STRUCTURAL STUDIES OF AMORPHOUS MATERIALS USING X-RAY ANOMALOUS SCATTERIN

The use of synchrotron radiation sources promises to provide new experimental options for determining atomic arrangements in amorphous materials and to provide more definitive structural analysis than has been obtained with conventional techniques such as x-ray and neutron radial distribution function (RDF) analysis. Using conventional techniques, the structural analysis of a binary (A-B) amorphous material is frequently ambiguous because the RDF is a weighted sum of the A-A, A-B and B-B partial distribution functions (PDF's). For example, two types of models have been proposed for the structure of amorphous GeSe. In one, each Ge(Se) is coordinated by three Se(Ge) atoms while in the other each Ge is fourfold and each Se twofold coordinated. To distinguish between these two types of models a very simple question must be answered. Are there Ge-Ge near neighbors? Yet, both models are consistent with experimentally obtained RDF's and it has not been possible to answer this question.;Shevchik and Keating have proposed techniques to obtain more information about the structure of a binary amorphous material by changing the relative scattering amplitudes of atomic species in the material. With neutron diffraction similar modifications of the scattering amplitudes have been accomplished by isotopic substitution and used by various workers to study the structure of liquid alloys.;Since neutron scattering techniques are difficult to employ in the study of many amorphous materials (they require large samples and the availability of suitable isotopes) the development of similar techniques for use with x-ray analysis should prove useful. X-ray scattering amplitudes can be modified by tuning the incident wavelength extremely close to a characteristic absorption edge of an atom in the sample which results in large anomalous scattering effects. Since these techniques require tuning the incident photon energy close to an absorption edge, a synchrotron radiation source greatly facilitates these measurements.;For these techniques to yield accurate results, the anomalous scattering factors (ASF) must be well characterized including their magnitude and degree of chemical transferability. Measurements of the ASF have been performed using two techniques. First, the angular dependence of the total reflection of x-rays from an evaporated film of GeSe has been measured as a function of photon energy. By fitting a theoretical relation to this data the ASF have been determined. The second technique involves measuring the x-ray absorption coefficient of a thin film of GeSe which is then related to the ASF by using the optical theorem and the Kramers-Kronig dispersion relationship.;To demonstrate the usefulness of these techniques we have applied them to determining the structure of amorphous GeSe. Data collection techniques have been developed and experimental data measured near the Ge and Se K absorption edges in amorphous GeSe. These measurements are complicated by the extremely high fluorescent rates near the absorption edges. Data analysis techniques have been developed and these data analyzed to obtain the average of the Ge-Ge and Ge-Se PDF's of amorphous GeSe.;The results of this analysis support the threefold coordinated Ge models of the GeSe structure. The dissertation contains an analysis of proposed improvements to the data collection techniques and alternate data analysis procedures that, coupled with the techniques used in the analysis of the GeSe data, probably will provide reliable solutions to the structure of many binary amorphous alloys. We believe that these techniques will become very useful but should currently be viewed as being in a developmental period.