Abstract
The equations of state for two uniaxial systems: the first with the polar direction identical with one of the crystallographic axes and the second with the polar direction within a crystallographic plane giving a realistic Curie–Weiss constant ratio and the experimentally obtained non-classical critical exponents are used to describe the behaviours of spontaneous polarization and of the susceptibility. The theories are exemplified by the behaviour of molecular-ionic crystals (CH3NH3)5Bi2Cl11 (MAPCB), (CH3NH3)5Bi2Br11 (MAPBB) and (C3N2H5)5Bi2Br11 (IBB).
Acknowledgement
This study was supported by the Polish State Committee for Scientific Research: project register number: N N204 147837.