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Iron oxide is commonly found in natural or industrial glass compositions and can exist as ferrous (Fe2+) or ferric (Fe3+) species, with their ratios depending on glass composition, temperature, pressure and the redox reactions during the glass forming process. The iron redox ratio plays an important role on silicate glass structures and consequently various properties. This work aims to study the effect of iron oxide, and particularly the iron redox ratio, on the structures of borosilicate and boroaluminosilicate glasses using molecular dynamics simulations with newly developed iron potential parameters that are compatible with the borosilicate potentials. The results provide detailed cation coordination states of both iron species and the effect of redox ratio on boron coordination and other structural features. Particularly, competition for charge compensating modifier cations (such as Na+) among the fourfold‐coordinated cations such as B3+, Al3+, and Fe3+ is investigated by calculating the cation–cation pair distribution functions and coordination preferential ratios. The results show that the trivalent ferric ions, with a shorter Fe–O bond distance and better defined first coordiation shell with mainly four‐fold coordination, act as a glass former whereas the divalent ferrous ions mainly play the role of glass modifier. The ferrous/ferric ratio (Fe2+/Fe3+) was found to affect the glass chemistry and hence glass properties by regulating the amount of four‐coordinated boron, the fraction of non‐briding oxygen and other features. The results are compared with available experimental data to gain insights of the complex structures and charge compensation schemes of the glass system. [ABSTRACT FROM AUTHOR]

Network glass structures are commonly characterized by the network formers and their linkage but modifiers can also play an important role on various features of glass structures. In this work, we investigated the effect of cation field strength (CFS) of common modifier cations with large differences of CFS on the structures of aluminoborosilicate glasses by performing molecular dynamics (MD) simulations with recently developed potentials. It was found that modifier cations with higher CFS such as Mg2+ significantly reduced the fraction of fourfold coordinated boron, suggesting that the cations with higher field strength favor nonbridging oxygen generation in the silicate network and are less effective for charge compensation. The findings from our MD simulations are compared with the results from NMR and Raman spectroscopy studies in the literature as well as those from other MD simulations. Insights of the CFS effect on glass structures and the structural role of Mg2+ ions are gained from these simulations results and related discussions. [ABSTRACT FROM AUTHOR]

The structures of a series of Na2O–FeO–Fe2O3–SiO2 melts with Si/Fe = 1, 2, or 3 have been characterized via synchrotron X-ray total scattering using aerodynamic levitation, a containerless technique, paired with laser heating. The melt structure has been simulated using empirical potential structure refinement (EPSR) based on X-ray scattering data and molecular dynamics (MD) simulations with effective partial charge potentials. Pair distribution functions and coordination numbers of cations in the melt were obtained from the data refinement and from the MD simulations. Iron redox and accompanying density were modeled for each composition assuming either (1) oxidized (all Fe3+) or (2) reduced (some Fe2+) to the level predicted for the Ar levitator environment from a literature model. The actual redox of the melts appears to be intermediate, with some Fe2+ but not as much as assumed from the literature model of redox. Comparison of EPSR and MD models at the same redox conditions indicates generally good agreement, and precise values of coordination numbers depend sensitively on the assumptions for cutoff lengths. Stepwise cooling of each melt in the series resulted in formation of magnetite on the free surface of the sample, but no silica-containing crystalline phases were observed. This study provides a comprehensive assessment of coupled factors (composition, redox, density) needed to assess high-temperature in situ structural measurements of simplified iron silicates. [ABSTRACT FROM AUTHOR]

• Molecular dynamics simulations of sodium iron silicate glasses with different compositions and measured redox ratios. • Short and medium range structures obtained and structural role of iron determined from simulation results. • Ferric iron is mainly four-fold coordinated and plays the role of glass former while ferrous iron has higher coordination and plays the role of modifier. • Iron local environments form simulations agree well with those measured from EXAFS measurements. • Glass structure were compared with the melt structure and the effect of temperature and redox ratio studied. Iron oxides are frequently found in natural and industrial glass compositions and can affect various physical and chemical properties of the glasses and their melts. Thus, a fundamental understanding of iron-bearing silicate melts and glasses is of both scientific and technological importance. This study investigates the structures of sodium iron silicate glasses with compositions of NaFeSiO 4 , NaFeSi 2 O 6 , NaFeSi 3 O 8 , and Na 5 FeSi 4 O 12 using molecular dynamics simulations in combination with Extended X-ray Absorption Fine Structure (EXAFS) characterizations. Short and medium range structural features of these glasses support that ferrous (Fe2+) and ferric (Fe3+) ions play the roles of network modifier and network former, respectively, with the Fe oxidation states playing an important role in the polymerization of the glass network. These simulation results agree well with newly measured room temperature EXAFS spectra. The simulated glass structures were also compared to the melts structures with the same composition but different redox ratios. The average coordination numbers of the cations were found to be affected both by the melt temperature and iron redox ratio. [ABSTRACT FROM AUTHOR]

This paper reports detailed short and medium range structural features of lanthanum oxide containing alkaline phosphate and aluminophosphate glasses revealed from molecular dynamics simulations. Two glass series xLa 2 O 3 -(100-x)NaPO 2 (x = 0–7.5) and xLa 2 O 3 -yAl 2 O 3 -(100-x-y) NaPO 3 (0 ≤ x ≤ 5 and 5 ≤ y ≤ 15) were studied and the cation local environments as a function of compositions in terms of partial distribution functions, bond angle distributions and oxygen coordination number investigated. It was found that La-O has a bond distance of ∼x223C2.44 Å and average coordination number 6.5-7, consistent with experimental findings based on neutron and X-ray diffractions. Medium range structure feactures such as Q n distribution, network connectivity and polyhedral linkages were studied. The clustering behavior of lanthanum ions in the glasses were investigated in detail and it was found that La3+ ions are mainly surrounded by PO 4 units in the second coordination shell that prevented La3+ ion from clustering. As a result, the addition of alumina does not significantly change the clustering behavior. [ABSTRACT FROM AUTHOR]

• Comparative study of MD simulations of borosilicate and aluminoborosilicate glasses by using three recently developed empirical potentials. • Systematic comparison made possible by selecting a series of ~20 borosilicate glasses with fixed K but varying R values to evaluate capability of potentials in wide composition ranges. • Boron coordination (B N4), bond distance, bond angle distribution for short range structure evaluation while ring size distribution and others for medium range structures. • Mechanical property prediction from the potential were compared with experimental data. • Valuable information of the potential behaviors of three sets of potentials were provided that will help future developments and improvements of boron potentials. Despite their practical importance, atomistic modeling of B 2 O 3 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 [SiO 2 ]/[B 2 O 3 ]) =2 and varying R (ratio of [Na 2 O]/[B 2 O 3 ] 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 N 4 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. Image, graphical abstract [ABSTRACT FROM AUTHOR]