Partial Static Structure Factors Research Articles

Chemically ordered structure and dynamics in Al80Ti20 liquids

We conducted ab initio molecular dynamics simulations to study the structural and dynamical properties in Al80Ti20 liquids. The prepeak that represents some chemically ordered structure, emerges in the partial static structure factors at low temperatures. By calculating coordination numbers, we found a preferred connection for Al–Ti pairs, which induces an excess Ti/Al concentration around the central Al/Ti atoms. An excess Ti concentration is even found in the second neighbour shells around Ti atoms, rationalizing the prepeak as the result of chemical concentration fluctuation in these neighbour shells. Concomitantly, slow structural relaxation occurs below 1400K, exhibited as the second step relaxation in self-intermediate scattering function. The slow dynamics induces a non-Arrhenius transition for the temperature-dependent diffusivity and structural relaxation time, as well as the breakdown of the Stokes–Einstein relation. These results unveil the detailed ordering structure for the chemical interaction and the associated dynamical behaviour in Al80Ti20 liquids.

FPMD studies on the microstructures and transport properties of molten MgCl2-NaCl-KCl with addition of active metals

A purification protocol of molten MgCl2-NaCl-KCl (MNK) by adding active metals has been proposed to reduce the corrosion of alloy materials in the application of generation III concentrating solar power plants. In this work, the ionic transport properties and micro structural characteristics of metals (Mg, Na, and K) and molten MNK mixtures (metal-MNK) are investigated at 723 K by first principles molecular dynamics (FPMD) simulations. According to the evolution of total energy, the Mg-MNK is the most stable mixture, while the Na-MNK mixture has larger energy fluctuations resulting in a higher specific heat capacity. Studies have shown that the small amount of metal existing in the molten MNK has almost no effect on the ionic diffusion order, but total diffusion coefficients and thermal conductivities of metal-MNK mixtures are improved. Meanwhile, the intensity of diffracted light of molten MNK is weakened by metals, and the microstructures of molten MNK added with alkali metals are more susceptible than that with metallic Mg by analyzing the total and partial static structure factors. Furthermore, the intermediate range orders of metal-MNK are demonstrated, especially for Mg-Mg pairs hinting the formation of the Mg2+-void-Mg2+ structure or Mg2+-centered clusters. Overall, the FPMD results deeply elaborate the dynamic interactions of molten MNK and metallic particles, which give insights for the development of salt purification and corrosion inhibition technology.

Quasicrystal formation in binary soft matter mixtures

Using a strategy that may be applied in theory or in experiments, we identify the regime in which a model binary soft matter mixture forms quasicrystals. The system is described using classical density functional theory combined with integral equation theory. Quasicrystal formation requires particle ordering with two characteristic lengthscales in certain particular ratios. How the lengthscales are related to the form of the pair interactions is reasonably well understood for one component systems, but less is known for mixtures. In our model mixture of big and small colloids confined to an interface, the two lengthscales stem from the range of the interactions between pairs of big particles and from the cross big-small interactions, respectively. The small-small lengthscale is not significant. Our strategy for finding quasicrystals involves tuning locations of maxima in the dispersion relation, or equivalently in the liquid state partial static structure factors.

Dynamics Study of Superionic Conducting Glass Na3PS4 Using Quasi‐Elastic Gamma‐Ray Scattering: Analysis Based on Diffraction and Reverse Monte Carlo–Density Functional Theory Modeling

Superionic conducting glass Na3PS4 has been studied as a candidate of electrolyte for all‐solid‐state batteries. However, the microscopic dynamics of relaxation units, Na and PS4 ions, in Na3PS4 glass is not known despite its essential importance to the ionic conduction mechanism. Momentum‐transfer dependence data of the experimental intermediate scattering function values for Na3PS4 glass is measured at subpicosecond and nanosecond time scales, respectively, by a quasi‐elastic gamma‐ray scattering experiment using time‐domain interferometry. The dynamics data is analyzed based on Faber–Ziman partial static structure factors that are obtained using a model glass structure constructed from the combination of density functional theory calculation and reverse Monte Carlo modeling for diffraction spectra. The analysis successfully explains the experimental result and reveals the vibrational and angstrom‐scale translational motion of respective relaxation units, i.e., Na and PS4 ions in the unique time scales: the translational motion of some Na ions occurs at picosecond to nanosecond time scale, whereas the movement of PS4 ions rarely occurs in the superionic conducting Na3PS4 glass.

