Bond Angle Distributions Research Articles

Viscosity of molten MgF2-LiF-MgO system and structure investigation using classical molecular dynamics simulations

The effect of temperature and fluoride structure on the viscosity of a molten 32.8MgF2-67.2LiF system with varying MgO concentration was investigated for the environmentally sound electrolytic reduction process of magnesium extraction. Whereas the temperature dependence of the viscosity was not significant, increasing viscosity was observed with the increase in the MgO concentration. Using classical molecular dynamics simulation, the molten fluoride structure in the 32.8MgF2-67.2LiF with varying MgO concentration was investigated at 1123 K. From the result of the radial distribution function and coordination numbers, the formation of a characteristic bonding structure between the Mg-F and O-Mg was verified. The F-Mg-F bond-angle distribution results indicated the formation of a marginally distorted octahedral structure of MgF6 in the present MgF2-LiF-MgO system. Moreover, the Mg-F-Mg bond angle distribution revealed the formation of a three-dimensional MgF6 network structure by sharing one or two fluorine atoms. Owing to the increasing edge-sharing MgF6 structure unit and formation of a larger three-dimensional network associated by the oxygen atoms, the viscosity increased by the addition of MgO in the 32.8MgF2-67.2LiF system.

The effects of calcium content on molecular structure and mechanical properties of sodium aluminosilicate hydrate (NASH) gels by molecular dynamics simulation

The effects of calcium content on the molecular structure and mechanical properties of sodium aluminosilicate hydrate (NASH) gels were studied by molecular dynamics simulations. NASH gels with different calcium content were constructed, followed by optimization, equilibration, and production runs of molecular dynamics simulation. Properties such as energy profile, radial distribution function (RDF), distribution of bond length and bond angle, mean square displacement (MSD), elastic modulus and density were obtained. The results indicated that with the increase of Ca/Al, average bond lengths and total energy decreased monotonically, and the distributions of bond lengths and bond angles became narrower. Results from the MSD analysis showed that diffusion degree of both Si and Al in the gels decreased with the increase of the Ca/Al ratio. Furthermore, the elastic modulus, compressive strength and density of the gel systems increased monotonically with the increase of calcium content, and the trend is consistent with experimental findings.

Molecular Dynamics Simulations of the Interface between Porous and Fused Silica

Molecular dynamics simulations are performed to gain insights into the structural and vibrational properties of interface between porous and fused silica. The Si–O bonds formed in the interface exhibit the same lengths as the bulk material, whereas the coordination defects in the interface are at an intermediate level as compared with the dense and porous structures. Clustered bonds are identified from the interface, which are associated with the reorganization of the silica surface. The bond angle distributions show that the O–Si–O bond angles keep the average value of 109°, whereas the Si–O–Si angles of the interface present in a similar manner to those in porous silica. Despite the slight structural differences, similarities in the vibrations are observed, which could further demonstrate the stability of porous silica films coated on the fused silica.

The structural correlation and mechanical properties in amorphous hafnium oxide under pressure

The classical molecular dynamics (MD) technique was used to investigate the atomistic structure of amorphous hafnium oxide (HfO2) under pressure. The local atomic structure and the liquid-solid transition of HfO2 were analyzed for the pair radical distribution functions, bond angle distributions and coordination number. The simulation reveals that although the fractions of structural units HfO[Formula: see text] and OHf[Formula: see text] strongly change with the density, the partial bond angle distributions of these structural units are almost identical for all constructed models. This result has enabled us to establish a relationship between the bond angle distributions and the fractions of structural units.

