Disorder and Diffuse Scattering

Group leader: PhD Students:

Dr. Thomas Weber Arkadiy Simonov    

Disorder is a common feature in crystal structures. For example, more than 20% of the ca. 500'000 structures deposited in the 'Cambridge Structural Database' are disordered. Real-structure properties and thus the basic principles of self-organization, however, are known in only a few cases. This is surprising, because modern experimental possibilities, like third generation synchrotrons, powerful area detectors etc., not only allow easy access to the disorder-related diffuse scattering, but also deliver it almost for nothing as a side product of a conventional Bragg data collection. The strong discrepancy between our knowledge about the average and disordered properties of crystals may be explained by the fact that current techniques for investigating disorder are complicated and time consuming. That is, the interpretation of diffuse scattering is strongly lagging behind experimental progress. Our goal is to develop new and improve existing techniques, so that the investigation of complex real-structure properties becomes a matter of routine.

Current projects

3D Pair distribution function analysis (3D-PDF)

Pair distribution function analysis is a well-established method for analyzing disorder in powder samples. In this project we extend this method to disordered single crystals. We take advantage of the fact that single-crystal diffraction patterns not only carry full three-dimensional information, but also allow an easy separation of Bragg and diffuse scattering. Recently, we determined the disordered superstructure of the decagonal quasicrystalline phase d-Al65Cu20Co15. Real-structure properties could be identified to an extent that is hardly possible with traditional techniques (Fig. 1). To make this method available to a broad community, we are currently developing a computer program for the quantitative refinement of 3D-PDF models of periodic and quasiperiodic disordered structures.


Fig. 1: Sections from diffuse scattering (left) and 3D-PDF patterns (right) of d-Al65Cu20Co15 (see P. Schaub, T. Weber, W. Steurer, J. Appl. Cryst. (2011) 44, 134-149.).

Monte Carlo modeling

In the frame of an international cooperation between our group, the University of Zurich and Oak Ridge National Laboratories, we are further developing Monte Carlo simulation techniques for modeling disordered crystal structures. A major goal of this project is to develop the Monte Carlo simulation computer program ZODS (Zurich Oak Ridge Disorder Simulation program), which is designed to be not only user friendly, but also fast and flexible. Special focus is on the efficient use of modern super-computer architectures and of multi-core desktop computers to take full advantage of current trends in computing technologies.


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