Microscopic model of phase transition in the crystals of DMAAlS and DMAGaS type

The four-state model is proposed for description of phase transition in ferroelectric crystals of DMAGaS and DMAAlS type. Thermodynamical functions of the model are obtained in the mean field approximation. The phase transition between paraelectric and ferroelectric phases is investigated. It is established that order of the phase transitions depends on relations between model parameters.