase equilibria and formation mechanism of REEs in the CaO–SiO₂–CaF₂–P₂O₅–Ce₂O₃ system are supplied to provide the data required for sustainable utilization of REE-bearing slag.     

Interfacial reaction between magnesia refractory and “FeO”-rich slag: Formation of magnesiowüstite layer

This study investigated the reaction between CaO-SiO₂-Al₂O₃-xFeO-MgO-MnO (CaO/SiO₂?=?1.2, x?=?20–50?wt%) slag and magnesia refractory. Using SEM-EDS analysis, we confirmed the formation of a (Mg,Fe)O_{ss(solid_solution)}, called magesiowüstite (MW), intermediate layer at the slag-refractory interface. MgO dissolved from refractory and reacted with the bulk slag to form MW layer at the interface. Simultaneously, slag penetrated through micro-pores and reacted with the refractory to form MW layer. In other words, the MW layer built up in both directions from initial refractory-slag interface. The thickness of the MW layer increased as the FeO content in the slag increased, and using EDS line scanning, a Mg and Fe concentration gradient was confirmed within the MW layer. The slag, which penetrated into the refractory, had a chemical composition of the CaO-SiO₂-Al₂O₃-MgO system without FeO, indicating that FeO was consumed by forming a MW layer at the refractory hot face. The slag-refractory interfacial reaction was simulated using thermochemical software, FactSage?7.0. The results predicted a MW monoxide composed of MgO and FeO. A spinel phase was formed when FeO was greater than 40?wt%. These thermochemical computations were comparable to our experimental findings.     

Structural dependence of crystallization in phosphorus-containing sodium aluminoborosilicate glasses

The article reports on the structural dependence of crystallization in Na₂O–Al₂O₃–B₂O₃–P₂O₅–SiO₂-based glasses over a broad compositional space. The structure of melt-quenched glasses has been investigated using ¹¹B, ²⁷Al, ²⁹Si, and ³¹P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, while the crystallization behavior has been followed using X-ray diffraction and scanning electron microscopy combined with energy dispersive spectroscopy. In general, the integration of phosphate into the sodium aluminoborosilicate network is mainly accomplished via the formation of Al–O–P and B–O–P linkages with the possibility of formation of Si–O–P linkages playing only a minor role. In terms of crystallization, at low concentrations (≤5 mol.%), P₂O₅ promotes the crystallization of nepheline (NaAlSiO₄), while at higher concentrations (≥10 mol.%), it tends to suppress (completely or incompletely depending on the glass chemistry) the crystallization in glasses. When correlating the structure of glasses with their crystallization behavior, the MAS NMR results highlight the importance of the substitution/replacement of Si–O–Al linkages by Al–O–P, Si–O–B, and B–O–P linkages in the suppression of nepheline crystallization in glasses. The results have been discussed in the context of (1) the problem of nepheline crystallization in Hanford high-level waste glasses and (2) designing vitreous waste forms for the immobilization of phosphate-rich dehalogenated Echem salt waste.     

Structural variations of bioactive glasses obtained by different synthesis routes

Two bioactive glasses with different chemical compositions (mol%) 46.2SiO₂–26.9CaO–24.3Na₂O–2.6P₂O₅ (45S5) and 40SiO₂–54CaO–6P₂O₅ (A2) were synthesized by the use of sol–gel and melt–quenching techniques. The effect of synthesis method on glass structure was investigated using X-ray diffraction, FTIR, Raman, XPS, ²⁹Si and ³¹P MAS–NMR spectroscopic methods. The results show that the synthesis route has significant influence on the glass structure. Both melt–derived A2 and 45S5 glasses exhibit fully amorphous structure, while gel–derived ones, stabilised at 700 °C, reveal the presence of crystalline silicate and phosphate phases. Gel–derived glasses exhibit more polymerized structure compared to melt–quenched ones. Phosphorus is present in the orthophosphate type environment (Q⁰) together with some pyrophosphate (Q¹) species and it does not take part in the formation of Si–O–P bonds. This indicates that phosphorus acts as a glass structure modifier and forms phosphate-rich phase separated from a silica-rich one. The theoretically predicted network connectivity is consistent with the experimental determination only for melt–derived glasses, assuming silicon as the only network former.     

