The powder pattern of disordered structures consists of spherically integrated reciprocal space averaged over many powder particles which each may have an individual defect structure. The respective PDF is the average over all individual PDF distributions. Accordingly, the powder patter/ the PDF is best calculated by the simulation of many individual crystals, whose individual powder diffraction pattern are calculated by use of the Debye formalism, respectively, whose individual PDF's are averaged. In this presentation, a refinement algorithm that uses an evolutionary approach is presented. It allows to refine arbitrary disorder models and – for nanoparticles - arbitrary particles shapes. Besides disorder parameters, parameters that define a disorder distribution such as the size of nanoparticles can equally be refined.

As an example, the structure refinement of nanoparticles is discussed. Data were collected at the high energy beamline BW5, HASYLAB, Germany at 15K to Q_max = 25 Å^-1, and transformed to the corresponding PDF. The CdSe/ZnS core/shell particles were simulated with an elliptical CdSe core and semi-spherical ZnS shell particles, which were placed in a locally epitaxial position onto the CdSe core. Both structures were simulated as Zincblende/Wurtzite disordered structures. The refinement allows to determine structure and defect structure of the core and shell, the size and shape of the core, and the size distribution of the shell particles.

[1] R.B. Neder, V.I. Korsunskiy, Ch. Chory, G. Müller, A. Hofmann, S. Dembski, Ch. Graf and E. Rühl, phys.stat. sol. (c) 4, 3221-3233

[2] F. Niederdraenk, K. Seufert, P. Luczak, S.K. Kulkarni, Ch. Chory, R.B. Neder and Ch. Kumpf, phys.stat. sol. (c) 4, 3234-3243

[3] R.B. Neder and Th. Proffen, Diffuse Scattering and Defect Structure Simulation, Oxford University Press, in press

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