Ab Initio Study of Hydrolysis Effects in Single and Ion-Exchanged Alkali Aluminosilicate Glasses

Hydrolysis in alkali-doped aluminosilicate glasses is one of the most complicated mechanisms in glass science. There remain many fundamental and unresolved issues with implications on their potential applications. Herein, we address this challenge by carrying out detailed calculations on the structure and properties of both anhydrate (dry) and hydrated alkali aluminosilicate glasses on carefully constructed models. Specifically, the Na-, (Na + K)- and K-doped aluminosilicate glasses with compositions (SiO2)0.6 (Al2O3)0.2 (Na2O)0.2-x (K2O)x (x = 0, 0.10 and 0.20) are simulated using ab initio molecular dynamics (AIMD). The local short- and intermediate-range order in these glasses are analyzed in terms of atomic pair distribution, coordination number, bond length and bond angle distributions to delineate the subtle variations due to different alkali size and hydrolysis. The electronic structure, interatomic bonding, mechanical and optical properties for these models are calculated and validated with available experimental data. We use the novel concept of total bond order density (TBOD), the quantum mechanically derived metric, to characterize the internal cohesion and strength in the simulated glasses. Detailed analysis of the hydrolysis mechanism enables us to provide information on the complex interplay of various participating elements and their interactions at the atomic level. Such detailed information provides new platform of knowledge which is crucial for understanding the issues related to glass corrosion and durability and ways and means for their special applications in commercial glass products. Both un-dissociated molecular water and dissociated water in the form of hydroxyl group exist in the hydrated models in the presence of alkali ions. For the first time, we observed the opposite mixed alkali effect in the Poisson's ratio for anhydrate and hydrated glasses.

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.

Coadsorption of Na+ and H2O on the surface of hydroxylated silica

ABSTRACT Molecular simulation was used to investigate the coadsorption mechanism of Na+ and H2O on the hydroxylated silicon wafer. Water molecules can form a 3.5 Å-thickness bimolecular structure at the solid–liquid interface. Near the solid surface, the density of water can reach approximately 1300 kg/m3. Na+ does not bond with SiOH directly but adsorbs near SiOH in the form of hydrated ions. The adsorption of Na+ on the surface belongs to the ion hydration outer ring adsorption. The adsorption sites of Na+ are mainly located in the first water molecular layer. In the system, the peak value of the bond length distribution curve of water molecules is 0.98 Å, while the peak value of the bond angle distribution curve is 102°. Due to the polarisation of Na+, the peak value of the bond angle distribution curve of the water molecules decreases, especially at the interface water. Additionally, Na+ can destroy the hydrogen bonds in the system. AFM imaging confirmed that NaCl is adsorbed on the surface in the form of granular aggregates and the adsorption of Na+ by SiOH is weak in aqueous solution.

Structural modifications of soda‐lime silicate glasses using femtosecond pulse‐laser irradiation

AbstractFemtosecond pulse laser radiation (250 fs, 1030 nm) of alkali and alkaline earth modified soda‐lime silicate (SLS) glasses resulted in densification. The degree of density changes depends on pulse energy and glass‐network connectivity. Raman spectroscopy study of the high‐frequency region (800‐1200 cm−1) confirmed that surface densification was the result of interatomic changes in the distribution of silicon‐nonbridging oxygen (NBO) bond lengths and Si‐O‐Si bond angle distribution (BAD). Modifier valency contributes to structural rigidity and the degree of disorder a glass can incur during the deposition of high‐energy laser pulses. Varying soda content caused larger shift in the laser modified Q3 band position and width compared to the calcium modified system. Increasing laser fluence was found to affect the position of Q3 Si‐O stretching mode as well as the band width, indicating the increased degree of disorder laser pulses induce. These results underscore the role of modifier ions in the laser‐induced densification of the SLS glasses.

Effect of interatomic potentials on modeling the nanostructure of amorphous carbon by liquid quenching method

Extending from our previous work (Li et al., 2013), eight different carbon interatomic potentials, Tersoff1989, Tersoff1994, REBO-Ⅱ, AIREBO, ReaxFF, COMB3, REBO-II + S and Tersoff + S, are applied to conduct a comparative study on the nanostructure of amorphous carbon films. The amorphous carbon films at different density are modeled by LAMMPS with liquid quenching method. The atomic structure of a-C film is analyzed in terms of coordination number, radial distribution function, the bond angle distribution and Voronoi polyhedra. The effect of interatomic potentials on atomic cohesive energy and time consumption are analyzed and compared. No two potentials model the a-C film with the same atomic structure. The a-C films modeled by REBO-II + S and Tersoff + S potentials are consistent with experimental data and DFT results at density from 2.2 to 3.2 g/cm3. High sp3 carbon rate is modeled by Tersoff1994, and high sp2 carbon rate is modeled by REBO-Ⅱ and AIREBO. Lot of three-atom-rings are found when COMB3 is used. The sp3 and sp2 carbon rate are similar modeled by Tersoff1989 and ReaxFF. But there are sp1 carbon chains in a-C films modeled by ReaxFF at low density. Those results are conducive for researchers to evaluate and choose of interatomic potentials.

