The concepts of symmetry and phase transition are closely related. A change of symmetry in a solid necessarily implies a thermodynamic phase transition. In addition, ferroic properties (and in general multistability) are connected with a real or virtual symmetry break, ferroic structures being necessarily pseudosymmetric. Under this perspective, a brief review of the basic features governing the structural properties of ferroic and distorted materials in general will be presented. Using a few examples we pretend to show the predictive power of a complete characterization of the symmetry break implicit in any ferroic, and in general in any distorted structure. Basic points to be reviewed are:

• Symmetry break and order parameter. Landau postulate: active irrep.
• Order parameter direction and isotropy subgroups.
• Multiestability. Domains. Ferroics and multiferroics. Ferroic species.
• Crystallographic meaning of the order parameter.
• Hierarchy of distortion modes: primary and secondary modes.
• Proper and improper ferroics.
• Transitions with several active irreps.

By means of a set of worked out examples and some exercises an overview of the tools and programs available in the Bilbao Crystallographic Server for this type of problems will be given. Also the application of some other programs available in internet [ISOTROPY, H. T. Stokes, D. M. Hatch, and B. J. Campbell, (2007), stokes.byu.edu/isotropy.html] will be briefly discussed. The main tasks and related programs included in this practical session are:

• Prediction of symmetry of distorted phase: CELLSUB (INVARIANTS, ISODISPLACE).
• Prediction of ferroic properties (primary and secondary variables): SYMMODES (COPL).
• Mode decomposition: identification of transition mechanism: SYMMODES.

Supplementary Material