Qn Distribution Research Articles

Temperature effects on the structure of lead metasilicate glass, supercooled liquid, crystal, and liquid: Vibrational anharmonicity and configurational transformations analyzed by molecular dynamics and Raman scattering

Raman spectroscopy and Molecular Dynamics (MD) simulations were employed to explore in detail the effects of temperature on the four stages of the PbO∙SiO2 system: glass, supercooled liquid, crystalline phases, and liquid. The Qn distribution up to 1153 K and the wavenumber temperature coefficients derived from vibrational anharmonicity were determined from the Raman data, whereas the structural transformation reflected in changes in the Qn population distribution up to 2400 K was inferred from MD simulations. The results indicate that reversible configurational transformations occur only in the higher temperature ranges of the supercooled liquid and more notably in the liquid state. During these transformations, the increase in the Q0 and Q1 populations occurs at the expense of the Q3 and Q4 populations. On the other hand, only vibrational anharmonicity was observed in the glass and crystalline states.

Insights on the structure and properties of sodium iron phosphate glasses from molecular dynamics simulations

Iron phosphate glasses are promising nuclear waste forms while more detailed understanding of their structures and structure-property relations are still needed to better design waste glass compositions. In this work we report studies of three series of sodium iron phosphate (NFP) glasses: 60P2O5-(40-x)Fe2O3-xNa2O (x = 0→40), (100–2x)P2O5-xFe2O3-xNa2O (x = 5→17.5) and one with different iron redox ratio, to understand the composition as well as the iron redox effects on the structure and properties of these glasses using molecular dynamics simulations with effective two-body and three-body potentials. Structural analyses, including pair distribution function, bond angle distribution, Qn distribution, and polyhedral connectivity, were performed to obtain in-depth information on short-range and medium-range structural features. The P-O pair distributions showed a first peak splitting with phosphorus-bridging and non-bridging oxygen contributions. This and the average P-O and other cation-oxygen bond distances are in excellent agreement with experiments. The coordination number of P5+ remained four while that of Fe3+ increased from 4.30 to 4.72 with decreasing Fe/Na ratio. Polyhedral linkage analysis showed [PO4] units linked with [PO4] and [FeOx] through corner-sharing while the [PO4]-[FeOx] linkages become dominant for compositions with Fe2O3 larger than 15 mol%. The effect of iron redox ratio on the structure of NFP glasses was also studied and it was found that bond lengths and coordination numbers were not strongly affected, while the reduction of iron introduced higher network distortions, as evident by O-P-O bond angle and Qn distribution. The glass transition temperature (Tg) showed a monotonic increase with Fe2O3 in the first series, in good agreement with experiments, while those of the second series showed a maximum at P2O5 = 82 mol%. Calculated elastic moduli were found to increase with Fe2O3 in the first glass series, which was be explained by the increase of network connectivity, while those of the second series decrease with Fe2O3 due to decrease of P2O5.

A new viscosity model for the high-iron coal slag based on the bond distribution

The coal slag viscosity, intrinsically determined by the slag structure, is the key to the smooth slag tapping process of an entrained flow coal gasifier. However, the slag structure parameters obtained by the experiments (Raman, NMR, and XPS) or by the theoretical calculation (Molecular dynamics simulation and First-principles calculation) are very time-consuming. This study employed thermodynamic modeling as a convenient method to determine the short-range-ordered structure of high-iron coal slag. The viscosity prediction performance of the structure parameters obtained by the thermodynamic modeling was compared with that obtained by the experiments (Raman and XPS). Our results demonstrated that the short-range-ordered structure of coal slag was described as the distribution of the seven bonds determined by thermodynamic modeling. A new structure parameter (RNF/NM) based on the bond distribution was proposed to quantify the network depolymerization degree of slag. The activation energy (Eη) for the viscous flow, which was predicted by RNF/NM, was much more accurate than the structure parameters (Qn distribution, Al coordination, and NBO/(BO + NBO)) that were determined by the experiments, even the effect of Fe3+-tetrahedron was considered. Finally, a novel slag viscosity model as a function of RNF/NM was proposed. This study provides new insights into the structure analysis and viscosity modeling of coal slag by thermodynamic modeling. Additionally, this method can be applied to the viscosity prediction of other slag systems, such as metallurgical slag, glass, etc.

