Partial Structure Factors Research Articles

Nanoscale heterogeneity in alkyl-methylimidazolium bromide ionic liquids

High-energy x-ray diffraction measurements and atomistic molecular dynamics (AMD) numerical simulations have been carried out on 1-alkyl-3-methylimidazolium bromide ionic liquids, C(n)mimBr, with n = 2, 4, and 6. Excellent agreement between experiment and simulation is obtained, including the region of the low-Q peak that has proved problematic in previous work in the literature. In the partial structure analysis of the AMD results, a distinct peak develops at the leading edge of the ring-ring pair distribution function and shifts to lower r with increasing alkyl chain length, indicating that the preferential parallel and antiparallel alignment of neighboring cation rings plays a larger role with increasing chain length. The ring-ring, anion-anion, and ring-anion partial structure factors are dominated by strong charge-ordering peaks around 1.1 Å(-1), corresponding to a distance between neighboring polar entities of D(2) = 5.7 Å. In contrast, the tail-tail S(Q) is dominated by the low-Q peak that rises and moves to lower Q with increasing chain length; the length scale of this structural heterogeneity D(1) increases from about 10 Å in C(2)mimBr to 14.3 Å in C(4)mimBr and 18.8 Å in C(6)mimBr. Both the length scale of the structural heterogeneity and its anomalous temperature dependence in the C(n)mimBr liquids studied here show considerable similarity to results in the literature for C(n)mimPF(6) liquids, indicating a remarkable insensitivity to the form and size of the anion. Our results are consistent with the concept of nanoscale heterogeneity with small, crystal-like moieties.

Static structure, microscopic dynamics and electronic properties of the liquid Bi–Pb alloy. An ab initio molecular dynamics study

We perform an ab initio molecular dynamics study of the static, dynamic and electronic properties of the liquid Bi x Pb 1− x alloy at three concentrations, including the eutectic one, x = 0.55. This alloy is of particular technological interest for its possible use as coolant in fast reactors. Our predictions are in good agreement with the available experimental data. In particular, the computed total static structure factors reproduce accurately the neutron diffraction results, and the predicted adiabatic sound velocity and shear viscosity compare well with the experimental values. The partial dynamic structure factors exhibit clear side peaks indicative of propagating density fluctuations, and the longitudinal and transverse dispersion relations show several branches, which we analyze in detail. The electronic density of states show that the liquid Bi–Pb alloy is a good metal, but with strong deviations from the free-electron parabolic curve.

A polarizable ion model for the structure of molten CuI

The results are reported of the molecular dynamics simulations of the coherent static structure factor of molten CuI at 938 K using a polarizable ion model. This model is based on a rigid ion potential to which the many body interactions due to the anions induced polarization are added. The calculated structure factor reproduces the clear sharp prepeak observed in neutron diffraction data. The corresponding partial structure factors and the related radial distribution functions calculated by molecular dynamics are compared with those found in the literature derived from a combination of neutron and x-ray diffraction data with the aid of the reverse Monte Carlo simulation technique, as well as those calculated by ab initio MD simulations.

The structure of molten AgCl, AgI and their eutectic mixture as studied by molecular dynamics simulations of polarizable ion model potentials

The structure of molten AgCl, AgI, and their eutectic mixture Ag(Cl(0.43)I(0.57)) is studied by means of molecular dynamics simulations of polarizable ion model potentials. The corresponding static coherent structure factors reproduce quite well the available neutron scattering data. The qualitative behavior of the simulated partial structure factors and radial distribution functions for molten AgCl and AgI is that predicted by the reverse Monte Carlo modeling of the experimental data. The AgI results are also in qualitative agreement with those calculated from ab initio molecular dynamics.

Structure study of chalcogenide glasses from high Q-range neutron diffraction experiment and RMC modelling

A neutron diffraction structure study has been performed on newly synthesized Ge20As20Se50Te10 and Ge27As13Se50Te10 chalcogenide glasses. Oscillations in the structure factor, S(Q) have been measured with good sign-to-noise up to 35 Å−1. The reverse Monte Carlo simulation was used to model the 3-dimensional atomic configuration. The partial atomic correlation functions and structure factors have been revealed. Several first and second neighbour distances, and coordination numbers have been calculated. We have established that several first neighbour atomic distances are overlapping at two characteristic distances, namely the Ge-Ge, Ge-Se and As-Se are centred at 2.42 Å, while the Ge-Te, As-Te and Se-Te are centred at 2.60 Å.