A comparative study of thermodynamic, conformational, and structural properties of bottlebrush with star and ring polymer melts.

Thermodynamic, conformational, and structural properties of bottlebrush polymer melts are investigated with molecular dynamics simulations and compared to linear, regular star, and unknotted ring polymer melts to gauge the influence of molecular topology on polymer melt properties. We focus on the variation of the backbone chain length, the grafting density along the backbone, and the length of the side chains at different temperatures above the melt glass transition temperature. Based on these comparisons, we find that the segmental density, isothermal compressibility, and isobaric thermal expansion of bottlebrush melts are quantitatively similar to unknotted ring polymer melts and star polymer melts having a moderate number ( f = 5 to 6) of arms. These similarities extend to the mass scaling of the chain radius of gyration. Our results together indicate that the configurational properties of bottlebrush polymers in their melt state are more similar to randomly branched polymers than linear polymer chains. We also find that the average shape of bottlebrush polymers having short backbone chains with respect to the side chain length is also rather similar to the unknotted ring and moderately branched star polymers in their melt state. As a general trend, the molecular shape of bottlebrush polymers becomes more spherically symmetric when the length of the side chains has a commensurate length as the backbone chain. Finally, we calculate the partial static structure factor of the backbone segments and we find the emergence of a peak at the length scales that characterizes the average distance between the backbone chains. This peak is absent when we calculate the full static structure factor. We characterize the scaling of this peak with parameters characterizing the bottlebrush molecular architecture to aid in the experimental characterization of these molecules by neutron scattering.

Structure factors in a two-dimensional binary colloidal hard sphere system

ABSTRACTHard disks are one of the simplest interacting many-body model system in two dimensions (2D). Here, we present a comprehensive set of measurements of the static structure factors for quasi-2D monodisperse fluids and two different binary colloidal hard sphere mixtures: a small size ratio (SSR) system with a negligibly small negative non-additivity and a large size ratio system with a significantly larger non-additivity. We compare the experimental results for the monodisperse and SSR systems to those calculated using density functional theory (DFT) for additive mixtures. Furthermore, we determine the zero-wavevector limits of the static structure factors for the monodisperse and binary hard sphere fluids directly from an analysis of number and concentration fluctuations. For the monodisperse case, this leads to the isothermal compressibility, which agrees very well with DFT, and is consistent with the scaled particle theory equation of state for hard disks. For the binary fluids, the partial static structure factors are used to calculate the Bhatia–Thornton structure factors, and we find qualitative agreement with DFT for the SSR mixture. Finally, the zero-wavevector limits of the Bhatia–Thornton structure factors are determined and directly related to the thermodynamic factor, the dilatation factor and the isothermal compressibility.

Partial structure factors reveal atomic dynamics in metallic alloy melts

We investigate the dynamical decoupling of the diffusion coefficients of the different components in a metallic alloy melt, using a combination of neutron diffraction, isotopic substitution, and electrostatic levitation in Zr-Ni melts. We show that excess Ni atoms can diffuse more freely in a background of saturated chemical interaction, causing their dynamics to become much faster and thus decoupled than anticipated from the interparticle interactions. Based on the mode-coupling theory of the glass transition, the averaged structure as given by the partial static structure factors is able to explain the observed dynamical behavior.

First-principles molecular dynamics modeling of UCl3 in LiCl-KCl eutectic

The first principles molecular dynamics (FPMD) simulations are applied to the study of LiCl-KCl eutectic and UCl3 in LiCl-KCl eutectic. The densities of the molten salt mixtures are optimized by FPMD simulations in NVT ensemble, and the local coordination structures are evaluated in terms of radial distribution functions and partial static structure factors. Both self-diffusion coefficients and ion conductivities are evaluated from the mean square displacements using Einstein relation for diffusion coefficients. It is conformed that the FPMD simulations give satisfactory structural and transport characteristics compared with experimental observations. Therefore, the predictability of FPMD simulations for structural and transport characteristics of molten salt mixtures is confirmed, and the FPMD simulations can be useful measurement in the establishment of database of actinide behavior in molten salt mixtures essential for the pyroprocessing of spent nuclear fuels.