Interpretation of experimental findings on the structure of glass in the CaO–MoO3–P2O5 system using a thermodynamic model including oxidation–reduction equilibria

AbstractThis work extends the thermodynamic model of associated solutions used in the past to describe the structure and properties of glasses to the area of complex multicomponent glasses with polyvalent elements, where it has not been applied until now either due to the absence of Gibbs energies of formation of the necessary compounds or due to oxidation–reduction equilibrium in the presence of a gas phase containing oxygen. While the fitting of unknown Gibbs energies based on experimental data has already been applied to some extent in our previous work, the implementation of redox is, to the best of our knowledge, new. Four concentration series were taken from the published data from the glass-forming ternary system CaO–MoO3−P2O5: A) xMoO3−(0.5–0.75x)CaO−(0.5–0.25x)P2O5; B) xMoO3−(0.5–0.875x)CaO−(0.5–0.125x)P2O5; C) xMoO3−(0.5−x)CaO−0.5P2O5; M) xMoO3−(1−x)P2O, for which the distributions of Qn units were also published (Q denotes the PO4 tetrahedral unit with n bridging oxygens) by the 31P MAS NMR method and the Mo[V]/ΣMo fraction by the ESR method [Černošek et al. (J Solid State Chem 303:122522, 2021); Holubová et al., (J Non-Cryst Solids 607:122222, 2023)]. The following compounds were considered in the TD model: P2O5, CaO, Mo[VI]O3, Ca(PO3)2, Ca2P2O7, (Mo[VI]O2)(PO3)2, (Mo[VI]O2)2(P2O7), (Mo[VI]O2)3(PO4)2, (Mo[V]O)2(PO3)2(P2O7), (Mo[V]O)PO4. All except the hypothetical compound (Mo[VI]O2)3(PO4)2 exist, and their structure is known. Binary phosphate compounds with molybdenum lack Gibbs energies of formation. Therefore, one of the series, namely A, was used to determine these energies by nonlinear regression with the help of a genetic algorithm, without/with redox, and then the distribution of Qn units and the fraction of Mo[V]/ΣMo was predicted for the remaining series. It was found that the distribution of Qn units can be described by the TD model with redox only. During the reduction of molybdenum, the distribution of Qn unit’s changes, and thus also the connectivity of the phosphate network, for example, according to the reactions: (MoO2)2(P2O7)—> 2(MoO)PO4 + 1/2O2, in which Q1—> Q0 and 2(MoO2)(PO3)2—> (MoO)2(PO3)2(P2O7) + 1/2O2 in which Q2—> Q1. Despite the fact that the TD model with redox gives excellent agreement in the case of the Qn distribution, the agreement with the ESR measurements of the Mo[V]/ΣMo ratio is not good. The TD model predicts significantly more pentavalent molybdenum in the glass.

Short range order of glassy KSb5S8 by diffraction, EXAFS, vibrational spectroscopy and DFT calculations

We report on a detailed experimental and simulation study of the short- and medium-range order of a potential phase change material – glassy KSb5S8. On the experimental side, diffraction techniques and EXAFS have been employed to record accurate structural data. Structural models have been generated by fitting multiple datasets simultaneously with the reverse Monte Carlo simulation technique. In addition, density functional theory was employed to study the structure and vibrational modes of selected clusters, representative of the glass structure. Unconstrained RMC simulation runs revealed that the average Sb-S coordination number is 3.18 ± 0.2, thus Sb is mostly threefold coordinated in the glassy state. The fraction of edge and corner sharing SbSn polyhedra and distribution of bridging S atoms (Qn distribution) have also been obtained. Distribution of bridging S atoms around Sb is similar in the crystalline and glassy states. DFT calculations assisted in the identification of a Raman mode at ∼468 cm−1, assigned to hypervalent bonding (quasi-tetrahedral units) in the glass structure.

