  A four-state model is proposed for description of the sequence of phase
transitions in ferroelectric crystals of DMAGaS and DMAAlS type. The ordering
processes in the subsystems of DMA groups are considered as a main reason of
such transformations. The interaction between groups in their various
orientational states is taken into account in the dipole-dipole approximation.
Obtained thermodynamical characteristics of the model (spontaneous
polarization, occupancy of orientational states, dielectric susceptibility,
phase diagram) are in good agreement with experimental data. The experimental
fact of suppression of ferroelectric phase at increase of hydrostatic pressure
is explained under assumption that pressure changes the difference between
energies of various orientational states of DMA groups mostly.
