Ryszard Cach

Characterization of ferroelectric phase transition in GPI crystal

The dielectric behaviour of a glycine phosphite [GPI: Gly.H 3 PO 3 ] crystal is investigated around its ferroelectric phase transition. The temperature dependence of the dielectric constant is measured for dc bias fields up to 200 kV/m. The dielectric constant is also measured in the paralectric phase as a function of electric field at constant temperatures. The results are discussed on the basis of the classical state equation and the equation coefficients are estimated. The obtained results lead to the conclusion that the ferroelectric phase transition in GPI has an intermediate character between critical and tricritical one.

Dielectric measurements and domain structure in LiKSO4 at low temperatures

Dielectric measurements in the temperature range 33-300K with simultaneous optical observations on LiKSO4 slabs perpendicular to the c hexagonal axis have been performed. A few dielectric anomalies and domain textures are obtained according to the different thermal treatments. A specific thermal treatment has been found allowing reproducible results. Previous contradictory results in the literature can therefore be explained.

Phase transitions in LiKSO4 at low temperature

Abstract First optical observations of the domain textures and simultaneous dielectric measurements in LiKSO4 at low temperature are presented. Reproducible results were obtained under specific thermal treatment.

Determination of landau free-energy parameters by dielectric measurements in (ch3NH3)5Bi2CI11 crystals+

Abstract The adiabatic dielectric permittivity in the biasing electric field and spontaneous polarization of (CH3NH5)5Bi2Cl11 crystals are measured near the high temperature phase transition. Coefficients of the dielectric state equation are determined.

Phase front and dielectric measurements in KD2PO4

Abstract The ferroelectric properties and the ferroelastic ones interact strongly in the KH2PO4 compounds. The competition between the elastic energy and the electrostatic energy is clearly illustrated by the shapes of the phase front at the KD2PO4 first order transition. The correlation between the dielectric constant e′c and the loss constant ϵ″c measurements, and the coexistence of the two phases are shown in this paper.

Unusual structural phase transition in [N(C2H5)4][N(CH3)4][ZnBr4]

ABSTRACT The new hybrid organic–inorganic crystal [N(C2H5)4][N(CH3)4][ZnBr4] was grown and its physical properties and structural phase transition are presented. On the basis of thermal analysis (DSC (differential scanning calorimetry), DTA (differential thermal analysis), DTG), X-ray structural, dilatometric and dielectric studies as well as optical observation, the reversible first-order phase transition at 490/488 K on heating and cooling run, respectively, has been found. An appearance of domain structure of ferroelastic type gives evidence for an untypical lowering of crystal symmetry during the phase transition. At room temperature, the satisfying crystal structure solution was found in the tetragonal system, in the P21m space group.

Monodomain ferroelectric region near the factory-roof-shaped phase front in the KD2PO4 218 K transition

Monodomain regions are observed in the uppermost ferroelectric parts of the factory-roof-shaped phase front during the KD2PO4 218 K first-order transition. It is also demonstrated that the existence of the phase front induces high values of dielectric and loss constants (along the c axis). An explanation is proposed assuming specific properties for the observed monodomain regions.

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.