Date: Thursday, Feb 20th
Presenter: Dr. Jun Li, OSU Chemistry Department
The understanding of crystal structures is an important prerequisite for understanding materials properties. Techniques like X-ray and neutron powder diffraction are often employed by materials scientists in phase identification and structure determination for poly-crystalline samples. X-rays and neutrons provide complementary information for crystal structure analysis. X-rays interact with the spatial distribution of the valence electrons, while neutrons are scattered by the atomic nuclei through the strong nuclear forces. In addition, neutrons have an intrinsic magnetic moment that is useful for magnetic structure study. Structure refinement by Rietveld method is to fit structural models to X-ray and neutron diffraction data and find the right structural information, such as verifying the space groups, calculating lattice parameters and the amount of disorder or mixing between atomic sites, refining the atom positions and thermal displacement parameters, quantitatively determining the percentages of different phases, analyzing the crystallite size and strain and crystalline defects. Many software packages are available for structure refinement, among which GSAS (General Structure Analysis System) with a graphical user interface EXPGUI is most widely used especially in the United States. Various examples of structure refinement will be given and discussed, and the application of GSAS software will be briefly introduced.