Medium-range Structure Research Articles

Revealing the medium-range structure of glassy silica using force-enhanced atomic refinement

The medium-range order structure of silicate glasses remains poorly known as it is not directly visible from conventional experiments. In turn, although atomistic simulations offer a direct access to the structure of glasses, they face several limitations, e.g., extremely high cooling rates. Here, we adopt the force-enhanced atomic refinement (FEAR) method to overcome these limitations and reveal the atomic structure of glassy silica, both at the short- and medium-range length scales. We find that FEAR yields a glass structure that simultaneously exhibits higher thermodynamic stability and enhanced agreement with experimental structure data as compared with molecular dynamics and reverse Monte Carlo simulations. Overall, we show that the increased stability enabled by FEAR primarily arises from the fact that the generated atomic configuration exhibits a more ordered medium-range structure and a lower fraction of unstable small silicate rings.

Al addition on the short and medium range order of CuZrAl metallic glasses

Appropriately addition of Al element can significantly enhance CuZr binary system mechanical properties and glass forming ability (GFA). In this work, the role of minor addition of Al atoms in CuZrAl metallic glasses is investigated by molecular dynamics (MD) simulation. The short and medium range structure of Cu50Zr50 and Cu45Zr50Al5 are systematically investigated from the liquid at 1600 K to amorphous solid at 100 K. Our results show that Al atoms tend to act as the centers of icosahedron (ICO), and it can remarkably increase the fraction of full ICO. More importantly, Al atoms can significantly facilitate the formation of Bergman medium range order, which accounts for the higher GFA of CuZrAl metallic glasses. The enhancement mechanism of Al addition on Bergman MRO can be explained by its adhesive effect on icosahedral clusters.

The Structure of Gd3+-Doped Li2O and K2O Containing Aluminosilicate Glasses from Molecular Dynamics Simulations

Understanding the atomic structure of glasses is critical for developing new generations of materials with important technical applications. In particular, the local environment of rare-earth ions and their distribution and clustering is of great relevance for applications of rare earth-containing glasses in photonic devices. In this work, the structure of Gd2O3 doped lithium and potassium aluminosilicate glasses is investigated as a function of their network modifier oxide (NMO–Li2O, K2O) to aluminum oxide ratio using molecular dynamics simulations. The applied simulation procedure yields a set of configurations, the so-called inherent structures, of the liquid state slightly above the glass transition temperature. The generation of a large set of inherent structures allows a statistical sampling of the medium-range order of the Gd3+ ions with less computational effort compared to other simulation methods. The resulting medium-range atomic structures of network former and modifier ions are in good agreement with experimental results and simulations of similar glasses. It was found that increasing NMO/Al ratio increases the network modifier coordination number with non-bridging oxygen sites and reduces the overall stability of the network structure. The fraction of non-bridging oxygen sites in the vicinity of Gd3+ ions increases considerably with decreasing field strength and increasing concentration of the network modifier ions. These correlations could be confirmed even if the simulation results of alkaline earth aluminosilicate glasses are added to the analysis. In addition, the structure predictions generally indicate a low driving force for the clustering of Gd3+. Here, network modifier ions of large ionic radii reduce the probability of Gd–O–Gd contacts.

Comprehensive understanding of the microstructure and volatilization mechanism of fluorine in silicate melt

Fluorine plays an important role in glass, ceramic, metallurgy, welding, etc. Few studies have investigated the role of fluorine in the molten aluminium-silicate slags. Herein, classical molecular dynamics (MD) simulation combined Raman spectroscopy technique was employed to investigate the influence of Al2O3/SiO2 (A/S) ratio on short- and medium-range order structures of molten CaO-SiO2-Al2O3-Na2O-F (CSANF) slags. Then, partial least squares (PLS) modeling was used to explore the latent relationship between the microstructure and diffusion coefficient of ions was further analyzed. The results show that in the molten CSANF slags, the F− mainly coordinates with Na+ and Ca2+, and the coordination of Ca2+ with F− is most stable. The volatilization of fluoride in the molten CSANF system can be prevented by increasing Ca2+, reducing Na+, and reducing the polymerization of Si-O network. Collectively, this study provided a comprehensive understanding of the microstructure and volatilization mechanism of fluorine in silicate melt.

