Considerations of Partially Ferroelastic and Partially Antiferroelastic Crystals and Partially Ferroelectric and Partially Antiferroelectric Crystals

It was reasoned that in normal ferroelastic crystals, if their prototypic (or peraelastic) phase transitions are of the second kind, the dependence of the spontaneous strain on temperature T should be proportional to ( T _c - T ) ^1/2 right below the transition temperature T _c and the T dependence of the elastic compliance should obey the so-called “Curie-Weiss law” above and below T _c . However, it is also considered that possible are ferroelastic crystals in which, whereas their prototypic phase transitions are of the second kind, the T dependence of the spontaneous strain should be proportional to T _c - T right below T _c and the T dependence of the elastic compliance should disobey the Curie-Weiss law above and below T _c . In this paper, two models of such ferroelastic crystals are presented and thermodynamically investigated. One of these models is not only ferroelastic but also feeroelectric; its spontaneous polarization and electric susceptibility should so peculiarly depend on T as its spontaneous strain and elastic compliance. These crystals are a kind of partially ferroelastic and partially antiferroelastic crystals and a kind of partially ferroelectric and partially antiferroelectric crystals; the ferroelasticity and ferroelectricity are “weaker” than the antiferroelasticity and antiferroelectricity respectively.