On the First Principles Calculation of Redox Potential in Molten LiCl-KCl Eutectic Based on Adiabatic Substitution

A first principles approach based on adiabatic substitution is proposed to evaluate the redox potentials of a series of solute cations Mm + (Cu+, Au+, Tl+, Nd2+, Sc3+, Y3+, Ce3+, Pr3+, Nd3+, Gd3+, Tb3+, U3+, Pu3+, Np3+ and Am3+) in molten LiCl-KCl eutectic solvent at 723 K. A thermodynamic cycle is designed by adiabatic substitution of cation and neutral atom in molten salt with reference cation and reference neutral atom (Ag+ and Ag0 for monovalent cations, Ni2+ and Ni0 for divalent cation, and La3+ and La0 for trivalent cations) and vice versa. The redox potentials of these cations are evaluated relative to theAg+(sol)|AgCl(s)|Ag(s), Ni2 +(sol)|NiCl2(s)|Ni(s) and La3 +(sol)|LaCl3(s)|La(s) reference electrodes, respectively. Good agreements with experimental values are obtained with maximum absolute error of 0.354 V and standard deviation of 0.226 V. In addition, the first principles molecular dynamics (FPMD) is applied to the study of solvation structures of MClm in molten LiCl-KCl eutectic in terms of radial distribution functions and partial static structure factors in satisfactory agreement with X-ray and neutron diffraction characterizations. It is revealed that the solute cation affects significantly the intermediate-range order of Li+–Li+ but slightly of Cl−–Cl− and K+–K+.

Non-equilibrium dynamics of glass-forming liquid mixtures

The non-equilibrium self-consistent generalized Langevin equation theory of irreversible processes in glass-forming liquids [P. Ramírez-González and M. Medina-Noyola, Phys. Rev. E 82, 061503 (2010)] is extended here to multi-component systems. The resulting theory describes the statistical properties of the instantaneous local particle concentration profiles nα(r, t) of species α in terms of the coupled time-evolution equations for the mean value n̄α(r, t) and for the covariance σ(αβ)(r, r'; t) ≡ δn(α)(r, t)δn(β)(r', t) of the fluctuations δn(α)(r, t) = n(α)(r, t) - n̄α(r, t). As in the monocomponent case, these two coarse-grained equations involve a local mobility function bα(r, t) for each species, written in terms of the memory function of the two-time correlation function C(αβ)(r, r'; t, t') ≡ δn(α)(r, t)δn(β)(r', t'). If the system is constrained to remain spatially uniform and subjected to a non-equilibrium preparation protocol described by a given temperature and composition change program T(t) and n̄α(r, t), these equations predict the irreversible structural relaxation of the partial static structure factors Sαβ(k; t) and of the (collective and self) intermediate scattering functions Fαβ(k, τ; t) and F(αβ)(S)(k, τ; t). We illustrate the applicability of the resulting theory with two examples involving simple model mixtures subjected to an instantaneous temperature quench: an electroneutral binary mixture of equally sized and oppositely charged hard-spheres, and a binary mixture of soft-spheres of moderate size-asymmetry.

Thomson scattering from a three-component plasma.

A model for a three-component plasma consisting of two distinct ionic species and electrons is developed and applied to study x-ray Thomson scattering. Ions of a specific type are assumed to be identical and are treated in the average-atom approximation. Given the plasma temperature and density, the model predicts mass densities, effective ionic charges, and cell volumes for each ionic type, together with the plasma chemical potential and free-electron density. Additionally, the average-atom treatment of individual ions provides a quantum-mechanical description of bound and continuum electrons. The model is used to obtain parameters needed to determine the dynamic structure factors for x-ray Thomson scattering from a three-component plasma. The contribution from inelastic scattering by free electrons is evaluated in the random-phase approximation. The contribution from inelastic scattering by bound electrons is evaluated using the bound-state and scattering wave functions obtained from the average-atom calculations. Finally, the partial static structure factors for elastic scattering by ions are evaluated using a two-component version of the Ornstein-Zernike equations with hypernetted chain closure, in which electron-ion interactions are accounted for using screened ion-ion interaction potentials. The model is used to predict the x-ray Thomson scattering spectrum from a CH plasma and the resulting spectrum is compared with experimental results obtained by Feltcher et al. [Phys. Plasmas 20, 056316 (2013)].

