Mode crystallography of distorted structures

Acta Cryst. (2010). A66, 558-590 (doi.org/10.1107/S0108767310016247)

Antiferrodistortive primary distortion mode (from a parent P6₃22 structure) present in the P2₁ structure of SrAl₂O₄.

Many crystals can be considered distortions of configurations of higher symmetry. Their description in terms of symmetry-adapted modes has many advantages. Free computer tools that closely follow crystallographic conventions are now available for a straightforward application of this approach. The methodology is reviewed and several examples illustrate its possibilities, as for instance, detection of hidden correlations, identification of fundamental and marginal degrees of freedom, reduction of the number of positional parameters, comparison of structures, detection of false refinement minima, and analysis of thermal behaviour or phase diagrams.