IMPROVED MODELING OF RESIDUAL STRAIN/STRESS AND CRYSTALLITE-SIZE DISTRIBUTION IN RIETVELD REFINEMENT, D. Balzar, N.C. Popa, pp. 152-157

We propose two improvements to the microstructural modeling in Rietveld refinement. The first is the model that yields the complete texture-weighted strain and stress tensors as a function of crystallite orientations, as well as the average values of macroscopic strain and stress tensors for all Laue classes. Implementation in the Rietveld refinement program can be made through the set of refinable parameters that allow the calculation of strain and stress values. Another topic discussed is the problem of determination of coherently diffracting domain size and domain-size distributions. We present an approach where the exact size-broadened profile is obtained by averaging the interference function with a lognormal size distribution of spherical crystallites. We show that obtained size-broadened profile can be successfully modeled by a combination of Gaussian and Lorentzian functions, which permits for an analytical convolution with strain and instrumental profiles in Rietveld programs. This model can successfully fit “super-Lorentzian” line profiles that originate from a large dispersion of crystallite sizes.