Several physical models for description of size broadening that are convenient for implementation into the Rietveld programs are described. First, isotropic models are discussed and illustrated on powder patterns of cerium oxide. Second, a new anisotropic size-broadening model, based on spherical-harmonics representation allowing determination of both volume- and area-averaged apparent crystallites, is presented. The model effectiveness is demonstrated on a zinc oxide powder pattern exhibiting strongly anisotropic size broadening and pronounced super-Lorentzian peak shapes. It is shown how the apparent crystallites can be interpreted in terms of physical models by using ellipsoidal and cylindrical crystallites with lognormal size distributions.

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1. X-ray Characterization of Crystalline Defects and Strains in Thin Films