Structural densification of lithium phosphoaluminoborate glasses

Lithium aluminoborate glasses have recently been found to undergo dramatic changes in their short-range structures upon compression at moderate pressure (~1 GPa), most notably manifested in an increase in network forming cation coordination number (CN). This has important consequences for their mechanical behavior, and to further understand the structural densification mechanisms of this glass family, we here study the effect of P₂O₅ incorporation in a lithium aluminoborate glass (with fixed Li/Al/B ratio) on the pressure-induced changes in structure, density, and hardness. We find that P₂O₅ addition results in a more open and soft network, with P-O-Al and P-O-B bonding, a slightly smaller fraction of tetrahedral-to-trigonal boron, and an unchanged aluminum speciation. Upon compression, the cation-oxygen CNs of both boron and aluminum increase systemically, whereas the number of bridging oxygens around phosphorous (Qⁿ) decreases. The glasses with higher P₂O₅ content feature a larger decrease in Qⁿ (P) upon compression, which leads to more non-bridging oxygen that in turn fuel the larger increase in CN of B and Al for higher P₂O₅ content. We find that the CN changes of Al and B can account for a large fraction (around 50% at 2 GPa) of the total volume densification and that the extent of structural changes (so-called atomic self-adaptivity) scales well with the extent of volume densification and pressure-induced increase in hardness.     

Anionic effect of chloride,fluoride, and sulfide ions on the viscosity of slag melt

The effect of the addition of CaX (X=Cl₂, F₂ and S) on the viscous behavior and structure of CaO–SiO₂–Al₂O₃–MgO–CaX slag was investigated by measuring its viscosity. The viscosity of the slag without CaX gradually decreased with an increase in the C/S ratio because of the depolymerization of the silicate groups in the slag. While the viscosity of the CaX‐bearing slag decreased with an increase in the CaX content, depolymerization was not observed in this case. Three distinct compositional regions for the activation energy of the viscous flow were observed because of the effect of the equilibrium of the polymeric silicate groups. The relaxation effect of the CaX groups on the activation energy was also observed. Raman spectroscopic analysis indicated that the relaxation in the viscosity and activation energy by CaX addition stemmed from the breaking of the NBO‐M²⁺‐NBO linkage to form NBO‐M²⁺‐F^?, NBO‐M²⁺‐Cl^?, or M²⁺‐S^2?. All these results are discussed in detail with the help of a viscous flow model based on the ionic interactions.     

Cation field-strength effects on ion irradiation-induced mechanical property changes of borosilicate glass structures

We examine the impact of the glass network-modifier cation field strength (CFS) on ion irradiation-induced mechanical property changes in borosilicate (BS) glasses for the ternary M₂O–B₂O₃–SiO₂ systems with M = {Na, K, Rb} and the quaternary [0.5M₍₂₎O–0.5Na₂O]–B₂O₃–SiO₂ systems with M = {Li, Na, K, Rb Mg, Ca, Sr, Ba}. ¹¹B nuclear magnetic resonance (NMR) experiments on the as-prepared BS glasses yielded the fractional population of four-coordinated B species (B^[4]) out of all {B^[3], B^[4]} groups in the glass network, along with the fraction of B^[4]–O–Si linkages out of all B^[4]–O–Si/B bonds. Both parameters correlated linearly with the (average) CFS of the M⁺ and/or {M⁽²⁾⁺, Na⁺} cations. Both the nanoindentation-derived hardness and Young's modulus values of the glasses reduced upon their irradiation by Si²⁺ ions, with the property deterioration decreasing linearly with increasing M^z+ CFS, that is, for higher M^z+⋅⋅⋅O interaction strength. The irradiation damage of the glass network also increased linearly with the fraction of B^[4]–O–Si linkages, which are the second weakest in the structure after the M^z+⋅⋅⋅O bonds. Our results underscore the advantages of employing BS glasses with high-CFS cations for enhancing the radiation resistance for nuclear waste storage.     

Effect of Niobium Oxide on the Structure and Properties of Melt‐Derived Bioactive Glasses

The effects of adding Nb₂O₅ on the physical properties and glass structure of two glass series derived from the 45S5 Bioglass^® have been studied. The multinuclear ²⁹Si, ³¹P, and ²³Na solid‐state MAS NMR spectra of the glasses, Raman spectroscopy and the determination of some physical properties have generated insight into the structure of the glasses. The ²⁹Si MAS NMR spectra suggest that Nb⁵⁺ ions create cross‐links between several oxygen sites, breaking Si–O–Si bonds to form a range of polyhedra [Nb(OM)_6?y(OSi)_y], where 1 ≤ y ≤ 5 and M = Na, Ca, or P. The Raman spectra show that the Nb–O–P bonds would occur in the terminal sites. Adding Nb₂O₅ significantly increases the density and the stability against devitrification, as indicated by ΔT(T_x ? T_g). Bioglass particle dispersions prepared by incorporating up to 1.3 mol% Nb₂O₅ by replacing P₂O₅ or up to 1.0 mol% Nb₂O₅ by replacing SiO₂ in 45S5 Bioglass^® using deionized water or solutions buffered with HEPES showed a significant increase in the pH during the early steps of the reaction, compared using the rate and magnitude during the earliest stages of BG45S5 dissolution.     

