Atomic Pair Correlation Functions Research Articles

Neutron diffraction and reverse Monte Carlo modelling of v-B 2O 3 and 75B 2O 3–25Na 2O glasses

A neutron diffraction study has been performed on newly synthesized v-B 2O 3 and 75B 2O 3–25Na 2O glasses. The structure factor has been measured over a broad momentum transfer range, 0.4–30 Å −1, which made fine r-space resolution available for real space analyses. The reverse Monte Carlo (RMC) method was successfully applied to calculate a possible 3-dimensional atomic configuration that is consistent with the measured structure factor. The partial atomic pair-correlation functions, nearest neighbor atomic distances, coordination number distributions, average coordination number values and three-particle bond angle distributions have been revealed. It was established that addition of 25 mol% Na 2O to v-B 2O 3 transforms 41% of the 3-fold oxygen coordinated boron atoms into 4-fold coordinated ones. The first neighbor B–O distance is at 1.37 Å in v-B 2O 3, while two well resolved distances are at 1.38 and 1.50 Å in the 75B 2O 3–25Na 2O glass. The three-particle 〈O–B–O〉 and 〈B–O–B〉 bond angle distributions show an extra peak at around 90° in the 75B 2O 3–25Na 2O glass, which may be attributed to the BO 4 units, in addition to the peak at 120° characteristic for BO 3 trigonal units. The ring size distribution of the v-B 2O 3 network indicate that only 12% of boron atoms form boroxol group (B 3O 6), while the fraction of the larger 7–11 boron membered rings is 52%, and a tail of the distribution extends up to 20-membered rings.

Sodium bis(2-ethylhexyl)sulfosuccinate self-aggregation in vacuo: molecular dynamics simulation

Molecular dynamics (MD) simulations were conducted for systems in vacuo consisting of n AOT(-) anions (bis(2-ethylhexyl)sulfosuccinate ions) and n+/- 1 or n Na(+) ions up to n = 20. For n = 15, positively charged systems with Li(+), K(+), and Cs(+) cations were also considered. All systems were observed to form reverse micelle-like aggregates whose centre is occupied by cations and polar heads in a very compact solid-like way, while globally the aggregate has the form of an elongated and rather flat ellipsoid. Various types of statistical analyses were carried out on the systems to enlighten structural and dynamical properties including gyration radius, atomic pair correlation functions, atomic B-factor and moment of inertia tensor. For completeness and comparison the stability of reverse micelle is tested in the case of neutral n = 20 system in CCl(4) solution.

Inference on carbon atom arrangement in the turbostatic graphene layers in Tikak coal (India) by X-ray pair distribution function analysis

This paper communicates the distribution of carbon atoms in a single poly-cyclic aromatic (PCA) layer (graphene) in Tikak coal from Assam, India. The pair distribution function (PDF) analysis performed indicates no evidence of any graphite like structure in this coal. The aromatic fraction is observed to be 74% with the aliphatic fraction correspondingly estimated to be 26% in this coal. The average carbon atom has 2.5 nearest carbon atom neighbours at an average bond distance of 1.50 Å. The average stacking height of the parallel aromatic layers (Lc) and the average diameter of the aromatic layers (La) are estimated to be 9.86 Å and 4.80 Å, respectively. For this coal, the average number of stacking layers and the average number of atoms per layer are estimated to be four and eight respectively. In addition, the gamma band is observed at a d-value of 4.34 Å. The comparison of the atom-pair correlation function to simulated one-dimensional structure function calculated for a model compound benzene (C6H6) also indicates that C6 unit is the major components in this coal. The average carbon atom has at least one and one nearest aryl and alkyl C-C atom pairs separated by 1.39 and 1.54 Å respectively. [Received: January 30, 2010; Accepted: March 21, 2010]

Structural evolution of Cu–Zr metallic glasses under tension

The structural behaviour of Cu 50Zr 50 and Cu 65Zr 35 metallic glasses under uniaxial tensile stress was investigated in situ by high-energy X-ray synchrotron diffraction. The components of the elastic strain tensor were determined from both the change of positions of first maximum of the structure factor in reciprocal space as well as from the maxima of the atomic pair correlation function in real space. The atomic scale strain agrees with the macroscopic strain values. The topological and chemical short-range order of the Cu–Zr glasses changes upon loading. The number density of Cu–Zr and Zr–Zr nearest neighbour atomic pair becomes oriented along the loading direction whereas the partial nearest neighbour distances are only weakly influenced.

