Differential Pair Distribution Function Research Articles

D-48 Investigation of Surface Structures by Powder Diffraction: A Differential Pair Distribution Function (PDF) Study into Arsenate Sorption on Ferrihydrite

D-48 Investigation of Surface Structures by Powder Diffraction: A Differential Pair Distribution Function (PDF) Study into Arsenate Sorption on Ferrihydrite

Application of high-energy X-rays and Pair-Distribution-Function analysis to nano-scale structural studies in catalysis

We investigate the structure of supported Pt catalysts using high-energy X-ray scattering coupled with Pair-Distribution-Function (PDF) analysis. Recently, experimental approaches that enable the collection of PDF data in situ have been developed with time-resolution sufficient to study the structure of Pt nano-particles as they form. The differential PDF approach is utilized which allows the atom–atom correlations involving only Pt to be selectively recovered, enabling structural investigation of the supported particles and the mechanism of their formation. In parallel to the in situ analysis, we have examined samples prepared ex situ. Data collected on the ex situ samples show that the initial deposition of Pt 4+ occurs as the PtCl 6 2− species which are retained even when annealed in an oxygen atmosphere. The Pt differential PDFs of the samples reduced in hydrogen at 200 and 500 °C indicated nano-crystalline face-centered-cubic (fcc) metallic Pt particles. The ex situ reduced samples also contain a weak correlations at 2.1 Å, which we assign to Pt–O interactions between the particles and the support surface. The in situ experiments, following the reduction of Pt 4+ from 0 to 227 °C, indicate that the initial Pt nano-particles formed are ca. 1 nm in size, and become larger and more crystalline by 200 °C. The data suggest a particle growth mechanism where the initial particles that form are small (<1 nm), then agglomerate into ensembles of many small particles and lastly anneal to form larger well-ordered particles. Lastly, we discus potential future developments in operando PDF studies, and identify opportunities for synchronous application of complementary methods.

Partial structure of Pd42:5Ni7:5Cu30P20 bulk metallic glass

In order to study local structure in Pd42.5Ni7.5Cu30P20 excellent bulk metallic glass-former, an anomalous x-ray scattering (AXS) experiment was performed at energies close to the Pd, Ni and Cu K absorption edges together with the reference Pd40Ni40P20 and Pd40Cu40P20 glasses at the beamline BM02 of the ESRF. The differential structure factors, ΔiS(Q), were obtained with a good statistical quality. A small shoulder observed at about 19 nm-1 in S(Q) becomes prominent in ΔPdS(Q), indicating a Pd-Pd atomic correlation of 0.32 nm. This pre-shoulder was not observed in Pd40Ni40P20 bulk glass having a slightly worse critical-cooling-rate, but in Pd40Cu40P20 glass ribbon available by only ultrafast quenching. It is suggested that covalently bonded Pd-P-Pd connection with an almost right bond angle would be formed by replacing the Ni with the Cu atoms. From the obtaining differential pair distribution function around the Pd K edge, ΔPdg(r), the first peak position in Pd42.5Ni7.5Cu30P20 shows a longest atomic distance among the glasses and the width of the first peak is slightly narrower than that in Pd40Cu40P20. Thus, a more uniform atomic arrangement around Pd atoms with a loose packing, i.e., a longer and uniform Pd sublattice, is expected for Pd42.5Ni7.5Cu30P20 compared to the other two reference glasses.

Direct observation of adsorbed H2-framework interactions in the Prussian Blue analogue MnII3[CoIII(CN)6]2: The relative importance of accessible coordination sites and van der Waals interactions

Selective recovery of the guest-framework interactions for H(2) adsorbed in a nanoporous Prussian Blue analogue, through differential X-ray and neutron pair distribution function analysis at ca. 77 K, suggests that the H(2) molecule is disordered about a single position at the centre of the pore, ((1/4),(1/4),(1/4)), without binding at accessible Mn(II) sites.

Selective Recovery of Dynamic Guest Structure in a Nanoporous Prussian Blue through in Situ X-ray Diffraction: A Differential Pair Distribution Function Analysis

We use in situ X-ray diffraction to obtain data suitable for differential PDF analysis, providing unique insight into the structure of weakly bound, dynamic N2 molecules in the Prussian blue system Mn3[Co(CN)6]2.x{N2}. The differential PDF shows a distributed orientation of N2 molecules constrained to sites centered about the (1/4,1/4,1/4) positions within the pores. The results show a subtle response of the framework to guest loading which corresponds principally to the perturbation of the Mn ion coordination.

