LONG-WAVELENGTH ORDER-PARAMETER FLUCTUATIONS NEAR THE TRANSITION TO POLYDOMAIN STATE IN FREE FERROELECTRICS AND FERROMAGNETS

In the absence of external fields and currents, ferroelectrics and fer romagnets are known to undergo phase transitions to inhomogeneous (pol ydomain) structures induced by the dipole-dipole interaction. The long -wavelength order parameter fluctuations near such a transition are st udied for T > T-c in the mean-field and scaling regions by considering the quadratic part of the free energy functional for a thin plate of a uniaxial crystal with the polar axis perpendicular to the plate surf ace. It is shown that the specific anomalies in the long-period respon ses of a crystal appear due to condensation of the inhomogeneous (peri odic) soft mode with period proportional to root L (L is the plate thi ckness) in accordance with the phenomenological result of Landau and L ifshitz for the c-domain width and the microscopic result of Tarasenko , Chensky and Dickshtein for T --> T-c-0. The behavior of X-ray and ne utron scattering and light propagation in ferroelectrics near such a t ransition is described.