Composition Profile Reconstruction From Simulated X-Ray Diffraction Patterns of Nanometer-Sized Core/Shell Nanoparticles

A computation approach to the reconstruction of composition profiles at various stages of diffusional mixing in core/shell nanoparticles has been developed and tested on simulated x-ray diffraction data. The algorithm consists of a two-step approach. In the first step a set of atomic coordinates based on an arbitrary initial composition profile is constructed and relaxed using a Monte Carlo algorithm and realistic Lennard-Jones interaction potentials. In the second step the Debye formula is used to calculate the corresponding diffraction profile which is compared to the target diffraction pattern. An updated concentration profile is then constructed and the entire process repeated with the best concentration profile at each iterative step used as the starting point for the iteration. This approach does not assume that the actual composition profile is a solution to the diffusion equation and, therefore, can be used in the analysis of x-ray diffraction data on nanostructured diffusion couples and at very early diffusion times and/or in the presence of steep composition gradients.