W0381

Structural Analysis of Modulated Structures – An Introduction. Charles F. Campana¹, Václav Petrícek², Michal Dusek² and Sander van Smaalen³; ¹Bruker AXS, Inc., 5465 East Cheryl Parkway, Madison, WI 53711, USA; ²Institute of Physics, Academy of Science, Na Slovance 2, 182 21 Praha 8, Czech Republic; ³Laboratory of Crystallography, Univ. of Bayreuth, 95440 Bayreuth, Germany.

Aperiodic crystals are solid-state materials that lack translational symmetry in 3 dimensions, but which have a state of perfect long-range order. Their crystal structures cannot be described on the basis of a unit cell and lattice periodicity. Instead, a superspace description of the structure must be used to analyze incommensurately modulated structures and incommensurate composite crystals.

In the past decade, use of two-dimensional detectors bases upon CCD or image plate technology has become very common in crystallography laboratories. Service crystallographers often collect at least a hemisphere of reflection data, even in cases where no preliminary unit cell has been obtained. Some these datasets often turn out to be incommensurate structures.

We will provide an introductory overview of the tools which are now available to non-specialists for the analysis of modulated structures. The presentation will include examples of diffraction patterns and refined structures. The concepts of q-vectors, n-dimensional super-space and super-space groups will be discussed. Approaches for indexing and integration of experimental data will be presented. The public domain JANA2000 software package for refinement of modulated structures will be introduced.