The Definition and Classification of Ferroics

The phase transitions in solids still attract substantial attention of scientists and engineers due to many anomalies in their vicinity. Usually, at the phase transition point, the breaking of symmetries including translational, orientational and time inversion ones [1] takes place. 

Glinchuk et Nanoferroics, Springer Series in Materials Science 177, DOl 10.1007/978-94-007-5992-3-1, 

Characteristic feature of ferroics is the existence of at least two equivalent states which differ only in their orientations (either of some structural units or spontaneous electric/magnetic moment, or both) called orientation states. The term prototype phase means real or hypothetical phase of crystal where all of the orientational states are the same. It is clear that the prototype phase has higher point symmetry group, than real ferroic. Therefore there is a phase transition in a ferroic if [2]  

These conditions lead to four important features of the ferroics. 

In the phase, where some point group symmetry element is lost, there are at least two orientational states [3]. The regions, where these states occur, are called domains, and the regions separating the domains are called domain walls. 

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