Definitional boundaries

As with any technical field, learning about scattering involves absorbing a host of new definitions and jargon. As usual, definitions are not always as clear and consistent as we would like. This page tries to highlight some of the ambiguities.

In the scattering field, the core of the problem is that various techniques were developed to address different limiting cases. However, the fundamental interactions are the same in all cases. As more complex materials began being studied, the resultant data included a mixture of effects; it was no longer so easy to define which 'idealized' experiment one was performing. As instruments have become more versatile, a technique/dataset intended to measure a certain property can now be used to measure many different things. New kinds of samples (nano-materials, aperiodic crystals, etc.) have also upended historical assumptions.

Scattering

The terms diffuse scattering, scattering, diffraction, crystallography, etc. are used somewhat inconsistently. Traditionally, diffraction was used to

Crystallography

Crystallography typically refers to measuring a single-crystal sample to generate a 2D image with a large number of diffraction peaks. Peak indexing can be used to determine the symmetry and size of the unit cell. The peak intensities can then be used to fit for the probable electron-density distribution within the unit cell; i.e. to solve the crystal structure. Conceptually, a crystal is thus assumed to be a material that has a well-defined unit cell, which is repeated translationally throughout space (normally in all three dimensions). However, the more recent discovery of quasicrystals has forced a rethink of this definition. Quasicrystals do not have translational symmetry, yet they have well-defined local packing that is repeated throughout space, and indeed their diffraction patterns have well-defined peaks. Modernly, a crystal might instead be defined by as an ordered solid that exhibits an essentially discrete diffraction pattern.

• Carlos Basílio Pinheiroa and Artem M. Abakumov Superspace crystallography: a key to the chemistry and properties IUCrJ 2015, 2(1), 137-154. doi: 10.1107/S2052252514023550