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. The results obtained are compared with the experimental data.

Temperature dependences of the optical path difference by the variable part of refractive index D = (n − 1)l and the components ε ′ and ε ′′ of complex dielectric permittivity for NH 4 HSeO 4 single crystal are studied in the temperature range of 220-300 K. Anomalous behaviour of these dependences in the range of incommensurate phase 250-262 K was observed. Analysis of the results of dielectric investigation predicts a considerable decrease of the coefficient of elastic stiffness C in the narrow temperature range of 256-260 K at the transition from the paraelectric to incommensurate phase.