Bragg peak profiles and diffuse scattering in powder diffraction

Powder diffraction has evolved enormously over thelast twenty years, both in experimental techniques and in data analysis, butdespite the general trend towards increasingly holistic analysis methods, someaspects have not progressed as much. Most analyses, such as Rietveld structuralrefinement, still employ empirical functions to model the diffraction peakprofiles, with little regard for the implications of this choice; and littleregard for the missed opportunity to gain additional insights into themicrostructure through more in-depth, physically based modelling of theprofiles. A further neglected aspect concerns the diffuse scattering componentwhich is often improperly absorbed by the background, invariably treated as adisturbance, again with empirical functions. Yet, it is well known that theintensity "lost" in Bragg scattering due to thermal disorder istransferred to the diffuse component, carrying along physical information onthe local lattice dynamic structure, especially relevant in nanocrystallinesystems.

In this presentationI show the basics and some examples of an alternative and robust approach toprofile modelling, based on physical models of the microstructure with few andstable parameters to be refined (optimized) with the comparison and analysis ofexperimental data. I also present a diffuse scattering model that captures abasic aspect, namely the correlation between atomic thermal oscillationsbetween the closest atoms. The algorithms and procedures shown are fullycompatible with the Rietveld method approach, and in particular with the TOPAScomputing platform, among the most used in vast fields of research andindustrial technologies.