Study of As2Se3 and As2Se2Te glass structure by neutron- and X-ray diffraction methods

Neutron and high-energy X-ray diffraction measurements have been performed on As40Se60 and As40Se50Te10 glasses. Both the traditional Fourier transformation technique and the reverse Monte Carlo (RMC) simulation of the experimental data have been applied to model the 3-dimensional atomic configurations. From the analysis of the partial atomic correlation functions and structure factors the first and second neighbour distances, coordination numbers and bond-angle distributions were calculated. It is established that substitution of Se by Te does not change the basic glassy network structure. For the As-Se bonding is revealed that the first neighbour distance is at 2.42 Å, the average coordination numbers are CNAsSe=2.6±0.1 and CNSeAs=1.8±0.1 atoms and the three-atom-bond angle for <Se-As-Se>, <As-Se-As> configurations is 95°, the same value within limit of error, as obtained from RMC calculations.

Structure of liquid and glassyZnCl2

The method of isotope substitution in neutron diffraction was used to measure the structure of liquid ZnCl2 at 332(5)?°C and glassy ZnCl2 at 25(1)?°C. The partial structure factors were obtained from the measured diffraction patterns by using the method of singular value decomposition and by using the reverse Monte Carlo procedure. The partial structure factors reproduce the diffraction patterns measured by high-energy x-ray diffraction once a correction for the resolution function of the neutron diffractometer has been made. The results show that the predominant structural motif in both phases is the corner sharing ZnCl4 tetrahedron and that there is a small number of edge-sharing configurations, these being more abundant in the liquid. The tetrahedra organize on an intermediate length scale to give a first sharp diffraction peak in the measured diffraction patterns at a scattering vector kFSDP?1 A-1 that is most prominent for the Zn-Zn correlations. The results support the notion that the relative fragility of tetrahedral glass forming MX2 liquids is related to the occurrence of edge-sharing units.

Structural correlations of Ge 20Se 80− xTe x glasses based on reverse Monte Carlo simulation

Bulk chalcogenide Ge 20Se 80− x Te x (where x = 0, 10, 20 and 30 at.%) glasses were prepared using the melt-quench technique. The total structure factors of these alloys are obtained from the X-ray scattering data in the momentum transfer interval 0.61 ≤ K ≤ 16.45 Å −1. From reverse Monte Carlo (RMC) simulations of the X-ray scattering data, the short and intermediate-range order parameters are obtained. The simulations are useful to compute the partial pair distribution functions, g ij ( r), and the partial structure factors, S ij ( K). In Te-rich glass, the first sharp diffraction peak (FSDP) appears as a shoulder, instead of a peak for others, confirms that Se–Se bonds in addition to Ge–Ge bonds are responsible for the intermediate-range order inside these glasses. The partial coordination numbers and the bond angle distributions within the first coordination shell have been calculated. The ratio of the first to second peak positions ( r 1/ r 2) and the corresponding bond angle ( Θ) have confirmed that the Ge(Se 1/2) 4 tetrahedra, connected by Se–Se chains, can be considered as the main building units inside the investigated glasses.

Rheo-SANS Studies on Shear Thickening in Clay−Poly(ethylene oxide) Mixed Solutions

The structure evolution of network structure in clay−polymer aqueous solutions after shear thickening under shear deformation was investigated by means of small-angle neutron scattering (SANS; Rheo-SANS). Laponite (clay; C) platelet dispersions in poly(ethylene oxide) (PEO; P) aqueous solutions with various deuterious/hydrogeneous water compositions were prepared, and contrast-variation SANS experiments were carried out under flow field. The two-dimensional scattering contour patterns changed suddenly from isotropic to anisotropic patterns at the shear thickening transition point. The orientations of clay platelets and PEO chains were individually characterized by the partial structure factors, SCC(q), SCP(q), and SPP(q), which were obtained by decomposition of the SANS intensity functions. The SANS results show that PEO chains are adsorbed on clay platelets even in a flow field and are peeled off at the shear thickening transition point. Flow birefringence results suggest that the shear thickening is a c...

Glass transition of binary mixtures of dipolar particles in two dimensions

We study the glass transition of binary mixtures of dipolar particles in two dimensions within the framework of mode-coupling theory, focusing in particular on the influence of composition changes. In a first step, we demonstrate that the experimental system of König et al. [Eur. Phys. J. E 18, 287 (2005)] is well described by point dipoles through a comparison between the experimental partial structure factors and those from our Monte Carlo simulation. For such a mixture of point particles we show that there is always a plasticization effect, i.e. a stabilization of the liquid state due to mixing, in contrast to binary hard disks. We demonstrate that the predicted plasticization effect is in qualitative agreement with experimental results. Furthermore, also some general properties of the glass transition lines are discussed.

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.