Metastable States During Devitrification of a Zr70Pd20Cu10 Metallic Glass

ABSTRACTHigh energy synchrotron x-rays (124.63 keV) are used to investigate the initial stages of the devitrification for the Zr70Pd20Cu10 metallic glass prepared by melt-spinning (MS). Due to the excellent signal:noise, we are able to determine the initial nucleating phase by analyzing the differences in the total scattering function S(Q) as a function of time at a temperature ∼ 50 K below the crystallization temperature. The alloy undergoes a structural relaxation prior to nucleation and growth. Devitrification proceeds from nucleation of the icosahedral phase. The differential pair distribution function (dPDF) indicates that the as-quenched alloy may have icosahedral-like order which undergoes local rearrangement to true icosahedral order at an annealing temperature 50 K below the crystallization temperature. More importantly, time-resolved high-energy synchrotron is shown to have excellent sensitivity to the initial atomic rearrangements preceding nucleation in metallic glasses.

Metastable States During Devitrification of a Zr70Pd20Cu10 Metallic Glass

ABSTRACTHigh energy synchrotron x-rays (124.63 keV) are used to investigate the initial stages of the devitrification for the Zr70Pd20Cu10 metallic glass prepared by melt-spinning (MS). Due to the excellent signal:noise, we are able to determine the initial nucleating phase by analyzing the differences in the total scattering function S (Q) as a function of time at a temperature ∼ 50 K below the crystallization temperature. The alloy undergoes a structural relaxation prior to nucleation and growth. Devitrification proceeds from nucleation of the icosahedral phase. The differential pair distribution function (dPDF) indicates that the as-quenched alloy may have icosahedral-like order which undergoes local rearrangement to true icosahedral order at an annealing temperature 50 K below the crystallization temperature. More importantly, time-resolved high-energy synchrotron is shown to have excellent sensitivity to the initial atomic rearrangements preceding nucleation in metallic glasses.

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.

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.

Local atomic structure of semiconductor alloys using pair distribution functions. II.

A method of calculating the pair distribution function in semiconductor alloys has been extended to the differential and the partial pair distribution functions, and applied to pseudobinaries of the form ${A}_{1\ensuremath{-}x}{B}_{x}C$ with the zinc-blende structure. We have used a simple valence force model with bond-stretching and bond-bending forces. Results of calculations are presented for ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{As}.$ The differential pair distribution function provides a useful experimental way of determining the structural distortions in semiconductor alloys---specifically the mean nearest-neighbor and second-neighbor distances and widths can be obtained for chemically specific pairs of atoms.

Atomic structure of quasicrystalline Al65Ru15Cu20.

The atomic structure and chemical ordering of ${\mathrm{Al}}_{65}$${\mathrm{Ru}}_{25}$${\mathrm{Cu}}_{20}$ quasicrystals was studied by anomalous-x-ray-diffraction measurements of powder samples carried out at the Ru and Cu K edges. The differential atomic pair distribution function of Ru, and partial structure factor of Ru and Cu, were obtained. The results suggest that a strong covalent bonding between Al and Ru is contributing to the stability of this phase. A structural model obtained by a direct projection of a six-dimensional hypercubic lattice with the vertex decoration using a non-Penrose window is shown to account for the results quite well. Compositional ordering is specified by subdividing the window for each composition. The structure of this phasonless quasicrystal is fundamentally different from the structures of other quasicrystals with phasons.

Site of ruthenium in icosahedral AlMnRuSi

In order to investigate the sites occupied by ruthenium atoms in the Al 76Mn 14Ru 7Si 3 and Al 68Mn 20Ru 8Si 4 icosahedral alloys, the differential structure factors (DSF) and the differential pair distribution functions (DPDF) concerning ruthenium were obtained by the anomalous X-ray scattering technique with synchrotron radiation. On the assumption that ruthenium atoms occupy aluminum or manganese sites in the atomic structure model (Mackay-type; Quasiperiodic Configuration of Icosahedral Clusters (QCIC) model) of the AlMn icosahedral phase, it is concluded that the ruthenium atoms are preferentially substituted at manganese sites which almost correspond to the vertices of the three-dimensional Penrose tiling (3D-PT) in i-Al 76 Mn 14 Ru 7 Si i3. In the case of i-Al 68 Mn 20 Ru 8 Si 4, ruthenium atoms are substituted for two kinds of atomic sites, one of which is the manganese site and the other is that of aluminum atoms at the innermost shell of Mackay icosahedra.

Quasicrystallinity of icosahedral Pd58.8U20.6Si20.6.

The uranium differential atomic pair distribution function (DDF) of icosahedral ${\mathrm{Pd}}_{58.8}$${\mathrm{U}}_{20.6}$${\mathrm{Si}}_{20.6}$ is shown to compare well with the vertex-vertex distance distribution function of a quasicrystalline lattice up to 40 A\r{}. The DDF is also compatible, but less favorably, with the icosahedral glass model, which we point out has a local structure very similar to an ideal quasicrystalline structure up to 50 A\r{} or more. Based upon the comparison of theoretical and experimental values of physical density as well, we conclude that the icosahedral ${\mathrm{Pd}}_{58.8}$${\mathrm{U}}_{20.6}$${\mathrm{Si}}_{20.6}$ is better described by the quasicrystalline model.