Analytical static structure factors for the restricted primitive model

The mean static structure for the restricted primitive model formed by m ionic species is presented. Explicit expressions for the partial static structure factors are obtained from the Blum’s solution for the primitive model, which depend on an accumulative parameter, γ, expressed in terms of the Debye length. Additionally, the number–number, number–charge and charge–charge structure factors are provided. An equation to calculate the radial distribution functions which tends to the contact values when r→σ+ is also given. Finally, molecular dynamics simulation is performed to assess the accuracy of the theoretical results.

Structural relaxation of polydisperse hard spheres: Comparison of the mode-coupling theory to a Langevin dynamics simulation

We analyze the slow glassy structural relaxation as measured through collective and tagged-particle density correlation functions obtained from Brownian dynamics simulations for a polydisperse system of quasi-hard spheres in the framework of the mode-coupling theory (MCT) of the glass transition. Asymptotic analyses show good agreement for the collective dynamics when polydispersity effects are taken into account in a multicomponent calculation, but qualitative disagreement at small q when the system is treated as effectively monodisperse. The origin of the different small-q behavior is attributed to the interplay between interdiffusion processes and structural relaxation. Numerical solutions of the MCT equations are obtained taking properly binned partial static structure factors from the simulations as input. Accounting for a shift in the critical density, the collective density correlation functions are well described by the theory at all densities investigated in the simulations, with quantitative agreement best around the maxima of the static structure factor and worst around its minima. A parameter-free comparison of the tagged-particle dynamics however reveals large quantitative errors for small wave numbers that are connected to the well-known decoupling of self-diffusion from structural relaxation and to dynamical heterogeneities. While deviations from MCT behavior are clearly seen in the tagged-particle quantities for densities close to and on the liquid side of the MCT glass transition, no such deviations are seen in the collective dynamics.

Structure and dynamics of liquidNi36Zr64studied by neutron scattering

We report on investigations on the atomic dynamics and the static structure factors of binary ${\text{Ni}}_{36}{\text{Zr}}_{64}$ alloy melts. In order to undercool the melts deeply below the melting temperature and to avoid reactions with crucible materials, the liquids are containerlessly processed by the application of the electromagnetic levitation technique. This technique is combined with quasielastic neutron scattering at the time-of-flight spectrometer TOFTOF of the Munich research reactor (FRM II) and neutron diffraction at the diffractometer D20 of the Institut Laue-Langevin. Partial static structure factors of liquid ${\text{Ni}}_{36}{\text{Zr}}_{64}$ have been derived via isotopic substitution. The quasielastic neutron-scattering experiments indicate a large activation energy for Ni self-diffusion in liquid ${\text{Ni}}_{36}{\text{Zr}}_{64}$ of 0.64 eV. This may result from a peculiar short-range order of the Ni-Zr melts that differs from the icosahedral short-range order that was previously found to prevail in most melts of pure metals and of metallic alloys with a small difference of the atomic radii of the components and that is characterized by a high nearest-neighbor coordination number of $⟨Z⟩\ensuremath{\approx}13.9$, as found by elastic neutron scattering.

Structural relaxation in a binary metallic melt: Molecular dynamics computer simulation of undercooledAl80Ni20

Molecular dynamics computer simulations are performed to study structure and structural relaxation in the glassforming metallic alloy ${\text{Al}}_{80}{\text{Ni}}_{20}$. The interactions between the particles are modeled by an effective potential of the embedded atom type. Our model of ${\text{Al}}_{80}{\text{Ni}}_{20}$ exhibits chemical short-range order (CSRO) that is reflected in a broad prepeak around a wave number of $1.8\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$ in the partial static structure factor for the Ni-Ni correlations. The CSRO is due to the preference of Ni atoms to have Al rather than Ni atoms as nearest neighbors. By analyzing incoherent and coherent intermediate scattering functions as well as self-diffusion constants and shear viscosity, we discuss how the chemical ordering is reflected in the dynamics of the deeply undercooled melt. The $q$ dependence of the $\ensuremath{\alpha}$ relaxation time as well as the Debye-Waller factor for the Al-Al correlations show oscillations at the location of the prepeak in the partial static structure factor for the Ni-Ni correlations. The latter feature of the Debye-Waller factor is well reproduced by a calculation in the framework of the mode coupling theory (MCT) of the glass transition, using the partial static structure factors from the simulation as input. We also check the validity of the Stokes-Einstein-Sutherland formula that relates the self-diffusion coefficients with the shear viscosity. We show that it breaks down already far above the mode coupling critical temperature ${T}_{c}$. The failure of the Stokes-Einstein-Sutherland relation is not related to the specific chemical ordering in ${\text{Al}}_{80}{\text{Ni}}_{20}$.