Topology of SiOx-units and glassy network of magnesium silicate glass under densification: correlation between radial distribution function and bond angle distribution

Topology of SiOx units and glassy network of magnesium silicate glass at different pressures are investigated by molecular dynamics simulation to clarify its microstructure under compression. Results show that SiOx-topology and glassy network structure are significantly dependent on pressure. At ambient pressure, the –Si–O– glassy network in Mg2SiO4 glass is split into subnets/clusters. Under compression, the small subnets tend to merge each other forming larger ones. The decrease of Si–O–Si bond angle under compression that accompanies a formation of edge- and face-sharing bonds between SiOx units results in the first peak splitting of Si–Si PRDF at high pressure. In particular, the investigation also reveals a tight correlation between PRDFs (Si–Si, Mg–Mg, Si–Mg, O–O) and BADs (Si–O–Si, Mg–O–Mg, Mg–O–Si, O–T–O (T = Si, Mg)), respectively. The spatial distribution of corner-, edge- and face-sharing bonds is not uniform but forming subnets/clusters. The clusters of face-sharing bonds form rigid particles embedding into mixture clusters of corner- and edge-sharing bonds. Size distribution of subnets/clusters (SiOx-cluster as well as clusters of corner-, edge- and face-sharing bonds) under compression also has been investigated to clarify the intermediate range order. The characteristic change of PRDFs under compression in the relationship with microstructural change and the mechanism of magnesium ions incorporation into –Si–O– network is also discussed in detail.

2D approximant lattice model: A framework for the simulation of amorphous film deposition

A 2D approximant lattice model has been developed for Monte Carlo simulation of amorphous film growth via physical vapor deposition at low temperature. The model allows for structural randomness of the deposited film by introducing a distribution of bond angle and bond length for the bond between incoming and substrate atoms, and it reduces the computational cost by discretizing the simulation domain into lattice points. The model has been tested by investigating the effects of bond angle and bond length ranges and surface relaxation distance on microstructure and roughness evolution of the deposited film. Simulation results show that the increase in bond-angle and bond-length ranges increases the amorphousness of the deposited film, as expected, while surface relaxation distance mainly affects the film density and growth exponent β: the film density increases while β decreases with increasing surface relaxation distance. The advantage of this model is in its ability to model the deposition of both crystalline and amorphous structures in one framework. In addition, its flexibility allows for the incorporation of other growth-process details such as surface diffusion, which may be crucial, depending on the material and the process, as well as extension to 3D for investigating and controlling interfacial roughness in realistic multilayered systems.

First-principles molecular dynamics studying the solidification of Ti-6Al-4V alloy

Solidification process plays a significant role in macroscopic properties of metallic materials. However, the understanding of this process at the atomic level is still far from complete. In this work, for the first time, the first-principles molecular dynamics simulations are carried out to systematically explore the structural evolution and transport properties of Ti-6Al-4V alloy, one of the commonly used alloys for selective laser melting technology, upon cooling from 2400 K to 1300 K. Firstly, the melting range of this alloy is predicted by simulated density to be about 1850–1950 K, which is in good agreement with the experimental value of 1875–1925 K. Afterwards, the local atomic structure variation with temperature is statistically analyzed by structure factors, partial radial distribution functions, coordination number, bond angle distributions, and Voronoi tessellation. It reveals that Ti-6Al-4V alloy experiences a phase transition from 2400 K to 1300 K. The system changes from a liquid dominated by the icosahedral-like cluster to a solid with body-centered cubic structure, where the 〈0, 5, 2, 6〉 and 〈0, 4, 4, 6〉 polyhedra play an important role. Eventually, the diffusion coefficients of Ti, Al, and V are estimated to investigate the kinetic property of liquid Ti-6Al-4V alloy. From 2400 K to 1950 K, the diffusion coefficients of atoms all obey Arrhenius function, and V atom has the largest diffusion coefficient, followed by the Ti atom, which is very close to the self-diffusion coefficient of pure liquid Ti, and finally the Al atom. This abnormal mass dependence of diffusion coefficient is well explained by activation energy. In conclusion, the present work provides insights into the understanding of structural dynamics and the kinetic properties of such alloy melts, which could guide selective laser melting technology to obtain desired microstructure.