Determination of atomic pair correlation functions for nickel oxide films via extended energy-loss fine structure spectroscopy

The extended energy-loss fine structure (EELFS) spectra for pure nickel samples (M 2,3 EELFS spectra), a NiO stoichiometric film (nickel M 2,3 EELFS and oxygen K EELFS spectra), and an “inhomogeneous” oxide film (Ni-O system) on the surface of nickel have been obtained. The calculations of amplitudes and intensities of electron transitions are performed for the corresponding inner levels of atoms taking into account the multipolarity of the excitation of inner atomic levels by an electron impact. The normalized oscillating components are extracted from EELFS spectra using the results of calculations. Close agreement between experimental results and theoretical data obtained for test Ni samples and NiO films indicate that the theoretical approaches applied to the description of EELFS spectra and the results of calculations are good approximations. Atomic pair correlation functions are obtained from the experimental normalized oscillating components of EELFS spectra with the use of the Tychonoff regularization technique.

Determination of the parameters of atomic pair correlations in nickel oxide films by the electron energy loss fine structure method

The electron energy loss extended fine structure (EELFS) spectra were obtained from the pure nickel surface (M 2,3 EELFS) of a stoichiometric NiO film (NiM 2,3 and OK EELFS spectra) and the “nonhomogeneous” oxide film on the surface of nickel Ni-O (NiM 2,3 and OK EELFS spectra). The amplitudes and intensities of electron transitions for the core levels of atoms were calculated with regard for the multiplicity of electron impact excitation of the corresponding core levels of atoms. The corresponding normalized oscillating terms were isolated using the results of calculations based on the experimental EELFS spectra. Agreement between the experimental and calculated (on Ni and NiO test objects) data showed that the theoretical approaches used and the calculated data for describing the EELFS spectra are good approximations. Using the results of calculations and the parameters of secondary electron elastic scattering (FEEF-8 data) we obtained the atomic pair correlation functions from the experimental normalized oscillating parts of the EELFS spectra by Tikhonov’s regularization method.

Spectroscopy of extended electron energy loss fine structures for quantitative analysis of the local atomic structure in ultrathin oxide films on the surface of 3d-metals

The work presents the theoretical description of formation processes of extended electron energy loss fine structure (EELFS) spectra. Simple analytical formulas are obtained to calculate EELFS spectra. Methods are proposed to compute amplitudes and intensities of electronic transitions of different multipolarity in electron-impact excitation of the inner shell of an atom. The corresponding test calculations are performed. We present experimental M 2,3 EELFS spectra of 3d-metals obtained from clean surfaces of Fe, Ni, and Cu and ultrathin stoichiometric oxide films of Fe2O3, NiO, and CuO as well as K EELFS spectra of oxygen from thin stoichiometric oxide films. Techniques are proposed to extract normalized oscillating parts from experimental EELFS data based on the use of calculated intensities of the corresponding electronic transitions. Atomic pair correlation functions are obtained for the objects under study from experimental EELFS data with regard to multipolarity of electron impact excitation of the atomic inner shell. Experimental results correspond well to known crystallographic data (partial interatomic distances, coordination numbers, and Debye-Waller factors).

Structure study of multi-component borosilicate glasses from high-Q neutron diffraction measurement and RMC modeling

A neutron diffraction structure study has been performed on multi-component borosilicate glasses with compositions (65−x)SiO2·xB2O3·25Na2O·5BaO·5ZrO2, x=5–15mol%. The structure factor has been measured up to a rather high momentum transfer value of 30Å−1, which made high r-space resolution available for real space analyses. Reverse Monte Carlo simulation was applied to calculate the partial atomic pair correlation functions, nearest neighbor atomic distances and coordination number distributions. The Si–O network consists of tetrahedral SiO4 units with characteristic first neighbor distances at rSi–O=1.60Å and rSi–Si=3.0Å. The boron environment contains two well-resolved B–O distances at 1.40 and 1.60Å and both 3- and 4-fold coordinated B atoms are present. A chemically mixed network structure is proposed including [4]B–O–[4]Si and [3]B–O–[4]Si chain segments. The O–O and Na–O distributions suggest partial segregation of silicon and boron rich regions. The highly effective ability of Zr to stabilize glassy and hydrolytic properties of sodium-borosilicate materials is interpreted by the network-forming role of Zr ions.

Structural behavior of Cu xZr 100−x metallic glass ( x = 35−70)

The structural behavior of rapidly quenched amorphous Cu x Zr 100− x alloys was investigated in a wide composition range between 35 and 70 at.% Cu content. High-energy X-ray diffraction patterns, atomic pair correlation functions, mass density and the thermal stability behavior of the alloys all exhibit monotonic changes with composition. Partial pair correlation functions were determined assuming that they remain unchanged in the different amorphous Cu x Zr 100− x alloys and only the weight fractions become altered with changes in composition. The experimental results can be well described by a solid solution-like replacement of Cu and Zr atoms in the whole composition range. No indications are observed neither for the existence of a special atomic arrangement at a particular chemical composition nor for the presence of phase separation in the glassy state of the binary Cu–Zr system.