Molecular dynamics computer simulation of amorphous silica under high pressure

The structural and dynamic properties of silica melts under high pressure are studied usingmolecular dynamics (MD) computer simulation. The interactions between theions are modelled by a pairwise-additive potential, the so-called CHIK potential,that has been recently proposed by Carré et al 2008 Europhys. Lett. 82 17001. Theexperimental equation of state is well reproduced by the CHIK model. With increasingpressure (density), the structure changes from a tetrahedral network to a networkcontaining a high number of five- and six-fold Si–O coordinations. In the partialstatic structure factors, this change of the structure with increasing density isreflected by a shift of the first sharp diffraction peak towards higher wavenumbersq, eventually merging with the main peak at densities around4.2 g cm−3. The self-diffusion constants as a function of pressure show the experimentally knownmaximum, occurring around a pressure of about 20 GPa.

Atomic dynamics of alumina melt: A molecular dynamics simulation study

The atomic dynamics of Al2O3 melt are studied by molecular dynamics simulation. The particle interactions are described by an advanced ionic interaction model that includes polarization effects and ionic shape deformations. The model has been shown to reproduce accurately the static structure factors S(Q) from neutron and x-ray diffraction and the dynamic structure factor S(Q,!) from inelastic x-ray scattering. Analysis of the partial dynamic structure factors shows inelastic features in the spectra up to momentum transfers, Q, close to the principal peaks of partial static structure factors. The broadening of the Brillouin line widths is discussed in terms of a frequency dependent viscosity �(!).

Collective and static properties of model two-component plasmas

Classical MD data on the charge-charge dynamic structure factor of two-component plasmas (TCP) modeled in Phys. Rev. A 23, 2041 (1981) are analyzed using the sum rules and other exact relations. The convergent power moments of the imaginary part of the model system dielectric function are expressed in terms of its partial static structure factors, which are computed by the method of hypernetted chains using the Deutsch effective potential. High-frequency asymptotic behavior of the dielectric function is specified to include the effects of inverse bremsstrahlung. The agreement with the MD data is improved, and important statistical characteristics of the model TCP, such as the probability to find both electron and ion at one point, are determined.

The structural properties of the liquid CdTe based on Reverse Monte Carlo modelling

The Reverse Monte Carlo (RMC) method only provides information on structure, not on dynamics, and so has restricted applicability. We have only attempted to describe the trends in structural terms. The static structure of liquid Cadmium Telluride is performed using RMC modelling technique fitting to neutron diffraction data. In order to retain its tetrahedral environment with the coordination number about four in its liquid state, we have used some plausible physical constraints. In this way, it is possible to derive static structure of liquid CdTe with analysing the experimental data. We have shown the results for the partial static structure factors S ij ( q) and the partial pair distribution functions g ij ( r). We have also determined the distribution of the number of neighbors within the first coordination shell and calculated the bond angle distribution. A good agreement with the experiment is observed for the structural properties of liquid CdTe.

Self-consistent generalized Langevin equation for colloidal mixtures

A self-consistent theory of collective and tracer diffusion in colloidal mixtures is presented. This theory is based on exact results for the partial intermediate scattering functions derived within the framework of the generalized Langevin equation formalism, plus a number of conceptually simple and sensible approximations. The first of these consists of a Vineyard-like approximation between collective and tracer diffusion, which writes the collective dynamics in terms of the memory function related to tracer diffusion. The second consists of interpolating this only unknown memory function between its two exact limits at small and large wave vectors; for this, a phenomenologically determined, but not arbitrary, interpolating function is introduced: a Lorentzian with its inflection point located at the first minimum of the partial static structure factor. The small wave-vector exact limit involves a time-dependent friction function, for which we take a general approximate result, previously derived within the generalized Langevin equation formalism. This general result expresses the time-dependent friction function in terms of the partial intermediate scattering functions, thus closing the system of equations into a fully self-consistent scheme. This extends to mixtures a recently proposed self-consistent theory developed for monodisperse suspensions [Yeomans-Reyna and Medina-Noyola, Phys. Rev. E 64, 066114 (2001)]. As an illustration of its quantitative accuracy, its application to a simple model of a binary dispersion in the absence of hydrodynamic interactions is reported.