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Below its ordering temperature at 105 K, perovskite-type SrTiO(3) exhibits a tetragonal phase with three different structural domains that are strongly influenced by the application of uniaxial mechanical stresses and electric fields. A careful neutron diffraction study of superlattice reflections provides full quantitative information about the varying domain distributions under external loads as a function of temperature. It is shown that electric field and uniaxial stress exhibit competitive effects and the simultaneous application leads to a complex redistribution behaviour of the tetragonal domains. The results are discussed in the context of the formation of a field induced ferroelectric phase at low temperatures. The experimental findings demonstrate that its polarization is always perpendicular to the tetragonal axis and the polar phase has orthorhombic symmetry.

The time dependence of switching multiferroic domains in MnWO4 has been studied by timeresolved polarized neutron diffraction. Inverting an external electric field inverts the chiral magnetic component within rise times ranging between a few and some tens of milliseconds in perfect agreement with macroscopic techniques. There is no evidence for any faster process in the inversion of the chiral magnetic structure. The time dependence is well described by a temperature-dependent rise time suggesting a well-defined process of domain reversion. As expected, the rise times decrease when heating towards the upper boundary of the ferroelectric phase. However, switching also becomes faster upon cooling towards the lower boundary, which is associated with a first-order phase transition.

The lock-in transition of K2SeO4 at 93.2 K was studied using high-resolution γ-ray diffraction, in situ dielectric measurements and neutron scattering. The phase transition shows a coexistence regime of 1.0 K with clearly resolved incommensurate and commensurate satellite reflections. Within a temperature interval of several K above the lock-in transition, strong third order satellites are observed, indicating a pronounced squaring of the modulation wave. Under the influence of high electric fields an intermediate phase is observed, which is characterized by a diffuse intensity distribution. In addition, the dielectric permittivity exhibits two maxima, indicating the existence of two separate phase transitions. This behaviour is attributed to distortions of the discommensuration lattice and pinning of the discommensurations by accumulated charged defects. It seems to be characteristic for modulated ferroelectrics of the A2BX4 group, since a similar behaviour is also found in isostructural Rb2ZnCl4 and K2ZnCl4. The soft-mode dynamics close to Tc is found to be essentially independent of the electric field and not affected by the discommensuration lattice.

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