The Microstructural Transformation and Dynamical Properties in Sodium-silicate: Molecular Dynamics Simulation

Molecular dynamics simulation of sodium-silicate has been carried out to investigate the microstructural transformation and diffusion mechanism. The microstructure of sodium silicate is studied by the pair radial distribution function, distribution of SiOx (x=4,5,6), OSiy (y=2,3) basic unit, bond angle distribution. The simulation results show that the structure of sodium silicate occurs the transformation from a tetrahedral structure to an octahedral structure under pressure. The additional network-modifying cation oxide breaking up this network by the generation of non-bridging O atoms and it has a slight effect on the topology of SiOx and OSiy units. Moreover, the diffusion of network- former atom in sodium-silicate melt is anomaly and diffusion coefficient for sodium atom is much larger than for oxygen or silicon atom. The simulation proves two diffusion mechanisms of the network-former atoms and modifier atoms.

Study of Structure Transition and Crystallization of Amorphous Silica under Compression

In this work, we use molecular dynamic (MD) simulation to study of the structure transition and crystallization of amorphous silica (SiO2) under compression. The structural evolution of amorphous SiO2 is explained through radial distribution function, coordination number distribution, bond angle distribution and visualization. Simulation result shown that there is a structural transformation from tetrahedral to octahedral network through SiO5 units. In the 5-15 GPa pressure range, structural transformation occurs powerfully and there are three structural phases corresponding to SiO4-, SiO5-, and SiO6- ones. At 15 GPa, octahedral-network (SiO6) is dominant. It is the first time we showed that when pressure is higher than 20 GPa, octahedral-network of amorphous SiO2 has a tendency to transform to stishovite crystalline phase.

Very sharp diffraction peak in nonglass-forming liquid with the formation of distorted tetraclusters

Understanding the liquid structure provides information that is crucial to uncovering the nature of the glass-liquid transition. We apply an aerodynamic levitation technique and high-energy X-rays to liquid (l)-Er2O3 to discover its structure. The sample densities are measured by electrostatic levitation at the International Space Station. Liquid Er2O3 displays a very sharp diffraction peak (principal peak). Applying a combined reverse Monte Carlo – molecular dynamics approach, the simulations produce an Er–O coordination number of 6.1, which is comparable to that of another nonglass-forming liquid, l-ZrO2. The atomic structure of l-Er2O3 comprises distorted OEr4 tetraclusters in nearly linear arrangements, as manifested by a prominent peak observed at ~180° in the Er–O–Er bond angle distribution. This structural feature gives rise to long periodicity corresponding to the sharp principal peak in the X-ray diffraction data. A persistent homology analysis suggests that l-Er2O3 is homologically similar to the crystalline phase. Moreover, electronic structure calculations show that l-Er2O3 has a modest band gap of 0.6 eV that is significantly reduced from the crystalline phase due to the tetracluster distortions. The estimated viscosity is very low above the melting point for l-ZrO2, and the material can be described as an extremely fragile liquid.

Molecular dynamics analysis of the effect of laser and defects on the micro-structure of fused silica

In this work, the classic molecular dynamics simulations are employed to investigate the atomic structural modification of fused silica with defects as laser irradiation. The dynamics evolution of the atomic structure of fused silica is modeled during energy deposition. The structure parameters such as pair distribution functions (PDFs), bond angle distributions (BADs), and the coordination number are given. The calculated results reveal that fused silica undergoes significant changes in terms of Si–O, Si–Si, and O–O bond lengths, Si–O–Si and O–Si–O bond angles, and the Si and O coordination numbers during laser irradiation. The effects of different surface defects on the micro-structure of fused silica are discussed too. The simulation results of molecular dynamics may help to understand the role of defects in the radiation effect of fused silica.

