ABSTRACT
The practical importance of ferroic crystals stems from the occurrence of ferroic phase transitions in them. We have defined a ferroic phase transition as one which entails a nondisruptive change of the point-group symmetry of the prototype of the crystal. A reduction of the point-group symmetry is necessarily accompanied by the emergence of at least one macroscopic property coefficient, which was forbidden by the point-group symmetry of the crystal from being nonzero in the parent phase. The symmetry restrictions on physical properties of crystals come from the Neumann theorem (cf. §C.l of Appendix C).
