Zdenek Kvitek

The zigzag interface shape at the ferroelastic phase transformation

The aim of this article is to analyse the shape of an interface between the paraelastic and ferroelastic phases in the static case when the sample is placed in a temperature gradient. It is shown that the resulting shape which is often of a zigzag form follows from the competition between the free energy induced by the gradient, the elastic-strain energy caused by lattice misfits along the interface and the interface surface energy. The evaluation of these energy contributions is unavoidable when their magnitudes are comparable. In the first step we evaluate the change in elastic energy, which is due to the lattice misfit, for an anisotropic medium as the function of growth of the zigzag interface between the phases. In the second step we perform a complete analysis of the zigzag interface in the approximation of an isotropic medium. As the main result of our analysis we show that the formation of a zigzag interface is energetically favourable when the ratio of the product of surface and chemical energies to the square of elastic-strain energy is small. When this condition is not satisfied, the interface remains flat. The analysis shows that depending on the value of a transition between the zigzag interface and the flat interface may exist which is discontinuous. This means that there exists a range of angles characterizing the zigzag shape which is forbidden.

Influence of the thermal gradient during the DKDP phase coexistence

Abstract DKDP phase coexistence processes are described for samples subjected to a thermal gradient G perpendicular to the c ferroelectric axis. Depicted are the situations for different values of G and t (the sample thickness in the G direction), and especially a particular zig-zag phase front case with edges reaching the sample boundaries.

Para-ferro phase coexistence in DKDP crystals

Abstract The coexistence between the paraelectric (paraelastic) and the ferroelectric (ferroelastic) phases in DKDP crystals near 215 K is an interesting example of competition between chemical, electrostatical and mechanical energies. Without applied electric field or stress, the phase front shapes in different thermal conditions are experimentally observed and explained. The domain texture in the ferroelectric region appears as quasi monodomain during the phase coexistence in particular situations. The dielectric constant is demonstrated as the sum of three contributions: a monodomain one, a polydomain one and another due to the existence of the phase front in the sample.

Phase front shapes and thermal distributions for samples in a rotating thermal gradient

The shape of the DKDP phase front is observed along three perpendicular directions during the coexistence of the paraelectric and ferroelectric phases for different orientations of an external thermal gradient . Quasi-planar and zigzag shapes are then described. The thermal distribution inside the sample can be well represented by a thermal gradient in the middle of a non-plate-shaped sample. The important effect of the samples corners is also demonstrated.

Dielectric measurements during phase coexistence in DKDP crystals under rotating thermal gradient

The dielectric properties of DKDP samples under a thermal gradient are studied versus the orientation of relative to the c ferroelectric axis. Simple models allow us to explain the variation of the dielectric constant in the paraelectric phase, and also demonstrate a contribution due to the phase front existence for the first time.

Light emission in KDP ferroelectric sample

Measurements of light emission from KDP (KH2PO4) single crystals during continuous repolarization by an alternating voltage have been performed. Simultaneous switching current measurements or optical observations of the domain texture in various experimental conditions allow to correlate the light emission and the domain motion. This light emission seems to be the result of an electron exoemission out of the sample faces followed by an excitation of the Helium gas surrounding the sample.

Phase Coexistence Phenomena and Experimental Conditions in the Ferroelectric-Ferroelastic DKDP 218 K Transition

The coexistence between the paraelectric and the ferroelectric (ferroelastic) phases in DKDP crystals near 218 K is a good example of competition between chemical, electrostatic and mechanical energies, the latter being often the most important one. The interaction between the domain texture and the phase front shape is demonstrated. Are presented the existence of monodomain regions whose dimensions along c are smaller than critical values, a thermal hysteresis explanation and special phase front shapes in samples under applied thermal gradient and electric fields.