Comparison of five empirical potential models for aluminosilicate systems: Albite and anorthite as test cases

In this work we have carried out a systematic investigation of the performance of different force fields in correctly reproducing the structure, densities as a function of temperature and the properties (Young's and Bulk moduli and the coefficient of thermal expansion) of both albite and anorthite crystals and glasses. After this first step of comparison, we selected the potential which gave the better overall performances (the Bertani-Menziani-Pedone potential) and investigated the effect of the cooling rate on structure and properties of albite and anorthite glasses and verified the predicting power of MD simulations for the simulation of the coefficient of thermal expansion (CTE) for a series of several aluminosilicate glasses whose compositions go from the albite-like one to the anorthite-like one.The tested interatomic potentials provide reliable short-range order around the constituting cations, but some differences are detected in the medium-range order described by the T-O-T angles (T= network formers cations, Si and Al), the Al/Si distributions and Qn distributions. In general, all the potentials overestimate the CTE because of the high quenching rate used but we have shown that extrapolated properties at room temperature provide a more reliable comparison with experiments. The CTE trend of the albite-anorthite plagioclase glass series is well reproduced and associated with the substitution of Na with Ca ions rather than the Al/Si one. The error associated with the computed CTE values decreases with box dimension and we found that simulation boxes containing 50k atoms are necessary to reliably estimate this property.

Molecular dynamics simulations to structure-properties relationship of MgO−BaO−CaO−Al2O3−B2O3−SiO2 glass-ceramic for intermediate temperature solid oxide fuel cell

The mechanism of MgO inhibiting phase precipitation and phase transformation of BaAl2Si2O8 in MgO−BaO−CaO−Al2O3−B2O3−SiO2 glass-ceramic is systematically investigated and analyzed. The molecular dynamics simulation results show that the addition of MgO makes the transformation of [BO3] to [BO4], bridge oxygen to non-bridge oxygen, and Qn distribution of [SiO4] tetrahedrons from Q3 to Q1, which is confirmed by infrared and Raman spectroscopy analysis of melt-quenched glass. Consequently, the precipitation process of the BaAl2Si2O8 phase is inhibited by the silicate structure transformation from metasilicate (SiO3)2− to pyrosilicate (Si2O7)6− to protosilicate (SiO4)4−, resulting in the BaAl2Si2O8 phase is gradually replaced with MgSiO3, Mg2SiO4, BaMgSiO4, Ba2MgSi2O7 and BaCa2Mg(SiO4)2 phase with increment of the MgO in the MBCASB glass. It means that the Mg ions are more competitive reacting with silicate structure to form the new crystalline phase than Al and Ba ions.

Composition Dependence of the Atomic Structures and Properties of Sodium Aluminosilicate Glasses: Molecular Dynamics Simulations with Reactive and Nonreactive Potentials.

Understanding the composition-structure-property relations of glass materials is essential for their technological applications. In this study, the structures and properties of a series of sodium aluminosilicate glasses with varying Al2O3/Na2O ratios ((35 - x)Na2O-xAl2O3-65SiO2, x = 0, 5, 10, 15, 17.5, 20) covering peralkaline to peraluminous compositions, have been studied by using molecular dynamics simulations with two types of interatomic potentials: a fixed partial charge pairwise potential (Teter) and a reactive diffusive charge reactive potential (DCRP). The short and medium structural features such as bond lengths, coordination numbers, Qn distributions, and ring size distributions were obtained and compared with experimental data. It was found that silicon remained fourfold-coordinated throughout the compositional range, while a noticeable amount of fivefold-coordinated aluminum together with oxygen triclusters (TBO) are present in compositions with higher Al2O3 contents (RAl/Na > 1). In addition, the simulation results from both potentials show a certain level of violation of the Al avoidance rule by exhibiting a non-negligible amount of [AlOx]-[AlOx] polyhedral connections. Neutron and X-ray diffraction structure factors of the simulated glasses were calculated and compared with available experimental data. The mechanical properties, including Bulk, Shear, and Young's modulus, were calculated and found to increase with increasing RAl/Na, in good agreement with the experiments. Correlations of the properties with glass structures as a function of glass compositions and the advantages as well as potential issues of the two sets of potentials in modeling sodium aluminosilicate glasses are discussed in the context of features of glass structures and the prospect of future simulations of glass-water reactions.

