Mode crystallography of distorted structures

摘要: The description of displacive distorted structures in terms symmetry-adapted modes is reviewed. A specific parameterization the symmetry-mode decomposition these pseudosymmetric defined on setting experimental space group proposed. This approach closely follows crystallographic conventions and permits a straightforward transformation between conventional descriptions structures. Multiple examples are presented showing insight provided by approach. methodology shown at work, illustrating its various possibilities for improving characterization structures, example: detection hidden structural correlations, identification fundamental marginal degrees freedom, reduction effective number atomic positional parameters, quantitative comparison with same or different group, false refinement minima, systematic thermal behavior, rationalization phase diagrams symmetries families compounds etc. close relation superspace formalism applied to commensurate superstructures also discussed. Finally, application this field ab initio first-principles calculations outlined. At present, there several freely available user-friendly computer tools performing automatic analyses. use programs does not require deep knowledge theory can be either posteriori analyze given structure priori parameterize determined. It hoped that article will encourage tools. All here have been worked out using program AMPLIMODES [Orobengoa et al. (2009). J. Appl. Cryst. 42, 820–833].