Mechanistic Insights on the Formation of High-Valent MnIII/IV=O Species Using Oxygen as Oxidant: A Theoretical Perspective

High-valent metal−oxo species are of great interest as they serve as a robust catalyst for various organic transformations, and at the same time, they offer significant insight into the reactivity of various metalloenzymes. Formation of Mn−Oxo species is of great interest as they are involved in the Oxygen Evolving Complex of Photosystem II, and various bio-mimic models were synthesized to understand its reactivity. In this context, using urea decorated amine ligands, Borovik et al. have reported the facile formation of Mn^III=O and Mn^IV=O species from [Mn^IIH₂buea]²⁻ (here H2buea=tris[(N′-tert-butylureayl)-N-ethyl]amine) precursor complex using oxygen as the oxidant. While reactivity of these species is thoroughly studied, mechanism of formation of such species is scarcely explored. In this work, we have attempted to establish the formation of these species from the Mn^II precursor using the experimental conditions. Our calculations reveal the following fundamental steps in the formation of such species: i) O₂ activation by Mn^II lead to formation of Mn^II−superoxide species wherein the oxidation state of the Mn^II found to be intact upon O₂ binding facilitated by the deprotonated nitrogen atom present in the cavity (ii) in the second step, superoxo species is converted to Mn^II−hydroperoxo species, [Mn^IIH₂buea(OOH)]²⁻ using dimethylacetamide solvent as source for HAT reaction (iii) presence of water molecule found to aid the O−O bond cleavage in [Mn^IIH₂buea(OOH)]²⁻ species leading to the formation of the putative Mn^III=O species, [Mn^IIIH₃buea(O)]²⁻ (iv) one-electron oxidation of Mn^III=O, leads to the formation of [Mn^IVH₃buea(O)]⁻ species and this step is endothermic and need some external oxidants for its formation. While various spin-states and their roles are explored, our calculations reveal that the Mn atom prefers to be in the high-spin state across the potential energy surface studied. However, the nature of the formation is strongly correlated to the spin state arising from the radical nature present in the O₂ moiety and also in the deprotonated nitrogen atom. This offers a unique multistate reactivity channel for the formation these species easing various kinetic barriers across the potential energy surface. Further, we have also computed the spectral parameters for the experimentally observed species, which are in agreement with the reported data offering confidence on the mechanism established. To this end, our study unveils a facile formation of high-valent Mn−Oxo species using O₂ as oxidant and role of water molecules in the formation of such species, and these mechanistic insights are likely to have implications beyond the example studied here.     

Oxygen permeation mechanism in polycrystalline mullite at high temperatures

The oxygen permeability of polycrystalline mullite wafers, serving as a model environmental barrier coating layer on SiC fiber‐reinforced SiC matrix composites, was evaluated at temperatures above 1673 h with an oxygen tracer gas (¹⁸O₂). Oxygen permeation occurred by grain‐boundary (GB) diffusion of oxygen from the high oxygen partial pressure (high‐Po ₂) surface to the low‐Po ₂ surface, with simultaneous GB diffusion of aluminum in the opposite direction. This GB interdiffusion of both oxygen and aluminum proceeded without acceleration or retardation, maintaining the Gibbs‐Duhem relationship. Oxygen permeation related to the GB diffusion of silicon was negligibly small compared to that generated by aluminum GB diffusion, resulting in decomposition of the mullite near the low‐Po ₂ surface. The GB diffusion coefficients for oxygen in the vicinity of the high‐Po ₂ surface were determined directly from the SIMS‐¹⁸O line profiles along individual GBs, as assessed from cross sections of the exposed wafer. The coefficients thus obtained were comparable to those determined in the absence of an oxygen potential gradient and those calculated from an oxygen permeation trial under the assumption of nearly ionic conductivity.