Structural features and rare earth ion clustering behavior in lanthanum phosphate and aluminophosphate glasses from molecular dynamics simulations

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 xLa2O3-(100-x)NaPO2 (x = 0–7.5) and xLa2O3-yAl2O3-(100-x-y) NaPO3 (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 Qn 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 PO4 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.

Speciation and polymerization in a barium silicate glass: Evidence from 29Si NMR and Raman spectroscopies

Raman and NMR spectroscopies have long been employed in pursuing quantitative relationships between polymerization of silicate glasses and melts. However, varying assumptions and ambiguities have often meant structural models vary significantly depending on the probe used. This study investigates a 40BaO·60SiO2 glass to determine its polymerization using static and MAS 29Si NMR, and Raman spectroscopies to construct a coherent structural interpretation. The challenges and advantages of each technique are discussed as the structural model of 40BaO­-60SiO2 glass required partial evidence derived from each of these spectroscopies. The Raman scattering cross sections for Q2 and Q3 were found to be 0.85 and 1.22, respectively. The Qn distribution was shown to be 3, 35, 57, 6% (±4%) for Q1 to Q4, respectively. This distribution yields a polymerization consistent with the stoichiometry, Ba0.66SiO2.66, having a O:Si ratio of 2.66. The agreement between all Qn distributions, the composition, the polymerization, and equilibrium constants strongly indicate that this structural interpretation is robust.

High-pressure plastic deformation of lead metasilicate glass accessed by Raman spectroscopy: Insights into the Qn distribution

In this study, lead metasilicate glasses (PbSiO3) were densified at different maximum pressures in a diamond anvil cell (DAC) at room temperature. The densified glass samples were investigated ex-situ by Raman spectroscopy to probe their pressure-induced plastic deformation limit and permanent structural modifications on the Qn distribution. With a high Pb content, this glass exhibits a low elastic limit at ~4 GPa, which is linked to an initial compact structure. Spectral curve fitting of the high-frequency region of the Raman spectra, consisting of symmetric Si-O stretching modes, exhibit subtle Qn population modifications with maximum pressure. This reveals silica network depolymerization where the proportion of non-bridging oxygens (NBO) increases at the expense of bridging oxygens (BO). Possible densification mechanisms are discussed in contrast to those known in other silicate glasses.

Predicting the dissolution rate of borosilicate glasses using QSPR analysis based on molecular dynamics simulations

AbstractQuantitative Structure Property Relationship (QSPR) analysis based on molecular dynamics (MD) simulations is a promising approach for establishing the composition‐property relationships of glasses and other materials with complex structures. A series of 20 borosilicate, aluminosilicate, and boroaluminosilicate glasses have been modeled using MD simulations with recently developed effective potentials. Short‐ and medium‐range structures of these glasses were analyzed and, based on the structural information, QSPR analysis of the initial dissolution rates (r0) of these glasses that were measured at 90°C and pH 9 by using various structural descriptors such as percentage of bridging oxygen species, network connectivity, and average ring size. The structural descriptor, Fnet, which contains both energetic information such as single bond strength and structural information such as cation coordination number and Qn distribution, was also used. It was found that while the overall network connectivity, average ring size and Fnet provide reasonable correlations with r0 of studied glasses, Fnet gives the best correlation among the descriptors. For glasses that show incongruent dissolution, it was found that modification of glass compositions to account for preferential release of modifier cations is necessary to achieve best correlations. The findings were discussed with results of recent studies on evaluating the compositional dependence of glass dissolution behavior using the topological‐constraints‐based models.

Ab initio molecular dynamics simulation of the structural and electronic properties of aluminoborosilicate glass

AbstractIn this study, the structural and electronic properties of aluminoborosilicate glass, which has a wide range of applications in fields such as microelectronics and displays, were examined using ab initio molecular dynamic simulations. Computing models containing 220 atoms correctly described the local structure of the glass. The reliability of the computing models was verified by the consistency between the experimental results, obtained using high‐energy X‐ray diffraction and solid‐state nuclear magnetic resonance, and the simulation results pertaining to structural factors, pair distribution functions, Qn distribution, and elastic properties. The presence of B and Al increased the flexibility and asymmetry of the system, as shown by the bond angle and ring size distributions. Based on the electronic properties, we observed that the introduction of Al and B atoms into the network could also cause covalent interactions with the O atoms, similar to that with Si atoms. However, the Na and Mg atoms still interacted with all kinds of atoms in the network via charge transfer and exhibited highly non‐localized effects on the charge of the network formers. These results extend our understanding of the structure of aluminoborosilicate glass and have guiding significance for improving and designing new types of this glass.