Effect of Na Ions on Melt Structure and Viscosity of CaO–SiO<sub>2</sub>–Na<sub>2</sub>O by Molecular Dynamics Simulations

Molecular dynamics (MD) simulations have been used to study the effect of Na ions on the structure properties of CaO–SiO2–Na2O melts. The short-range structure and medium-range structure of CaO–SiO2–Na2O in this study are consistent with existing data. The replacement of Ca2+ with Na+ in CaO–SiO2–Na2O melts has almost no effect on the degree of polymerization and distribution of bond angles of Si–O tetrahedron. From micro perspective, Na ions enhance the mobility of CaO–SiO2–Na2O melts by multiple ways. Firstly, the modification effect of Na+ on the melt network structure is weaker than that of Ca2+, the Si–O tetrahedron around Na+ is sparser than Ca2+, which is more conducive to ions movement. Secondly, the diffusion capacity of Na+ is much greater than other ions in CaO–SiO2–Na2O system, which the overall diffusion capacity of the system can be improved by adding more Na+. Thirdly, since Na+ has only one charge, there is no electrostatic restraint on the depolymerized tetrahedron which happened in multivalent charges such as Ca2+, so that the mobility of CaO–SiO2–Na2O is stronger than that of CaO–SiO2. The micro changes provide an explanation for the improvement of macro liquidity.

Ab-initio simulation studies of chromium solvation in molten fluoride salts

Understanding molten salt chemistry is essential in ongoing research of the molten salt nuclear reactor (MSR). In this context, detailed understanding of the mechanisms underlying selective oxidation of metal species, such as Cr, is required to guide the design of effective corrosion mitigation strategies in molten salts. An important starting point for such mechanistic understanding is knowledge of the solvation structure and its role in controlling metal speciation. In this work, we use ab initio molecular dynamics simulations to study the short-range (on the scale of the nearest-neighbor bond lengths) and medium-range (over length scales of several neighbor spacings) structure in three different fluoride melts with and without Cr addition; namely, 2KF-NaF, 2LiF-BeF2, and 3LiF-AlF3. We find that Cr0,Cr2+,Cr3+ can each be coordinated by different numbers of F-, with the variance in coordination number decreasing as oxidation state increases, and that these coordination geometries are largely independent of solvent. The manner by which Cr changes the medium-range structure, however, is found to be solvent-dependent. While 2KF-NaF melts show short and medium range order that is highly dynamic, 2LiF-BeF2 and 3LiF-AlF3 are characterized by molecular associates that are relatively long-lived that organize into oligomer structures on larger length scales. Rather than being solvated by F- ions alone, we find that Cr can incorporate into and be solvated within this oligomer structure. Fluoroacidity, alone, may therefore prove too simple a metric for assessing the corrosivity of molten fluorides. As our work suggests, the ability of Cr to solvate must be understood in the context of the short- and medium-range structure of the solvent.

A Comprehensive Investigation on the Microstructure and Thermal Conductivity of CaO-Al2O3 Based Mold Slags: Equilibrium Molecular Dynamics Simulations

The microstructure and thermal conductivity of the CaO-Al2O3-based (CA-based) mold slags are crucial for casting high aluminum steel. Herein, the microstructure and thermal conductivity of the CA-based mold slags were systematically investigated by using molecular dynamics (MD). Moreover, the effects of the CaO/Al2O3 mole (C/A) ratio and CaO/SiO2 mole (C/S) ratio on the microstructure and thermal conductivity of CA-based mold slags were studied. The results showed that the Al-O structure plays a skeleton role in the CA-based mold slags. The thermal conductivity of molten CA-based mold slags increases first and then decreases with increasing C/A ratio. The thermal conductivity of molten CaO-Al2O3-SiO2-MgO-Na2O (CASMN) slag is affected by the combination of short- and medium-range order structures.

