A Novel Approach to Discovering and Describing Planar Disorder in Close-Packed Structures from X-Ray Diffraction Spectra

Seminar, Center for the Study of Complex Systems

University of Michigan

November 2002


Polytypism is the phenomenon of a substance that can assume two or more layer-like structures--each having the same stoichiometry--but differing only in the manner of stacking. A substance particularly rich in observed structures is zinc sulphide, with some crystalline structures having unit cells extending over 100 layers. Also of interest are specimens that show considerable disorder, often as a result of an arrested solid state transformation between two crystalline structures. There can be regions in the crystal that largely preserve the original crystalline structure interspersed between regions containing more disorder or even different crystal structures. It has been a challenge to characterize these disordered samples on the basis of their diffraction spectra. I present a novel method of detecting and describing planar disorder in close-packed structures from their diffraction spectra which I demonstrate on experimental ZnS diffraction patterns. Incorporating ideas originating from the study of dynamical systems, I outline a method that provides the unique, minimal and most compact model of the stacking disorder possible. From this model, I am able to calculate physical parameters such as the average stacking fault energy and the stacking entropy. I contrast this approach to more traditional views of disorder such as the fault model, Jagodzinski's disorder theory and reverse Monte Carlo techniques.

Last updated: 26 April 2009. Copyright © 2008-2010 by Dowman P. Varn. Contact: dpv@ComplexMatter.org

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