Immobilization of uranium-contaminated soil into glass waste by microwave sintering: Experimental and theoretical study

To dispose of uranium-contaminated soil, the microwave sintering technology was used. A series of uranium-contaminated soils were successfully immobilized via microwave sintering for 30 min. When the amount of uranium ranges from 0 wt % to 1 wt %, glass phase occurred in saline-alkali soil, purple soil and red soil, while glass with presence of a crystalline phase occurred in yellow soil. The FT-IR analysis showed that the uranium-contaminated soils were mainly consist of [SiO4] units. The Qn distribution of Si-O tetrahedrons and network connectivity of 1 wt % uranium doped sintered soils were related to the composition of soils. The uranium ions were almost immobilized and evenly distributed in sintered soils. Theoretical results showed that the structures of uranium doped sintered soils were made of Si-O, Al-O and Fe-O tetrahedrons. The O-Si-O linkage was the most preferred one among all linkages and the network forming ability of Si was higher than Al.

Structures of fluoride containing aluminosilicate low activity nuclear waste glasses: A molecular dynamics simulations study

Fluorine and other halides commonly exist in nuclear waste forms, and due to their volatile nature, halide retention poses an issue affecting waste loading during vitrification. The compositional effect on fluorine incorporation in aluminosilicate glasses is investigated through molecular dynamics simulations. Oxygen and fluorine coordination numbers around glass former and modifier cations, bond angle distributions, and medium range structure features such as Qn distributions, ring size distributions and neutron diffraction structure factors were calculated. It was found that fluorine has higher preference to bond to Ca2+ than to Na+, both in the melt and the glass, and there is no Si-F bond formation in the glass but they do exist in the melt. Consequently, CaO for Na2O substitution can be an effective way to help fluorine retention without significantly changing the glass chemistry. These results thus provide insights on fluorine incorporation in the aluminosilicate nuclear waste glasses and the strategy on how to improve fluorine retention both in the glass and the melt.

Thermal conduction mechanism based on microstructural transformations of molten slag: The role of calcium oxide

The built up of the slag layer is a character of the entrained-flow gasifier, and its thickness distribution and heat transfer characteristics affect the stable operation of gasification, which is closely related to the thermal conductivity of molten slag. A non-stationary hot wire experimental method and a reverse non-equilibrium molecular dynamics simulation method were combined to investigate the thermal conductivity of the SiO2-Al2O3CaO-Fe2O3−MgO molten slag with a mass ratio of CaO ranging from 5 to 25 wt.%. The slag microstructure information of the distribution of oxygen and tetrahedron Qn was calculated by molecular dynamics simulation. A good agreement within the deviation of 18% between the measured and calculated thermal conductivity over the entire operating temperature range is observed, and one can find that thermal conductivity decreases significantly as the CaO content increased. The principal pathway for CaO modifying the network structure is the connection between two tetrahedral structures by generation of the Si-O-Ca and Al-O-Ca through the fracture of Si-O-Si and Si-O-Al. The rise of CaO promotes the formation of non-bridging oxygen (NBO) and the decomposition of bridging oxygen. The existence of NBOs leads to an enhancement of anharmonicity and shortens the interatomic distance in the tetrahedral structure, which aggravates phonon scattering to reduce phonon mean free path, thereby decreasing the thermal conductivity. Since the phonon propagation is restricted to short-range tetrahedron separated by structural disorder, the Q4 structure is the most sensitive to the addition of CaO. An equivalent molar concentration of the Q4 tetrahedron (XQ4) is defined and the approximate positive linear relationships between the thermal conductivities and XQ4 are obtained.