Reverse Monte Carlo simulation of Ge xSe 100− x glasses

Amorphous Ge x Se 100− x (with x=10, 20 and 40 at%) alloys were prepared using the melt–quench technique. Two-dimensional Monte Carlo of the total pair distribution functions (MCGR) have been found and used to assemble the three-dimensional atomic configurations using the reverse Monte Carlo (RMC) method. The simulations are useful to compute the partial pair distribution functions g Ge − Ge r m c ( r ) , g Ge − Se r m c ( r ) , g Se − Se r m c ( r ) and the partial structure factors S Ge − Ge r m c ( r ) , S Ge − Se r m c ( r ) , S Se − Se r m c ( r ) of the studied glasses. The partial pair distribution functions indicate that the basic building units are GeSe 4 and Ge 2Se 6 tetrahedral units in the Se-rich and Ge-rich glasses, respectively. Some of these tetrahedral units are connected by the homopolar units as confirmed by the bond angle distribution functions. The partial structure factors have shown that not only the homopolar Ge–Ge bonds, but also Se–Se bonds are behind the appearance of the first sharp diffraction peak (FSDP) in the total structure factor.

Non-collinear ferrimagnetism in aFe64Er19B17 metallic glass

Polarized neutron beam measurements on aFe64Er19B17 metallic glass have shown directly that it has a non-collinear magnetic structure. It can bedescribed using a model in which the magnetic moments on the iron atoms point in arandom cone that is ferrimagnetically coupled to a random cone of erbium moments, in themanner suggested from bulk measurements. The spin-flip cross-sections weresuccessfully calculated using an optimized choice of the values of the magnetic momentsμFe,μEr and the randomcone angles θFe, θEr. The non-spin-flip cross-sections have an unusual variation with the scattering vectorQ, which has not been observed before with transition metal–metalloid glasses. At 1.5and 60 K the cross-section contains a pre-peak at a smaller value ofQ (1.3 Å − 1) than the pre-peaks which have been observed in the structure factors of some transitionmetal glasses. At 180 K the form of these cross-sections remains the same but the twochannels have interchanged, so contains the pre-peak. This interchange shows that a complete inversion of the magneticstructure occurs between 60 and 180 K—presumably at the compensation temperatureTcomp≈120 K. Attempts to simulate these cross-sections using the methods applied to (Fe,Tb)B glasseswere unsuccessful because none of the known partial structure factors contains a pre-peakwhich can imitate the observed one. The possible origins of the pre-peak arediscussed.

Aging to equilibrium dynamics ofSiO2

Molecular dynamics computer simulations are used to study the aging dynamics of SiO2 (modeled by the BKS model). Starting from fully equilibrated configurations at high temperatures Ti∊{5000 K,3760 K}, the system is quenched to lower temperatures Tf∊{2500 K,2750 K,3000 K,3250 K} and observed after a waiting time tw. Since the simulation runs are long enough to reach equilibrium at Tf, we are able to study the transition from out-of-equilibrium to equilibrium dynamics. We present results for the partial structure factors, for the generalized incoherent intermediate scattering function Cq(tw,tw+t), and for the mean-square displacement Δr2(tw,tw+t). We conclude that there are three different tw regions: (I) At very short waiting times, Cq(tw,tw+t) decays very fast without forming a plateau. Similarly Δr2(tw,tw+t) increases without forming a plateau. (II) With increasing tw a plateau develops in Cq(tw,tw+t) and Δr2(tw,tw+t). For intermediate waiting times the plateau height is independent of tw and Ti. Time superposition applies, i.e., Cq=Cq(t/trCq) where trCq=trCq(tw) is a waiting time-dependent decay time. Furthermore Cq=C(q,tw,tw+t) scales as Cq=C(q,z(tw,t)) where z is a function of tw and t only, i.e., independent of q. (III) At large tw the system reaches equilibrium, i.e., Cq(tw,tw+t) and Δr2(tw,tw+t) are independent of tw and Ti. For Cq(tw,tw+t) we find that the time superposition of intermediate waiting times (II) includes the equilibrium curve (III).

Extracting differential pair distribution functions usingMIXSCAT

Differently weighted experimental scattering data have been used to extract partial or differential structure factors or pair distribution functions in studying many materials. However, this is not done routinely partly because of the lack of user-friendly software. This paper presentsMIXSCAT, a new member of theDISCUSprogram package.MIXSCATallows one to combine neutron and X-ray pair distribution functions and extract their respective differential functions.

