Total Pair Distribution Function Research Articles

Atomic-Scale Structure of Nanosized Titania and Titanate: Particles, Wires, and Tubes

The atomic-scale structure of nanosized “TiO2-type” materials of different morphologies, including particles, wires. and tubes, has been determined by total X-ray diffraction and atomic pair distribution function analysis. Atoms in dense-filled materials such as nanoparticles and nanowires are found to form a three-dimensional network of TiO6 octahedra, similar to that occurring in bulk TiO2. Atoms in hollow materials such as nanotubes are arranged in corrugated layers of TiO6 octahedra. The study calls attention to the often overlooked fact that a materialʼs morphology and atomic ordering may be strongly correlated at the nanoscale.

Three-dimensional structure of CdX (X=Se,Te) nanocrystals by total x-ray diffraction

The three-dimensional structure of oleic acid-capped CdSe and thiol-capped CdTe nanocrystals used as quantum dots has been determined by total synchrotron radiation x-ray diffraction and atomic pair distribution function analysis. Both CdSe and CdTe are found to exhibit the zinc-blende-type atomic ordering. It is only slightly distorted in CdSe implying the presence of nanosize domains and very heavily distorted in CdTe due to the presence of distinct core-shell regions. The results well demonstrate the great potential of the experimental approach and thus encourage its wider application in quantum dot research.

PbZr1−xTixO3by soft synthesis: Structural aspects

The structural aspects of a soft synthetic route employed to obtain fine crystallite $\mathrm{Pb}{\mathrm{Zr}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{3}$ $(x=0,0.5,1)$ powders are revealed by total x-ray diffraction and atomic pair distribution function analysis. It is found that the atomic-scale structure of the intermediate, highly disordered phase the route passes through resembles but is not exactly of the targeted, perovskite-type structure. It is suggested that future synthesis efforts are directed toward closing the observed ``structure gap'' as much as possible.

X-Ray Anomalous Scattering Studies of Fe-Tb Amorphous Thin Films

In order to investigate the atomic structure of sputtered-deposited Fe76Tb24 amorphous film, we performed X-ray anomalous scattering (XAS) measurements at two energies near the Tb K-absorption edge using the synchrotron-radiations (SR). The total, Tb and Fe environmental pair distribution functions (PDF's) of the amorphous film were obtained from the normalized scattering intensities after the corrections for the structural analysis. The coordination numbers were calculated from the area under the fitted PDF profiles multiplied by 4πr 2 and near-neighbor distances were estimated from the positions of the peaks of the fitted profiles.

Titania Polymorphs by Soft Chemistry: Is There a Common Structural Pattern?

The structural aspects of a soft chemical route1 employed to obtain nitrogen-doped brookite, rutile, or anatase fine crystallite powders are revealed by total X-ray diffraction and atomic pair distribution function analysis. It is found that the route first passes through a heavily disordered phase consisting of nanosize layers of TiO6 octahedra, similar to those occurring in the lepidocrocite-type structure. That phase is then transformed into a particular titania polymorph by rearranging the layers in a controlled way. The lepidocrocite-type structure thus again proves to be a very useful template in the soft chemistry of titania.

Atomic-scale structure of nanocrystallineCeO2–ZrO2 oxides by total x-ray diffraction and pair distribution function analysis

Total x-ray diffraction and atomic pair distribution function analysishave been used to determine the atomic ordering in nanocrystalline(∼1.5 nm insize) CeO2–ZrO2 prepared by a sol–gel route. Experimental data show that the oxides are a structurally andchemically inhomogeneous mixture of nanoscale domains with cubic-type andmonoclinic-type atomic ordering, predominantly occupied by Ce and Zr atomicspecies, respectively. The study is another demonstration of the great potential ofnon-traditional crystallography in studying the structure of nanocrystalline materials.

Amorphous and partly ordered structures in SiO2 rich volcanic glasses. An ED study

Glass structures of obsidian and pumice samples were measured using electron diffraction. Amorphous, partly ordered, and nanocrystalline regions were distinguished and analysed separately. The deconvoluted atomic distances obtained from experimental diffraction patterns through total pair-distribution functions are consistent with distances for ideal SiO 4 tetrahedra. Partly ordered structures in pumices are composed of plate-like fragments of tridymite/cristobalite layers, whereas obsidian contains quartz nanocrystals with abundant moganite-like planar faults. The validity of the structure-model of Goodman for silica glasses is discussed and an alternative interpretation is proposed for silicic volcanic glasses.

