G. Godefroy

Proton implanted waveguides in LiNbO3, KNbO3 and BaTiO3

Abstract By using a Van De Graaf accelerator, planar waveguides in LiNbO3, KNbO3, and BaTiO3 are fabricated (at 300 K) by thermally controlled proton implantation. The waveguide properties are investigated by dark line mode spectroscopy. The reconstructed profiles closely follow the ion concentration profiles as determined by simulation (TRIM).

Esr identification of Fe containing defects in BaTiO3

Abstract We report the identification and analysis of Fe3+ - V0, Fe4+ - V0 and Fe5+, possibly associated with VBa, in rhombohedral BaTiO3. All these defects as well as Fe3+ and several unidentified centers can change their charge states under illumination. The photorefractive effect in Fe containing BaTiO3 might involve all these centers.

Crystallographic study of a family of Cs2BX4 compounds

Abstract Five compounds of the Cs2BX4 family with B = Zn, Co, Cu and X = Cl, Br were grown and studied. They belong to Pnma space group with 4 formula units per cell. From the full determination of the structure it appears that the ZnX4 tetrahedra are almost regular while the CuX4 tetrahedra are strongly distorted. This seems to be due to a strong Jahn-Teller effect induced by the Cu ions. This remark is extended to other compounds of the general family A2BX4.

On interpretation of thermal hysteresis near commensurate-incommensurate phase transition

Abstract In multidomdin approximation of incommensurate phase phenomenological equations are given for pinned domain walls, nucleation and growth of incommensurate nuclei. Thermal hysteresis is interpreted as resulting from these three mechanisms.

Phase transitions of tetra-methylammonium tetrahalogeno-metallate (ii) solid solution single crystals

Single crystals of tetramethylammonium tetrahalogenometallate (II) solid solution single crystals were grown and studied mostly by differential scanning calorimetry. Analogies between the temperature-pressure phase diagram of pure hydrogenated or deuterated tetramethylammoniuni chlorides and the solid-solution concentration phase diagrams are shown. The progressive substitution seems equivalent to a change of the pressure, it is understood as a distortion of the metal-halide tetrahedron and a change of interactions with the first, second and third neighbours.

Theoretical study of Fe doping and oxidation–reduction influence on the photorefractive effect in BaTiO 3

We numerically solve charge-transport and Poisson equations for photorefractive BaTiO3 single crystals with a band model, using four impurity levels, Fe2+–Fe3+, Fe3+–Fe4+, VO••–VO•, and VO•-VOx. Densities and photoinduced spatial distributions of each population are computed as a function of annealing O partial pressure. Space-charge field and beam-coupling gain are also computed as a function of annealing O partial pressure, temperature, Fe concentration, grating wave vector, and light intensity. We discuss the intervening mechanism of impurity centers and the correlations between experimental conditions of crystal growth, oxidation–reduction treatments, and measurement parameters.

Light sensitive paramagnetic defects in BaTiO3

Abstract Many paramagnetic defects were identified in nominally undoped BaTiO3; among them for the first time: O− trapped holes and Cr5+. All defects can change their charge states under illumination. They thus can take part in the photorefractive effect of BaTiO3. By monitoring the ESR defect concentrations in dependence of light wavelengths several charge conversion thresholds were identified.

Crystal growth and characterization of two kinds of Fe doped BaTio3 single crystals

Abstract Single BaTio3: Fe crystals are either grown in a KF flux or pulled in a Tio2 Flux; the transition temprature decreases in the same way in the two kinds of crystals. The optical measurements show an apperent reduction of the intrinsic gap. These two phenomena are related to that Fe3+ ions occupy the high symmetry site for Ti-ions, thus modifying the point and translation symmetry.

DiP223: Strongly temperature dependent electro-optic coefficients in BaTiO3

Abstract By means of a Senarmont set up and a very sensitive method, electrooptic coefficients in BaTiO3 are accurately determined from the phase measurement of the retardation Γ due to the electric field induced birefringence of the crystal. Electro-optic properties are investigated as a function of temperature around room temperature. A very large value of the coefficient r42 is found, in agreement with previous investigations. In addition, this coefficient is shown to be very sensitive to the temperature even for a very small change. The importance of both the thermo-optic effect and the thermal variation of electro-optic properties is pointed out for device applications of BaTiO3.

Dielectric and D.T.A. studies of the phase transitions in Rb2ZnCl4: Influences of crystal growth conditions

Abstract Phase transitions in ferroelectric Rb2ZnCl4 crystals grown by various methods are examined by dielectric measurements and D. T.A. technique in the temperature range from -120 to 100 degres C. For most of the studied samples, small heat anomalies are seen around both of the two transitions points, and the dielectric constant along the ferroelectric axis shows a large thermal hysteresis over a wide temperature range including the commensurate - incommensurate transition point. All these anomalies were absent in some other samples; their unexpected behavior is assumed to be connected with the presence of defects.