New Interpretation of X-ray Diffraction Pattern of Vitreous Silica

The striking feature of X-ray diffraction pattern of vitreous silica is that the center of its intense but broad ring is located at nearly the same position as the strongest diffraction ring of β-cristobalite. Two fundamentally different explanations to the diffraction patterns were appeared about 90 years ago, one based on the smallest crystals of β-cristobalite and the other based on the non-crystalline continuous random network. This work briefly outlines the facts supporting and objecting these two hypotheses, and aims to present a new interpretation based on a medium-range ordering structure on the facets of clusters formed in the glass transition process. It will be shown that the new interpretation provides a more satisfactory explanation of the diffraction pattern and physical properties of silica glass, and offers considerable valuable information regarding the nature of glass and glass transition.

Cryogenic- and ultrasonic-induced local martensite-like order in Cu47.5Zr45.5Al5Er2 bulk metallic glasses

This study analyzed the influence of low-temperature elastic ultrasonic stimulus on the local structural order of Cu47.5Zr45.5Al5Er2 amorphous alloy, using the ultrasonic pulse-echo technique with a longitudinal wave frequency of 10 MHz, between 100 K–300 K under consecutive cryogenic thermal cycles. Ultrasonic attenuation and wave velocity data of Cu47.5Zr45.5Al5Er2 sample revealed an evolution of the anomalies observed between cooling and heating cycles, being the progressive changes in the local amorphous structure related to the forming and grown of the locally ordered structures induced by the consecutive cryogenic and ultrasonic waves cycles. After the Cu47.5Zr45.5Al5Er2 sample maintained free of mechanical stress at RT for a week, their anelastic response exhibited the full recovery of the as-received condition ultrasonic behavior, denoting the metastable nature of the short- and medium-range order structures originated from the thermomechanical cycles. The amorphous sample ultrasonic response showed attenuation peaks compatible with those observed in a Cu36Zr59Al5 crystalline sample, ass-ociated with a typical reversible martensitic transformation between B2 CuZr to B19’ and B33 CuZr phases.

Probing Medium-Range Order in Oxide Glasses at High Pressure.

Densification in glassy networks has traditionally been described in terms of short-range structures, such as how atoms are coordinated and how the coordination polyhedron is linked in the second coordination environment. While changes in medium-range structures beyond the second coordination shells may play an important role, experimental verification of the densification beyond short-range structures is among the remaining challenges in the physical sciences. Here, a correlation NMR experiment for prototypical borate glasses under compression up to 9 GPa offers insights into the pressure-induced evolution of proximity among cations on a medium-range scale. Whereas amorphous networks at ambient pressure may favor the formation of medium-range clusters consisting primarily of similar coordination species, such segregation between distinct coordination environments tends to decrease with increasing pressure, promoting a more homogeneous distribution of dissimilar structural units. Together with an increase in the average coordination number, densification of glass accompanies a preferential rearrangement toward a random distribution, which may increase the configurational entropy. The results highlight the direct link between the pressure-induced increase in medium-range disorder and the densification of glasses under extreme compression.

Viscosity-temperature relation based on the evolution of medium-range structures of silica

The viscosity of glass is the most important technological property to glass manufactures and various applications. Practically, finding an accurate equation to express the glass viscosity behavior in the entire glass transition temperature range is tremendously challenge because it spans more than ten orders of magnitude. After a brief review of existing empirical viscosity equations, this work focuses on the correlating silica viscosity behavior with the evolution of glass medium-range structure, based on the recently proposed nanoflake model. From this new model, a new equation is constructed, which correctly describes the Arrhenius-type behavior of silica viscosity η above the melting temperature Tm, and non-Arrhenius-type behavior from Tm to a critical temperature Tc. At temperature lower than Tc, the equation predicts Arrhenius-type behavior again for the viscous flow with increased activation energy. The new equation agrees with experimental data in the entire temperature spanned from extremely high to extremely low. The application of the new equation is shown to extend to sodium silicate glasses as well.

