We have estimated the Ginzburg number G governing crossover from the asymptotic to the classical critical behaviour near the normal-incommensurate phase transition in [N(CH(3))(4)](2)ZnCl(4) (TMAZC) crystals and the dependence of G on the hydrostatic pressure (0.1-330 MPa), following from the experimental data for the optical birefringence and the quantitative analysis of temperature derivatives of the birefringence in the framework of the approximation of weak Gaussian fluctuations. The Ginzburg number found experimentally for TMAZC is G ∼ 8 × 10(-3) at the atmospheric pressure and a considerable part of it is assumed to refer to structural defects. It is shown that the G value for TMAZC decreases with increasing hydrostatic pressure and, based on analysis of the literature birefringence data for Cs(2)CdBr(4) and Cs(2)HgBr(4) crystals, this behaviour might be expected to be generally typical for all the A(2)BX(4) family. The results obtained are discussed using a phenomenological theory of second-order structural phase transitions. In particular, they provide a basis for estimating the limits of the asymptotic critical region in TMAZC and demonstrate that the major part of the incommensurate phase should belong to the crossover region.

Temperature dependences of spontaneous and induced electrogyration are studied for the crystals of lead germanate doped with Li, Eu, La, Nd and (Li, Bi). Critical fluctuations observed in the optical parameters near ferroelectric phase transition point are explained in frame of the first fluctuation correction to the Landau theory and the critical indices governing the order parameter are found. It is shown that the doped crystals may be promising for practical applications owing to their large induced electrogyration coefficients.