Long-Range Correlations in Polymer-Containing Ionic Liquids: The Case of Good Solubility

Ionic liquids containing macromolecules as solutes are of considerable interest for polymer materials science. However, these systems are at an early stage of investigation. In this study, the structural features of ionic liquids containing flexible polymer chains under good solubility conditions are investigated on the basis of the polymer integral equation method. The structural properties of the system are analyzed in reciprocal space by using partial structure factors. In the presence of attractive interaction between the polymer chains and polar cationic groups, there is a range of polymer concentrations at which the long-range correlations of polymer chains are observed. When these correlations are pronounced and the polymer chain size is much larger than the correlation length of pure ionic liquid, the decrease of the characteristic scale of the long-range polymer ordering with increasing polymer concentration fits the power dependence. The effect of such an increase on the ordering of the ions of the solvent is also considered.

Electronic Transport and Thermodynamic Properties of Binary Liquid Alloy: Ab Initio Approach

In this article, the Harrison’s first principle pseudopotential technique has been employed to study the electrical resistivity and other physical properties (i.e., the Knight shift, Fermi energy, and electronic density of states) of Zintl liquid alloy. Because there is a possibility of formation of a compound in the system, as inferred from the graph of a partial structure factor of unlike pairs of atom, we also have computed the thermodynamic as well as the electron transport properties of this system, taking the complex formation model into account. Useful interpretation based on inferences has been withdrawn.

Structure and electrical resistivity of liquid In–Sn alloy

A comparative study is presented for the electrical resistivity of liquid In 1− x Sn x metal alloys. In the electrical resistivity calculation, Faber–Ziman formula and modified Ziman’s formula suggested by Ferraz–March are used. Partial structure factors have been calculated from the solution of Ornstein–Zernike (O–Z) equation with the Hybridized Mean Spherical Approximation (HMSA) closure and the transferable electron–ion potential of Fiolhais et al. has been used as the input pseudopotential. Two different screening functions are used in calculation of form factors which are necessary for the calculation of resistivity. The results are compared with experimental ones and the effect of screening function on resistivity is discussed.

Simulation and reference interaction site model theory of methanol and carbon tetrachloride mixtures

We report molecular dynamics and reference interaction site model (RISM) theory of methanol and carbon tetrachloride mixtures. Our study encompasses the whole concentration range, by including the pure component limits. We majorly focus on an analysis of partial, total, and concentration-concentration structure factors, and examine in detail the k-->0 limits of these functions. Simulation results confirm the tendency of methanol to self-associate with the formation of ring structures in the high dilution regime of this species, in agreement with experimental studies and with previous simulations by other authors. This behavior emerges as strongly related to the high nonideality of the mixture, a quantitative estimate of which is provided in terms of concentration fluctuation correlations, through the structure factors examined. The interaggregate correlation distance is also thereby estimated. Finally, the compressibility of the mixture is found in good agreement with experimental data. The RISM predictions are throughout assessed against simulation; the theory describes better the apolar solvent than the alcohol properties. Self-association of methanol is qualitatively reproduced, though this trend is much less marked in comparison with simulation results.

Partial Pair Correlation Functions of Highly Concentrated Aqueous Urea Solutions Determined by Neutron Diffraction with 14N/15N and H/D Isotopic Substitution Methods

Abstract Neutron diffraction measurements were carried out on 14N/15N and H/D isotopically substituted aqueous 25 mol % urea (xurea = 0.25) solutions in order to obtain direct experimental information on the short-range structure on the urea–urea, urea–water, and water–water interactions in highly concentrated aqueous urea solutions near saturation. Scattering cross sections observed for isotopically distinct solutions were successfully combined to derive intermolecular partial structure factors, aNNinter(Q), aNHinter(Q), aNO(NC)inter(Q), aHHinter(Q), aXHinter(Q), and aXXinter(Q) (X: O, N, and C), and corresponding intermolecular partial pair correlation functions. The nearest neighbor intermolecular N···O and N···H distances have been obtained as 3.14(1) and 3.42(1) Å, respectively, indicating that the amino group of the urea molecule forms hydrogen bonds with neighboring water molecules in such a highly concentrated solution. The nearest neighbor intermolecular N···N interaction was found at intermolecular distance of 4.71(5) Å with a coordination number of 3.1(5), which evidences that the hydrogen-bonded urea oligomers exist in the present solution. The nearest neighbor intermolecular O···H distance has been found to be 1.90(1) Å, which indicates that hydrogen bonds among the solvent water molecules are still preserved in the present solutions. On the other hand, corresponding O···H coordination number 0.83(1) is much smaller than that reported for room temperature liquid water. The nearest neighbor intermolecular H···H and O···O distances were determined to be 2.37(5) and 2.86(2) Å, respectively. The corresponding H···H and O···O coordination numbers were obtained as 2.0(5) and 1.4(1), respectively. These values are also much smaller than those observed for the pure water. The present results suggest that the hydrogen-bonded network among water molecules is considerably modified due to the nearest neighbor urea–water and urea–urea interactions, which can be strongly related with protein denaturation ability of the concentrated aqueous urea solutions.