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.

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.

Hierarchical Short- and Medium-Range Order Structures in Amorphous GexSe1–x for Selectors Applications

In the upcoming process to overcome the limitations of the standard von Neumann architecture, synaptic electronics is gaining a primary role for the development of in-memory computing. In this field, Ge-based compounds have been proposed as switching materials for nonvolatile memory devices and for selectors. By employing the classical molecular dynamics, we study the structural features of both the liquid states at 1500K and the amorphous phase at 300K of Ge-rich and Se-rich chalcogenides binary GexSe1-x systems in the range x 0.4-0.6. The simulations rely on a model of interatomic potentials where ions interact through steric repulsion, as well as Coulomb and charge-dipole interactions given by the large electronic polarizability of Se ions. Our results indicate the formation of temperature-dependent hierarchical structures with short-range local orders and medium-range structures, which vary with the Ge content. Our work demonstrates that nanosecond-long simulations, not accessible via ab initio techniques, are required to obtain a realistic amorphous phase from the melt. Our classical molecular dynamics simulations are able to describe the profound structural differences between the melt and the glassy structures of GeSe chalcogenides. These results open to the understanding of the interplay between chemical composition, atomic structure, and electrical properties in switching materials.

Intra- and Interchain Interactions in (Cu1/2Au1/2)CN, (Ag1/2Au1/2)CN, and (Cu1/3Ag1/3Au1/3)CN and Their Effect on One-, Two-, and Three-Dimensional Order.

Mixed-metal cyanides (Cu1/2Au1/2)CN, (Ag1/2Au1/2)CN, and (Cu1/3Ag1/3Au1/3)CN adopt an AuCN-type structure in which metal-cyanide chains pack on a hexagonal lattice with metal atoms arranged in sheets. The interactions between and within the metal-cyanide chains are investigated using density functional theory (DFT) calculations, 13C solid-state NMR (SSNMR), and X-ray pair distribution function (PDF) measurements. Long-range metal and cyanide order is found within the chains: (−Cu–NC–Au–CN−)∞, (−Ag–NC–Au–CN−)∞, and (−Cu–NC–Ag–NC–Au–CN−)∞. Although Bragg diffraction studies establish that there is no long-range order between chains, X-ray PDF results show that there is local order between chains. In (Cu1/2Au1/2)CN and (Ag1/2Au1/2)CN, there is a preference for unlike metal atoms occurring as nearest neighbors within the metal sheets. A general mathematical proof shows that the maximum average number of heterometallic nearest-neighbor interactions on a hexagonal lattice with two types of metal atoms is four. Calculated energies of periodic structural models show that those with four unlike nearest neighbors are most favorable. Of these, models in space group Immm give the best fits to the X-ray PDF data out to 8 Å, providing good descriptions of the short- and medium-range structures. This result shows that interactions beyond those of nearest neighbors must be considered when determining the structures of these materials. Such interactions are also important in (Cu1/3Ag1/3Au1/3)CN, leading to the adoption of a structure in Pmm2 containing mixed Cu–Au and Ag-only sheets arranged to maximize the numbers of Cu···Au nearest- and next-nearest-neighbor interactions.

OSDA-free hydrothermal conversion of kenyaite into zeolite beta in the presence of seed crystals

