Jean Bornarel

Light deflection and birefringence in (NH4)2SbF5 ferroelastic crystals

The light deflection phenomenon is studied in ammonium pentafluoro antimonate (APFA) crystals versus the laser beam incidence related to the twinned sample. The variations of the orientation of the deflected beams, as well as of their polarization are measured and theoretically explained. A simplified model allows one to establish analytical relations between the deflected beam angles ( and ) and the measured optical properties of the crystal. With a normal light beam incident on the sample, a linear dependence between sin2 and the birefringence is theoretically and experimentally demonstrated, which is interesting for further research and applications.

Light deflection in gadolinium molybdate ferroelastic crystals

The deflection of a He-Ne light beam by polydomain gadolinium molybdate (GMO) crystals has been studied with respect to incidence angle i on the sample at room temperature. The A and B deflected beams do not cross each other during the i variation, in contrast to results and calculations previously published. The model using the Fresnel equation confirms this result. The model presented is more accurate for numerical calculation than that using the Huygens construction.

Phase Transitions and Twin Domains in (NH4)2SbF5. Dielectric and Optical Properties

The dielectric and optical properties of (NH 4 ) 2 SbF 5 crystals (APFA) have been studied simultaneously by optical observation in the temperature range 80 to 300 K. The existence of two phase transitions is demonstrated at temperature 293 and 169 K. The second order of the first one is confirmed. The continuous character of the lower one is demonstrated and the proportionality between electronic density deformation and NH 1 + orientations seems clear. A notable phenomenon of brilliant domain walls is discovered at about 145 K (when β = 0°).

The domain wall phenomena at the coercive state in KDP crystals

Abstract The dielectric properties of KDP crystals have been known for a long time: high values below the Curie temperature up to the domain freezing. We show the similarity of the domain wall phenomena in the dielectric measurements with natural textures below the transition and in the coercive state during a hysteresis cycle. The properties of these dense domain textures are studied by macroscopic measurements (dielectric, piezoelectric), but also by differential γ ray diffractometry, light and electronic emission, and diffraction of a laser beam. The strong lateral interactions between the domain walls explain the collective processes and the instable situation of the walls themselves.

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.

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.

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.