Magnetic phase transition of FeS at high pressures

24 European Crystallographic Meeting, ECM24, Marrakech, 2007 Page s59 Acta Cryst. (2007). A63, s59 RMC simulation of the experimental structure factor was successfully applied to generate a reliable 3-dimensional atomic configuration. Several partial atomic pair correlation functions, like the gSiO(r), gBO(r), gOO(r), gSiSi(r), gSiB(r), gNaO(r), gBaO(r), gZrO(r) and most of the corresponding coordination number distributions has been revealed providing information on the short and intermediate range order. The Si-O network proved to be highly stable consisting of slightly modified SiO4 units. The B-O surrounding proved to be more complex. We have found that two characteristic well resolved first neighbour B-O distances are present at 1.40 A and 1.60 A. The latter agrees with the Si-O first neighbour distance, which is also at 1.60 A. From the detailed analyses of the B-O and O-B coordination number distributions we have established that both 3and 4-fold coordinated boron atoms (denoted as B and B) are present. We suppose that the boron atoms partly form a mixed continuous network with Si-O network, where several different mixed B-O-Si and B-O-Si linkages are present in agreement with the findings of NMR [4,5] and Raman spectroscopy [6]. The work was supported by grants OTKA-42495, EC HPRI-RII3-CT-2003-505925, DE-AC52-06NA25396.

Simulation of atomic and electronic structure of liquid caesium by method of the first-principal molecular dynamics

Atomic and electronic structures of liquid caesium were investigated by means of the first-principle molecular dynamics method for a range of temperatures. The calculated atomic pair correlation function agrees well with experimental diffraction data. An analysis of the atomic configurations is carried out using the Voronoi polyhedron method. We present results for the atomic properties (pair correlation function and self diffusion coefficient) and electronic properties (s-, p-, d-density of states, including DOS at the Fermi level). An estimation of degree of electron localization for the entire temperature range is carried out. The obtained results are compared with those of classical molecular dynamics and reverse Monte Carlo methods, and the TB-LMTO method.

Neutron diffraction study of sodium borosilicate waste glasses containing uranium

The effect of uranium oxide on the structure of sodium borosilicate host glasses has been studied by neutron diffraction. The samples were prepared by quenching the melted mixtures of composition 70 wt% [(65 − x)SiO 2 · xB 2O 3 · 25Na 2O · 5BaO · 5ZrO 2] + 30 wt% UO 3 with x = 5, 10 and 15 mol%. It was found, that the U-loaded glasses posses good glass and hydrolytic stability. An enhanced probability for inter-mediate atomic correlations at around 4.8 Å has been established. The RMC simulation of the neutron diffraction data is consistent with a model where the uranium ions are incorporated into interstitial voids in the essentially unmodified network structure of the starting host glass. The U–O atomic pair correlation functions show a sharp peak at around 1.7 Å, and several farther distinct peaks are at 2.8, 3.6 and 4.1 Å. The uranium ions are coordinated by six oxygen atoms in the 1.6–3.4 Å interval.

Network structure of multi-component sodium borosilicate glasses by neutron diffraction

Neutron diffraction structure study has been performed on multi-component sodium borosilicate based waste glasses with the composition of (65 − x)SiO 2. · xB 2O 3 · 25Na 2O · 5BaO · 5ZrO 2, x = 5–15 mol%. The maximum momentum transfer of the experimental structure factor was 30 Å −1, which made available to determine the distribution function with high r-space resolution. Reverse Monte Carlo modelling was applied to calculate several partial atomic pair correlation functions, nearest neighbor distances and coordination numbers have been revealed. The characteristic features of Si–O and Si–Si distributions are similar for all glassy samples, suggesting that the Si–O network consisting of tetrahedral SiO 4 units is highly stable even in the multi-component glasses. The B–O correlations proved to be fairly complex, two distinct first neighbor distances are present at 1.40 Å and 1.60 Å, the latter equals the Si–O distance. Coordination number distribution analyzes has revealed 3 and four-coordinated boron atoms. The O–O distribution suggests a network configuration consisting of boron rich and silicon rich regions. Our findings are consistent with a structure model where the boron rich network contains mostly trigonal BO 3 units, and the silicon rich network is formed by a mixed continuous network of [4]Si–O–Si [4] with several different [4]B–O–Si [4] and [3]B–O–Si [4] linkages.