Effect of chain length on structure and dynamics in a melt of semiflexible rings

We investigate structure and dynamics of semiflexible rings in a melt by molecular dynamics simulation. Our research mainly focuses on chain length effect. We observe a power law of the intrachain structure factor of rings at intermediate length scale. The fractal dimensions of 1 and 3 at different length scales are reported. By analytical expression for the intrachain structure factor, the value of fractal dimensions is considered to be related to the features of rod and ring, respectively. With the increase of the chain length, the width of the Gaussian distribution of bond angles and the mean value of bond angles show significant increases. Further, we find that chain length induces ultra-soft sphere-like behavior in radial distribution function and short-range attractive effective potential. By combining this with the separation between self- and collective intermediate scattering function, we propose that the presence of the short-range attractive effective potential is the structural signal of the emergence of interpenetration and cluster in ring polymers. Finally, a significant increase of the decay exponent of diffusion with chain length in the melt of semiflexible rings is found. Interpenetration of semiflexible rings is supposed to be responsible for the large value of exponent.

Geometric configuration of five-coordinated Al and Si in tectosilicate calcium aluminosilicate glasses and its effect on plastic flow

Using molecular dynamics simulations of calcium aluminosilicate glasses, AlV, SiV, and their surrounding O environments were analyzed. Unlike a trigonal bipyramidal structure, which is common for five-coordinated species, the bond angle distributions suggest that AlV and SiV adopt configurations consistent with the range of geometries found in a Berry pseudorotation. This allows AlV and SiV to adopt any configuration between two extremes, which are trigonal bipyramids, with minimal energy cost. It is shown that this range of geometries, in combination with the ease with which Al changes coordination, facilitates mechanisms that lead to plastic deformation or viscous flow. This understanding of the geometry of these five-coordinated network formers, along with knowledge of plastic deformation mechanisms, can be used to explain the effects of pressure, temperature, and aluminum content on the plastic flow behavior of aluminosilicates.

SYNTHESIS OF TWO-DIMENSIONAL Fe-C ALLOY VIA MOLECULAR DYNAMICS SIMULATION

Formation of two-dimensional (2D) Fe-C alloy with square lattice structure from the liquid state is studied via molecular dynamics (MD) simulation. The researchers find that the crystallization of 2D Fe-C alloy exhibits a first-order-like phase transition. Evolution of structural and thermodynamic properties upon cooling from the melt of the model is investigated in details. Structural properties of the Fe50C50 model are investigated via the radial distribution function (RDF), coordination number, interatomic distance, and bond-angle distributions. The researchers find that Fe-Fe distance is 2.62Å, which is close to the value of DFT calculations and experiments. In addition, various types of structural defects are studied such as vacancies of different shapes and rings of several sizes clearly using the visualization’s software Visual Molecular Dynamics (VMD). Moreover, it can be proposed that the 2D Fe-C material would have many important applications in electronics and mechanic devices.

Disorder by design: A data-driven approach to amorphous semiconductors without total-energy functionals

X-ray diffraction, Amorphous silicon, Multi-objective optimization, Monte Carlo methods. This paper addresses a difficult inverse problem that involves the reconstruction of a three-dimensional model of tetrahedral amorphous semiconductors via inversion of diffraction data. By posing the material-structure determination as a multiobjective optimization program, it has been shown that the problem can be solved accurately using a few structural constraints, but no total-energy functionals/forces, which describe the local chemistry of amorphous networks. The approach yields highly realistic models of amorphous silicon, with no or only a few coordination defects (≤1%), a narrow bond-angle distribution of width 9–11.5°, and an electronic gap of 0.8–1.4 eV. These data-driven information-based models have been found to produce electronic and vibrational properties of a-Si that match accurately with experimental data and rival that of the Wooten-Winer-Weaire models. The study confirms the effectiveness of a multiobjective optimization approach to the structural determination of complex materials, and resolves a long-standing dispute concerning the uniqueness of a model of tetrahedral amorphous semiconductors obtained via inversion of diffraction data.