
Environmental Sciences
Seminar Abstract
Environmental and Geologic Materials: Probing local structure by Pair Distribution Function (PDF) analysis of total scattering data
Xray diffraction has been widely used over the past century or so to determine the position of atoms in relation to each other in solids. Routinely, crystal structures are determined by examination of the Bragg peaks diffracted from single crystals with dimensions in the order of tens of micro meters. In the last few decades, the improvement in powder instrumentation and development of Rievteld refinement techniques have led to the refinement of crystal structures from polycrystalline samples, a multigrain sample with grain sizes in the order of a micro meter, becoming commonplace. Both techniques scrutinize Bragg peak position and intensity, a consequence of long range order in the sample. More recently, with the advent of high energy synchrotrons and spallation neutron sources, Pair Distribution Function (PDF) analysis of powder diffraction data has been applied to crystalline material. This involves taking the Fourier transform of the total structure function (both the Bragg and diffuse scattering), giving a real space distribution of distances between pairs of atoms in a structure. This technique has proved powerful in systems where the local structure differs from the average structure; often the local structure is key in understanding the properties of complex materials. High values of the elastic scattering vector, Q = 4πsinθ/λ, are required as the real space resolution is directly related to the value of Q_{max} used in the Fourier transform. Since no periodicity in necessary for PDF analysis, it has proved a useful tool in the investigation into the structure of crystalline solids, glasses, amorphous materials and nanomaterials; particles whose size is in the order of tens of nanometers. Last updated: 03/02/2009 