Diffuse scattering from microstructures and mesostructures

Abstract

Mesoscopic features, such as thick twin walls, surface relaxations, or haloes around metamict regions in crystals have a lengthscale of the order of 1–100 A. Such features are studied experimentally using diffuse scattering signals; either between pairs of twin-related Bragg peaks (for twin walls) or around Bragg peaks (for features such as metamict haloes). Study of such features by X-ray diffraction has a number of advantages over the use of electronmicroscopy, many related to the reduced need for invasive sample preparation. The weak intensity of diffraction from mesostructures requires careful attention to experimental design, either using extremely strong incident radiation, or optimised collection geometry. Measurements of twin wall widths in a range of ferroelastic and co-elastic materials and minerals and their temperature dependence are reviewed, and found to be consistent with theoretical predictions and the results of other experimental methods. Determination of the size of metamict regions in zircon from diffuse scattering agree with the results of microscopy experiments.