General Consideration of Ferroelectrics and Ferroelastics such that the Electric Susceptibility or Elastic Compliance is Temperature Independent in the Prototypic Phase

Ferroelectrics and ferroelastics such that the principal element of electric susceptibility tensor or elastic compliance tensor is almost independent of temperature in the prototypic phase down to the Curie point are considered in a general and unified way. For these ferroelectrics and ferroelastics it is essential that the free energy function describing them contains terms of coupling between the first power of electric polarization or mechanical strain and the N th power of the quantities motivating the phase transformation, and that the smallest, n , of the permissible values of N is >1. It is found out how n (>1) influences the curves of spontaneous polarization and electric susceptibility or of spontaneous strain and elastic compliance versus temperature around the Curie point.