Dynamic and structural behavior of different rigid nonpolarizable models of water

The local structure and the dynamical behavior of water have been analyzed in two different regimes (the dense isochore of 0.995 g/cm2 and the supercritical isotherm of 673 K) through four rigid nonpolarizable models of water. An important change in the slope of temperature dependence of the self-diffusion coefficient at ρ=0.995 has been observed at T≈450 K, showing two main regions that are related to a change on the activation energy of the process (originated for a change of the structure of the first solvation shell from a tetrahedral to dodecahedral arrangement). The local orientational structure of water has been analyzed through the use of tetrahedral order parameters q. A direct relation between q and D has been observed for all models showing some kind of master curve up to 450 K at the 0.995 g/cm3 isochore. The structure of the system at short and large radial distances has been analyzed through a decomposition of the site–site radial distribution functions in terms of spherical harmonics, and a three-dimensional picture of the total pair distribution function has been reconstructed from this set of spherical harmonic projections.

Neutron diffraction study in amorphous Ni30Ta70 alloy powders by mechanical alloying

A mixture of elemental Ni and Ta powders with an atomic ratio of 3∶7 was subjected to mechanical alloying (MA). An amorphous Ni30Ta70 alloy was formed after 80 hrs of milling, the amorphization by rapid quenching technique of which has not been reported. The atomic structural changes were observed by neutron diffraction in the amorphization process during MA. The radial distribution function RDF(r) shows that peaks of fcc-Ni and bcc-Ta crystal broaden first and gradually approach those characteristic of an amorphous phase with increasing MA time. A local atomic environment around Ni and Ta atoms was studied by analyzing the first peak in the total pair distribution function g(r) after the completion of amorphization. We reach our conclusion from this analysis that the amorphization in the Ni30Ta70 alloy takes place by the penetration of smaller Ni atoms into the bcc-Ta lattice.

The liquid-state structure of difluoromethane

Neutron scattering experiments have been performed on liquid difluoromethane (CH2F2, R32) resulting in the first experimental determination of the total structure factor and total pair distribution function of difluoromethane. Analysis of the measurements has led to the liquidstate intramolecular structure of this fluid. The structure has been compared with that obtained for some other halogenated methanes. A comparison with results obtained for trifluoromethane (CHF3, R23) suggests that there is less long-range order in liquid-state difluoromethane than trifluoromethane. Subtraction of the intramolecular structure leads to an intermolecular structure factor and an intermolecular pair distribution function, both of which are crucially important for assessing the reliability of models of the pair potentials used in computer simulations.

Local structure of In0.5Ga0.5As from joint high-resolution and differential pair distribution function analysis

High resolution total and indium differential atomic pair distribution functions (PDFs) for In0.5Ga0.5As alloys have been obtained by high energy and anomalous x-ray diffraction experiments, respectively. The first peak in the total PDF is resolved as a doublet due to the presence of two distinct bond lengths, In–As and Ga–As. The In differential PDF, which involves only atomic pairs containing In, yields chemical specific information and helps ease the structure data interpretation. Both PDFs have been fit with structure models and the way in that the underlying cubic zinc-blende lattice of In0.5Ga0.5As semiconductor alloy distorts locally to accommodate the distinct In–As and Ga–As bond lengths present has been quantified.

Life Time of Structural Units and Viscosity of Borate Melt by Computer Simulation

Molecular dynamics simulation was carried out for B 2 O 3 -Na 2 O melt by using the modeled pair potential. The total pair distribution function derived was in good agreement with the experimental result. The structural units such as four- or three-coordinated B atoms and their life times were analyzed to characterize the network structure of melt. It was found the life time of four-coordinated B atom was very short. We also simulated the viscosity of the melt and discussed the relation of the viscosity and network structure.

PDFFIT, a program for full profile structural refinement of the atomic pair distribution function

The programPDFFITis designed for the full profile structural refinement of the atomic pair distribution function (PDF). In contrast to conventional structure refinement based on Bragg intensities, the PDF probes thelocalstructure of the studied material. The program presented here allows the refinement of atomic positions, anisotropic thermal parameters and site occupancies as well as lattice parameters and experimental factors. By selecting individual atom types one can calculate partial and differential PDFs in addition to the total PDF. Furthermore one can refine multiple data sets and/or multiple structural phases. The program is controlled by a command language which includes a Fortran style interpreter supporting loops and conditional statements. This command language is also used to define the relation between refinement parameters and structural or experimental information, allowing virtually any constraint to be implemented in the model.PDFFITis written in Fortran-77, and the source code and documentation are availableviathe World Wide Web.

Structural correlations in titanium dioxide

Molecular-dynamics simulations were performed, by using a phenomenological potential model, which takes into account steric repulsion due to atomic sizes, screened Coulomb interaction from charge transfer, charge-dipole interaction due to the large electronic polarizability of the anions, and dipole-dipole interaction, in order to investigate the short- and intermediate-order correlations in the amorphous phase of ${\mathrm{TiO}}_{2}.$ The results are presented for partial and total pair-distribution functions, coordination numbers, partial static structure factors, and density-density, charge-charge, and x-ray structure factors. Partial structure factors show that correlations at the length range $3\ensuremath{-}8$ \AA{} are responsible by the first sharp diffraction peak, which is almost absent in the charge-charge structure factor. Bond-angle distributions are also determined to analyze the orientational correlations in the various phases of the system. Our results are compared with experimental data inferred from x-ray spectroscopy.

