An introduction to powder diffraction crystallography and GSAS

Powder diffraction allows diffraction measurements to be made from crystalline materials that do not provide suitable single crystals for individual measurements. While powder diffraction avoids many problematic issues in single-crystal measurements and analysis, it does so at a net loss of information, since the three-dimensional reciprocal lattice is spherically averaged to a single dimension. Nonetheless, crystallographic analysis of powder diffraction data is a growing field, with many new techniques being developed.

The single greatest advance in powder diffraction crystallography came from the work of Hugo Rietveld and coworkers at the Petten reactor (Netherlands) in the 1960's. In the Rietveld method, structures are fit directly to powder diffraction patterns, along with instrumental/specimen dependent parameters, such as peak profile terms. This is in contrast to fitting to instrument- and specimen-independent structure factors, as done in other areas of crystallography. The Rietveld technique will be introduced, as well as the GSAS software system, which is the only package that implements crystallographic fitting for all types of measurements and materials.

If time allows, I will also introduce other topics essential for powder diffraction crystallography: indexing and structure solution approaches and discuss some of the ideas for improvements to GSAS being considered at Argonne, as well as our hopes to initiate a next-generation software package (GSAS-II).