Structural and thermal behavior of 45S5 Bioglass®‐based compositions containing alumina and strontium

AbstractThe present research exposes the influence of 2 mol% of Al2O3 and 2 mol% SrO in 45S5 Bioglass®‐based compositions. Four compositions were produced to elucidate the difference in how both oxides influence structure and thermal behavior separately and their synergy when together. Thermal properties, crystallization tendency, and sintering behavior was evaluated by differential scanning calorimetry, hot stage microscopy, and dilatometry. Changes of medium‐range structures were characterized by Qn distribution of Raman spectroscopy and evaluation of 31P, 27Al, 23Na, and 29Si environment obtained by magic angle spinning nuclear magnetic resonance. Despite Qn distribution was predominantly Q2 in all samples, the composition criteria used enabled improved processing and stabilibity characteristics. The addition of Al2O3 and SrO promoted larger sinterability parameter (Sc) which indicates better sintering behavior, the glass stability against crystallization doubled (KH) compared to 45S5 and the processing window enlarged from 106 to 171.

Structure and Physical Properties of 30MgSO4-(70-x) P2O5-xSm2O3 glasses

Glasses in the series 30MgSO4-(70-x) P2O5-xSm2O3 0.2 x 1.0 mol percent were prepared by single step melt-quench techniques, the structure, physical and optical properties of samarium oxide at different concentrations were studied. The amorphous phase of the samples was verified by X-ray diffraction spectroscopy, the structural units were investigated using Fourier Transform Infrared, Raman and NMR spectroscopy where the spectra reveal the presence of symmetric, asymmetric, torsoional and bending vibration of phosphate groups, the NMR spectra reveals the changes in Qn distribution here discussed, the compositional dependence on physical (density, average molar weight, dielectric constant, Inter-nuclear distance, Field strength, Molar volume, Molar refractivity etc) and optical parameters were determined. In terms of structural modifications, there is variation of density and molar volume, the glass sample has increasing refractive index values with decreasing molar volume content. The luminescence spectra were observed within UV-Vis-NIR region, the absorption spectra of the sample were recorded in the range of 350 to 1800 nm meanwhile, absorption edge the optical band-gap, and E was evaluated. Our result may be advantageous for many optical applications (e.g visible lasers or color displays).

Ionic self-diffusion of Na2O–Al2O3–SiO2 glasses from molecular dynamics simulations

The structure of alkali aluminosilicate glasses (NAS) as a function of the molar ratio of Al/Na (RAl/Na) were investigated by using classical molecular dynamics simulations and validated by comparing with the high energy X-ray diffraction experiments. General structural features, including the pair distribution functions, coordination number, Qn distribution, oxygen species and ring size distributions, were analyzed to elucidate the effects of Al content on the structure of the glasses. By inducing effective rings size as a new parameter, the RAl/Na dependence of the alkali ion diffusion ability below the glass transition temperature were analyzed on the basis of medium range structures instead of the calculated diffusion coefficient. The results suggest that the optimum aluminum content for NAS system exists at around RAl/Na = 1.2 not only with a high ionic self-diffusion but also with a good network connectivity, providing a theoretical guidance for the design and optimization of chemically strengthened glasses.

Effect of CeO2 Doping on the Structure and Properties of Titanium Barium Silicate Glass

Glasses with the composition 50SiO2 · 20TiO2 · 15BaO · 7.5SrO · 7.5CaO doped with the varied content of CeO2 were prepared by conventional melt quenching technique. The effect of CeO2 on the structure of glasses was investigated by X-ray photoelectron spectroscopy and Raman spectroscopy. The density, thermal stability, dielectric properties and acid resistance of glasses were studied by Archimedes method, differential thermal scanning analysis and X-ray diffraction, dielectric measuring as well as the powder method. The results showed that the coordination of Ti, which existed in glasses as [TiO4], did not change. An increase in the content of non-bridging oxygen atoms and a corresponding decrease in the content of bridging oxygens correlated. The depolymerization of the glass network was attributed to an increase in the number of Si–O–Ti bonds and the change of the Qn distribution. With an increase in the CeO2 content, the thermal stability and acid resistance of glass first increased and then decreased, and the dielectric constant and dielectric loss of glass decreased first and then increased.