The increased intensity of the first sharp diffraction peak of a melt

The most striking change in the x-ray diffraction pattern of glass which takes place during melting is the generation of a strong first sharp diffraction peak (FSDP). Starting from a model of glass and using reverse Monte Carlo simulations, the structure of molten is approximated by replacing the ions with ions and displacing them in a random manner within the extended cages originally occupied by the larger ions. In this way the increased vibrations and the additional diffusive motion of the ions are taken into account. This motion requires considerably more room in the melt than is occupied by the cations in the glass. The model reproduces the structure factor of the melt, including the FSDP, and explains the disappearance of the peak at 0.35 nm in the total pair distribution function.

Comparison of the Structure of Liquid Amides as Determined by Diffraction Experiments and Molecular Dynamics Simulations

The intermodular structures of liquid formamide, N-methylformamide and N,N-dimethylformamide at room temperature are studied by means of NVE molecular dynamics computer simulations. Newly developed flexible models are used. X-ray and neutron weighted structure and total radial pair distribution functions are computed from the simulated site-site pair distribution functions. They are compared with experimental results. The agreement is usually satisfactory as far as heavy atom pairs are concerned while the lengths of the hydrogen bonds are found to be systematically too long in the simulations.

ＭＡによって作製されたＣｕ‐ＴａおよびＮａ‐Ｔａアモルファス粉末の原子構造

Two mixtures of elemental powders Ni30Ta70 and Cu30Ta70 with a negative and positive heat of mixing, respectively, were subjected to mechanical alloying(MA) and the resulting structural changes have been studied by X-ray diffraction, neutron diffraction and DSC measurements. We found that the amorphization occurs by MA in both samples, in particular, despite the fact that the atomic pair of Cu and Ta is characterized by a positive heat of mixing. The coordination numbers and atomic distances of the Ni-Ni(Cu-Cu), Ni(Cu)-Ta and Ta-Ta pair correlations for the amorphous Ni30Ta70 and Cu30Ta70 phases was deduced by assuming the first nearest neighbors of the total pair distribution function g(r) to consist of the three partial ones, respectively. We concluded that the short range structure is fairly similar in both amorphous phases.

Structure of Amorphous Ge–S System by X-Ray Diffraction

An X-ray diffraction study has been carried out for characterizing the structure of the amorphous germanium-sulphur system. An essential feature of the total structural functions, structure factors and pair distribution functions is unchanged in the region between 40 and 15 at.% Ge. It is also found that their number densities are linearly increased on going from 40 to 15 at.% Ge and the number density of crystalline GeS 2 is considerably larger than that of amorphous GeS 2 in which there involve many vacant sites. The partial structural functions of a i j ( q ) and g i j ( r ) have also been estimated from measured total structural data using an assumption of their concentration independence. A possible model was proposed for explaining the short range ordering in amorphous Ge–S.

Definition of exchange based on cumulant expansion: Correlation induced narrowing of the exchange hole

Abstract Under the assumption, that the cumulant expansion of the reduced density matrices has a fundamental meaning, a corresponding decomposition of the pair distribution function of the homogeneous electron gas is considered: g(r) = 1 − 1 2 [f(r)] 2 −h(r) . Here the one-particle density matrix n(r, r′) = nf(|r-r′|) arises from the correlated momentum distribution function n(k) and h(r) is the irreducible part of g(r). In comparison with [f°(r)]2 arising from the uncorrelated Hartree-Fock momentum distribution function n°(k) the corresponding correlated function [f(r)]2 is slightly narrowed leading to a reduction of the exchange energy ϵ x . Normalization condition and virial theorem contain the mutual density dependence of the different parts of the total pair distribution function hole 1 - g(r). The situation is illustrated with the help of a simple model for the correlated momentum distribution function.

Molecular Dynamics Studies of Molten AgI.I. Structure and Dynamical Properties

The structural and dynamical properties of molten AgI are investigated using molecular dynamics calculations and pair potentials originally given by Parrinello et al . and modified by authors. The total pair distribution functions, thus obtained, are in good agreement with experimental results by Takahashi et al . and with theoretical results by Stafford and Silbert. The wave-number-dependent static dielectric function e( k ) is also calculated with use of the charge correlation function in the both α- and molten phases. In the α-phase, e( k ) is negative for small wave numbers and approaches zero at special wave numbers, which correspond to the positions of the Debye lines in the reciprocal lattice space. e( k ) in the molten phase shows the forerunning phenomenon to the crystalization, which is a very similar behavior to that in the strongly coupled one component plasma.