The X-ray Diffraction Small Research Facility: The Rietveld Method

The Rietveld Method

The Rietveld method for structural refinement of powder diffraction data has been developed over the last four decades and has proved indispensable in solving crystal structures. The process involves minimising the difference between a crystallographic model and experimental data, via a least squares refinement; such intricate modelling of the height, width and position of Bragg reflections in an X-ray diffraction pattern can yield a lot of information about the crystal structure of a material. The process involves:

• Recording the diffracted intensities at several thousand equal increments
• Developing a crystal structure model – i.e. symmetry, atomic positions, unit-cell size, site occupancies
• Varying these and other parameters by least squares refinement to get best fit between calculated and observed data
• Peak shape and diffractometer set-up are taken into account

The Rietveld Method is firmly established its position at the centre of structural physics, chemistry and materials science. This rigorous approach can also be applied to give the most accurate quantitative phase analyses possible.

Numerous software programs exist for subsequent structural refinement.  For most of our Rietveld analyses in the Department of Materials Science & Engineering, we use the `GSAS+EXPGUI´ software package.  You can find out a lot more about how EXPGUI works by watching the excellent tutorial videos at the APS website.