P. Bastie

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.

Optical observations of coexistence states during 1st order transition in KD2PO4, quartz and NH4C1.

Abstract At a 1st order transition the two phases are in equilibrium. In a solid, transition strain discontinuities induce elastic stresses which are of long range nature. In order to minimize elastic energy the interface takes definite orientations. The interface shapes, nucleation and motion have been optically observed in 3 materials : NH4C1 around the pressure induced tricritical point, quartz and deuterated KDP, with application of an electric field. In these 3 cases, observed interface orientations are in agreement with the predictions of the theory of coherent inclusions in anisotropic elastic materials.

Induced and spontaneous strain in ferroelectric ferroelastic tanane

Abstract Physical properties of the organic material tanane1 do not allow sufficiently accurate polarization measurements. As an alternative, the following order parameters were studied: spontaneous shear strain x 6, (linearly coupled to spontaneous polarization P 3), and spontaneous birefringence Δn 6, shown to be proportional to the spontaneous strain. Results of induced birefringence and strain, above and below the transition temperature (Tc = 13.5°C), are reported and discussed.

Dielectric response of the new ferroelectric: Tanane

We have studied the dielectric response of Tanane which is piezoelectric in the high-temperature phase. We report here the dielectric constant and the results of the studies of the piezoelectric resonance in the vicinity of the Curie point.

γ-ray diffractometry for the study of ferroelastic crystals: Application to KH2PO4

The shape of the rocking curves obtained by γ-ray diffractometry on KH2PC4 crystals is analysed. Perturbations near the surface [100] are shown. The applications of this technique to the study of domain switching processes are demonstrated.

Perturbated regions in the vicinity of domain wall in ferroelastic-ferroelectric crystals

Abstract Two different techniques are used to establish the strain distributions near the domain walls in ferroelastic-ferro-electric crystals (KDP, GMO, TAN ANE); classical optical techniques can give some results for domain structures not far from the monodomain state; γ-ray diffractometry is useful for dense domain structures (sample near zero macroscopic deformation).

Phenomenological interpretation of the critical behaviour of KDP and NH4C1 near their tricritical point

Abstract Recent order parameter measurements in KDP and NH4C1 near their pressure-induced tricritical point are interpreted using a Landau type free energy expansion of the following form: Good fits are obtained with the simple assumption that the coefficients a and b vary linearly with P and T near the tricritical point where they vanish simultaneously.

Temperature and electric field dependence of the order parameter and of the dielectric constant of KDP and RbDP near their transition point

Abstract We compare phenomenological and microscopic theories of the KDP transition. Dielectric constant of KDP and RbDP have been measured versus temperature and electric field and the results are discussed in the frame of the Landau theory.