Zeolite beta with high crystallinity has been successfully synthesized by using kenyaite conversion method with seed crystals under hydrothermal treatment at 125 °C for 36 h. The OSDA-free synthesis of zeolite beta could be obtained from the mixture composition of 0.96 NaOH: SiO2: 0.0167 Al2O3: 180 H2O by adding 25 wt % seeds. To study the crystallization behaviours and the medium-range structure changes during the conversion process, the prepared zeolite beta was characterized by powder X-ray diffraction, field-emission scanning electron microscope, Fourier transform infrared spectroscopy, energy dispersive spectroscopy, solid-state 27Al MAS and 29Si MAS NMR spectroscopy. The growth of zeolite beta happened on the surface of kenyaite. The five-membered rings and six-membered rings in kenyaite were partly preserved and the four-membered rings were formed by the connection of five-membered rings during the crystallization process. Meanwhile, the effects of reaction time, the initial mixture composition, the quantity of beta seed crystals and temperature in the synthesis were investigated. The as-synthesized zeolite beta, with relatively high thermal stability, exhibits good catalytic performance in the cyclohexanol dehydration reaction system.

Influence of short- to medium-range electronic and atomic structure on secondary relaxations in metallic glasses

Unusual secondary relaxations were revealed in metallic glasses (MGs) recently. In this paper, we explore the structural origins of secondary relaxations in La-based MGs alloyed with different species of elements. With the combination of synchrotron X-ray diffraction and ab initio molecular dynamics simulations, solute-atom-centered clusters with a string-like type of medium-range order are found in the MGs, the formation of which leaves dispersed low-electron-density regions. It is found the activation energy of fast secondary relaxation increases with the reduction of low-electron-density regions, while slow secondary relaxation relates to the distance of La in next nearest La-La atomic pairs and the size of the string-like solute-atom-centered clusters. The phenomena are interpreted within the framework of the generalized Maxell model and free volume model. Our results demonstrate fast secondary relaxation as the activation of a small concentration of liquid-like regions with extremely low viscosities preceding slow secondary relaxation, and provide evidence for the correlation of secondary relaxations with short- to medium-range electronic and atomic structure in MGs.

Icosahedral order in liquid and glassy phases of cyclohexane

ABSTRACT We performed all-atom molecular dynamics simulations for bulk cyclohexane and analysed the short- and medium-range structures in supercooled and glassy states by using the Voronoi tessellation technique. From the analyses of both the potential energy of the system and the radial distribution function of molecules, cyclohexane was found to be vitrified as the temperature decreased. Furthermore, the icosahedral-like local structures are dominant at all temperatures and grow in a supercooled liquid, whereas the face-centred cubic structures do not grow when the temperature decreases. It was also ascertained that the icosahedral-like structure is more dominant than the full-icosahedral one. The network of the distorted icosahedron spreads throughout the system at low temperatures. Our simulation demonstrates the stability of the icosahedral local structure even in a non-spherical molecule such as cyclohexane.

On the equivalence of vapor-deposited and melt-quenched glasses.

Vapor deposition can yield glasses that are more stable than those obtained by the traditional melt-quenching route. However, it remains unclear whether vapor-deposited glasses are "allowable" or "forbidden," that is, if they are equivalent to glasses formed by cooling extremely slowly a liquid or if they differ in nature from melt-quenched glasses. Here, based on reactive molecular dynamics simulation of silica glasses, we demonstrate that the allowable or forbidden nature of vapor-deposited glasses depends on the temperature of the substrate and, in turn, is found to be encoded in their medium-range order structure.

Hydrothermal conversion of kenyaite into zeolite omega in tetramethylammonium cations system

Zeolite omega has been synthesized through recrystallization of kenyaite. The X-ray diffraction, scanning electron micrograph, vibrational spectroscopy, 27Al magic angle spinning nuclear magnetic resonance (MAS NMR), and 13C cross-polarization (CP)/MAS NMR are used to explore the changes of medium-range structure and other crystallization behaviors. Parts of the 5 membered rings and 6 membered rings are remained as secondary building units (SBUs) of zeolite omega and the 4 membered rings are formed during the reaction period from 10 h to 48 h. Zeolite omega with high purity and crystallinity could be prepared from the substrates with molar composition: 3.6 Na2O: Al2O3: x SiO2: y H2O: 0.96 TMABr (x = 15–20, y = 120–220) by heating at 90 °C for 72 h, 100 °C for 48 h and 110 °C for 44 h.