A Molecular Dynamics Study of a Nafion Polyelectrolyte Membrane and the Aqueous Phase Structure for Proton Transport

A molecular dynamics simulation study of hydrated Nafion at water contents ranging from 5 to 20 wt % was performed to examine the structure and dynamics of the hydrated polyelectrolyte system. The simulations show that the system forms segregated hydrophobic regions consisting primarily of the polymer backbone and hydrophilic regions with an inhomogeneous water distribution. We find that the water clustering strongly depends on the water content. At low water content, only isolated small water clusters are formed. As the water content increases, it becomes increasingly possible that a predominant majority of water molecules form a single cluster, suggesting that the hydrophilic regions become connected. We characterize the atomic structures formed within the system by various atomic pair correlation functions. The water structure factor shows a peak at q values corresponding to an intercluster distance about 2.5 nm and greater. With increasing water content, the distance moves to larger values, consistent with findings from scattering experiments. We find that the degree of solvation of hydronium ions by water molecules is a strong function of water content. At 5 wt %, a majority of the hydronium ions are hydrated by no more than two water molecules, prohibiting structural diffusion. As water content increases, the hydronium ions continue to become increasingly hydrated, resulting in structures capable of forming eigen ions, a necessary step in structural diffusion. Addressing the experimentally observed fact that conductivity in these membranes abruptly drops near 5 wt %, we find that both the local structure of the poorly hydrated hydronium ions and the disconnected nature of the global morphology of the water nanonetwork at low water content should contribute to poor conductivity.

Phase Separation and Crystallization in Cu-Zr Metallic Glasses

The structural behavior of rapidly quenched Cu-Zr amorphous alloys was analyzed. High energy X-ray diffraction patterns and atomic pair correlation functions exhibit monotonic changes with composition. The experimental results can be well described by a solid solution-like replacement of Cu and Zr atoms in the whole composition range. No indications are observed that would support the existence of phase separation in the supercooled liquid state of the binary Cu-Zr alloys. For Cu60Zr30Ti10 and Cu60Zr20Ti20 bulk metallic glasses the formation of ultrafine nanostructures are proven upon heating. The transformation starts below the glass transition temperature. Phase separation in Cu-Zr-Ti bulk metallic glasses is related to primary crystallization. [doi:10.2320/matertrans.MJ200708]

X-ray diffraction studies on molten zinc bromide at high pressure

X-ray diffraction investigations into molten zinc bromide at temperatures of 450°C and 500°C and pressures up to 3000bar are presented. The distinct terms and the total atom pair correlation functions of ZnBr2 are presented and the effect of the pressure on the structure is discussed. The results are compared with previous experiments on molten ZnCl2.

Ab initio molecular-dynamics study of supercritical carbon dioxide.

Car-Parrinello molecular-dynamics simulations of supercritical carbon dioxide (scCO(2)) have been performed at the temperature of 318.15 K and at the density of 0.703 g/cc in order to understand its microscopic structure and dynamics. Atomic pair correlation functions and structure factors have been obtained and good agreement has been found with experiments. In the supercritical state the CO(2) molecule is marginally nonlinear, and thus possesses a dipole moment. Analyses of angle distributions between near neighbor molecules reveal the existence of configurations with pairs of molecules in the distorted T-shaped geometry. The reorientational dynamics of carbon dioxide molecules, investigated through first- and second-order time correlation functions, exhibit time constants of 620 and 268 fs, respectively, in good agreement with nuclear magnetic resonance experiments. The intramolecular vibrations of CO(2) have been examined through an analysis of the velocity autocorrelation function of the atoms. These reveal a red shift in the frequency spectrum relative to that of an isolated molecule, consistent with experiments on scCO(2). The results have also been compared to classical molecular-dynamics calculations employing an empirical potential.

High temperature–high pressure apparatus for neutron diffraction on molten salts: Structure factors of molten zinc chloride

An apparatus for neutron diffraction experiments on corrosive liquids at high temperatures and high pressures is presented. The cell is constructed for temperatures up to 900 K and pressures up to 5000 bar and tested on molten zinc chloride at five different thermodynamic states at 723 K up to pressures of 4000 bar. The distinct terms and the total atom pair correlation functions of all five thermodynamic states are determined and their density dependences are discussed.

High Pressure Studies on Molten Zinc Chloride.

AbstractFor Abstract see ChemInform Abstract in Full Text.

High pressure studies on molten zinc chloride

The structure of molten zinc chloride was investigated at pressures up to 3000 bar and temperatures up to 500°C with a new high pressure cell. The distinct terms and the total atom pair correlation functions were determined with high energy photons. The distinct terms show systematic change in the low κ-range between 0 and 3 A−1 as a function of the applied pressure. A reduction of the first sharp diffraction peak (prepeak) intensity of the distinct part is observed with increasing pressure. It is concluded, that this behaviour indicates a network breaking tendency of the corner shared [ZnCl4]2− tetrahedra with increasing pressure, while the tetrahedra themselves remain unchanged over the whole applied pressure and temperature range.