Theory of phase diagram of ferroelectrics with isostructural phase transformation

(NH3CH3)5Bi2X11(X = C1, Br) belong to a rich family of alkylammonium halogenobismuthate crystals showing ferroelectric properties and several successive structural phase transitions (IT) [l-2]. They are characterized by the same sequence of the phases symmetry changing [1,3]. Both of them display the isostructural PT; diffusive if X = C1, and abrupt if X = Br. Supposing that the nature of structural instability is the same in both substances and taking into account that the both compounds crystallizing in the form of pseudohexagonal prisms [4], we present a phenomenological theory of phase diagram to unify the description of both crystal properties, using a parent-phase approximation. As a parent phase for the both crystals we will choose a hexagonal degenerate structure which suffers the hypothetical proper ferroelastic phase transition to the real orthorombic paraelectric state.