Structural Complexity in Disordered, Layered Crystals
Science of Complex Systems Seminar
Center for Computational Science and Engineering
University of California, Davis
February 2006
Abstract:
A significant challenge in condensed matter science is the discovery and
characterization of structure in complex, disordered materials directly
from their x-ray diffraction spectra. A broad class of layered materials,
called polytypes, can exist in a wide range of both ordered and disordered
stacking configurations. Examples of polytypes include micas and kaolins,
and substances of technological importance, such as the wide band gap
semiconductor silicon carbide. While standard crystallographic techniques
can identify most ordered stacking structures, understanding the diffuse
diffraction spectra arising from disordered specimens has proven more
challenging. In this talk, I will briefly discuss the phenomenon of
polytypism at a level suitable for a general scientific audience. I will
introduce a novel technique for detecting and characterizing disordered
stacking structure directly from x-ray diffraction spectra. The resulting
expression for the structure is a directed graph. I will demonstrate the
technique on x-ray diffraction spectra obtained from zinc sulphide
crystals and show how it provides insight into the complex stacking
structure of these crystals as well as allows for the calculation of
material properties of physical import. The techniques introduced here are
quite general, and are applicable to the problem of inferring structure
(either spacial or temporal) given an experimental signal in the form of a
power spectrum.