Milen Gateshki

Structure of Nanocrystalline MgFe2O4 from X-ray Diffraction, Rietveld and Atomic Pair Distribution Function Analysis

The three-dimensional structure of nanocrystalline magnesium ferrite, MgFe{sub 2}O{sub 4}, prepared by ball milling, has been determined using synchrotron radiation powder diffraction and employing both Rietveld and atomic pair distribution function (PDF) analysis. The nanocrystalline ferrite exhibits a very limited structural coherence length and a high degree of structural disorder. Nevertheless, the nanoferrite possesses a very well defined local atomic ordering that may be described in terms of a spinel-type structure with Mg{sup 2+} and Fe{sup 3+} ions almost randomly distributed over its tetrahedral and octahedral sites. The new structural information helps explain the materials unusual magnetic properties.

Structure of Nanocrystalline GaN From X-ray Diffraction, Rietveld and Atomic Pair Distribution Function Analyses

The three-dimensional structure of nanocrystalline GaN has been studied by X-ray diffraction, Rietveld and atomic pair distribution function (PDF) analyses. The material is of very limited structural coherence, yet possess a well-defined atomic arrangement resembling the wurtzite structure. The study demonstrates the great power of X-ray diffraction and the PDF approach in determining the three-dimensional structure of nanocrystalline materials.

Second-order structural phase transition in Sr2CuWO6 double-perovskite oxide

In the present work we report results from neutron and synchrotron radiation diffraction measurements that confirm the presence of a very weak phase transition in Sr2CuWO6 at about 600 °C. This phase transition is continuous and changes the symmetry from I4/m at low temperature to another tetragonal phase at high temperature. On the basis of the experimental results we identify I4/mmm as the most probable space group for the high-temperature tetragonal phase.

Atomic-scale structure of nanocrystalline CeO2-ZrO2 oxides by total x-ray diffraction and pair distribution function analysis

Total x-ray diffraction and atomic pair distribution function analysis have been used to determine the atomic ordering in nanocrystalline (∼1. 5n m in size) CeO2‐ZrO2 prepared by a sol‐gel route. Experimental data show that the oxides are a structurally and chemically inhomogeneous mixture of nanoscale domains with cubic-type and monoclinic-type atomic ordering, predominantly occupied by Ce and Zr atomic species, respectively. The study is another demonstration of the great potential of non-traditional crystallography in studying the structure of nanocrystalline materials. (Some figures in this article are in colour only in the electronic version)

Structure of nanosized materials by high-energy X-ray diffraction: study of titanate nanotubes

High-energy X-ray diffraction and atomic Pair Distribution Function analysis are employed to determine the atomic-scale structure of titanate nanotubes. It is found that the nanotube walls are built of layers of Ti–O6 octahedra simular to those observed in crystalline layered titanates. In the nanotubes, however, the layers are bent and not stacked in perfect registry as in the crystal.

On the use of laboratory X-ray diffraction equipment for Pair Distribution Function (PDF) studies

Abstract At present synchrotron and neutron sources are the preferred choice for PDF analysis, but there is clearly an increasing need for a PDF solution based on inhouse diffraction equipment. Such a solution, though limited, will benefit areas where quick feedback about the materials properties is important and will allow the routine application of PDF analysis for materials characterization in university laboratories as well as industrial R&D departments. One interesting application is the use of the PDF technique for comparing amorphous pharmaceutical substances [1]. It is worth mentioning that the use of in-house diffraction equipment for PDF analysis has been reported on numerous occasions (see for example [2–4]). This article describes the latest developments of laboratory X-ray equipment for PDF analysis and discusses some results obtained with such equipment. Three examples — C60, amorphous MoS3 and crystalline CeO2 — were chosen to demonstrate different aspects of PDF data collection optimization.

GISAXS studies of mesoporous films using a standard laboratory diffractometer

With the increasing number of GISAXS (Grazing-Incidence Small-Angle X-ray Scattering) applications for the investigation of materials surface nano-structures, comes the demand for a mainstream laboratory capability to run alongside the more established synchrotron facilities. GISAXS poses considerable challenges when scaling the method to fit a multipurpose laboratory instrument, including the achievement of good angular resolution at small scattering radius, the reduction of scatter from the direct beam and the observation of low intensity signals. We have developed a hardware solution that addresses these challenges. The recent availability of small size pixel (55 micron) photon counting detectors with very low noise characteristics has enabled the implementation of new 2D imaging GISAXS hardware for a standard 1.8KW laboratory X-ray source. In this work we present a number of results that illustrate the capabilities of the new experimental set-up based on a standard multipurpose diffractometer. We present GISAXS images and analysis of a mesoporous silica thin film with close-packed hexagonal type ordering of the pores. In [1] we have reported reflectometry results and analysis of this sample structure. The addition of GISAXS information demonstrates the versatility of the multipurpose diffractometer and the strength in combining methods on one instrument. Strongly scattering Ti-filled silica mesoporous films illustrate the relative ease with which GISAXS signals can be recorded, including even the weak signal below the critical angle of the sample (fig.1). The scattering patterns from both samples exhibit subtle departures from a simple symmetry, suggesting that the films may exhibit residual strain. Thin films with vertical mesopores provide their own challenges in the observation of scatter close and parallel to the specularly reflected beam. We present results in which scattering from Co-filled mesopore structures with 37nm pitch can be clearly resolved.

PDF Analysis on a Laboratory System: Adapting the Experiment to the Material

The increased interest in recent years regarding the properties and applications of nanomaterials has also created the need to characterize the structures of these materials. However, due to the lack of long-range atomic ordering, the structures of nanostructured and amorphous materials are not accessible by conventional diffraction methods used to study crystalline materials. One of the most promising techniques to study nanostructures using X-ray diffraction is by using the total scattering (Bragg peaks and diffuse scattering) from the samples and the pair distribution function (PDF) analysis. The pair distribution function provides the probability of finding atoms separated by a certain distance. This function is not direction-dependent; it only looks at the absolute value of the distance between the nearest neighbors, the next nearest neighbors and so on. The method can therefore also be used to analyze non-crystalline materials. From experimental point of view a typical PDF analysis requires the use of intense high-energy X-ray radiation (E ≥ 20 KeV) and a wide 2θ range. After the initial feasibility studies regarding the use of standard laboratory diffraction equipment for PDF analysis [1-3] this application has been further developed to achieve improved data quality and to extend the range of materials, environmental conditions and geometrical configurations that can be used for PDF experiments. Studies performed on different nanocrystalline and amorphous materials of scientific and technological interest, including organic substances, oxides, metallic alloys, etc. have demonstrated that PDF analysis with a laboratory diffractometer can be a valuable tool for structural characterization of nanomaterials. This contribution presents several examples of laboratory PDF studies, in which the experimental conditions have been successfully adapted to match the specific requirements of materials under investigation.