  A simple description of thermodynamics of DMAAS and DMAGaS ferroelectric
crystals by means of Landau expansion is proposed. Conditions of occurrence of
phase transitions are established and their temperatures are obtained. The
influence of external hydrostatic pressure on phase transitions is described.
The temperature behaviour of dielectric susceptibility components and their
anomalies in the vicinity of phase transition points are investigated. Obtained
results are compared with experimental data.
