Special Issue-New Evolution of the Powder Diffraction Method. The Rietveld Method.

The outline of the Rietveld method is described where structure and lattice parameters are directly refined from powder diffraction data. Several functions such as extinction correction, preferred-orientation function, and profile-shape function, which are contained in the model function, have been improved to give physical meanings to parameters contained in them. Structure-refinement strategy is introduced which may be useful for beginners. Combined refinement of X-ray and neutron diffraction data is now being used more and more widely to extract structural information from powder patterns as much as possible.