Régnault Von Der Mühll

Phase transition and symmetry in BaAl2O4

Abstract Structural, optical, dielectric, and pyroelectric properties of crystals and ceramics of BaAl2O4 have been investigated. A refinement of the atomic positions has been carried out from X-ray powder diffraction data. BaAl2O4 is ferroelectric with TC=396K. The thermal behavior of Ps shows a broad maximum close to 220K without any phase transition. A model is proposed on the basis of the existence in the unit cell of two partial polarizations. A fit of the coefficients of the Landau free energy expansion allows the calculation of the thermal variations of these partial polarizations. A possible crystal structure is proposed for the paraelectric phase.

Structure cristalline de SrFeF5

Abstract The crystal structure of SrFeF5 has been determined by single crystal X-ray diffraction methods. The unit cell is monoclinic (space group P2 1 c ) with a = 7.062 ± 0.001 A, b = 7.289 ± 0.001 A, c = 14.704 ± 0.001 A, β = 95.40 ± 0.01° and Z = 8. The lattice is built up of spiral chains of octahedra parallel to the Oy axis. These chains are formed by (FeF6)3− octahedra sharing corners of the same edge. The strontium atoms bridge three neighbouring chains. The SrTiF5, SrVF5, SrCoF5, and BaInF5 phases are isostructural with SrFeF5.

Structural, ferroelectric and pyroelectric properties of nonstoichiometric ceramics based on BaAl2O4

Abstract The density and the ferroelectric and pyroelectric properties of ceramics derived from BaAl 2 O 4 have been investigated. The X-ray diffraction patterns and the variation of the density are consistent with the presence at 300 K of nonstoichiometric materials in the 44.4–50.0 mol% BaO range. They appear to be ferrielectric like BaAl 2 O 4 . The Curie temperature varies from 396 K (50%BaO) to 275 K (44.4%BaO) due to the composition change of the solid solution. The value of the spontaneous polarization, P s , is relatively small and shows anomalous behavior: the thermal variation presents a maximum but there is no structural change. The unit cell volume increases with decreasing BaO content. A substitution mechanism based on the replacement of two Ba 2+ for an [Al-O-Al] 4+ link, and a model accounting for the ferrielectririty are proposed.

Crystallographic and dielectric properties of a new non-stoichiometric phase with a LiTaO3-related structure

Abstract A new non-stoichiometric solid solution has been identified with the formula Li 1+y Ta 1−5y Ti 6y O 3 (0 ≤ y ≤ 0.028) and a LiTaO 3 -related structure. The cations in excess are localized in the octahedral vacancies. It shows ferroelectric properties. The Curie temperature is decreasing with rising x. The value of T C , the diffuse phase transition character and the relatively low melting point allow to expect eventual pyroelectric applications.

Synthese d'une phase de type SrTeO3 ferroelectrique a temperature ambiante

Abstract The Sr 1-x Pb x TeO 3 solid solution (0 ⩽ x ⩽ 0.30) has been prepared by substitution of strontium by lead in SrTeO 3 . The ferroelectric phase, which is limited for SrTeO 3 to a temperature range between 312 and 485°C, is extending with rising x. It overlaps the room temperature for x > 0.035 which allows to expect eventual applications.

Polarisation remanente dans les varietes de basse temperature de (NH4) 3AlF6 ET (NH4) 3FeF6

Abstract Dielectric and thermocurrent measurements have been carried out on (NH4) 3AlF6 and (NH4) 3FeF6 ceramic samples. A maximum of permittivity is observed close to the transition temperature (TT(NH4) 3AlF6 = 217K; TT(NH4 3FeF6 = 264K. In the low-temperature phase a polarization current of about 10-9A is obtained and can be reversed when the sign of the polarization field is changed, a property which could correspond to a ferroelectric behavior. However, no pyroelectric current is detected when the temperature decreases from TT. Another hypothesis, based on a field-induced polarization, has been considered : the depolarization current could be due to charge displacements from potential minima favored by rising temperature. In any way, the low-temperature phase is characterized by a remanent polarization.

Une nouvelle famille de materiaux ferroelastiques-ferroelectriques oxyfluores de type chiolite

Abstract A new family of ferroelastic-ferroelectric materials has been prepared with the composition (Na5−xNdx) (W3−2xNb2x)O9F5 (0 ⩽ x ⩽ 1,50), and a related chiolite structure. The Curie temperature decreases as x increases.

Deux nouvelles structures difluorine et trifluorine

Abstract The structures of the tetragonal SrCrF4 and Sr2CuF6 phases have been determined. The ordering of the cations imposes doubling and tripling of the unit cell. These cations occupy ideal positions; whereas the anionic sublattice is distorted. CaCrF4, CaCuF4, and SrCuF4 have the same structure as SrCrF4.

Electrical and optical properties of mixed aurivillius phase Bi7Ti4NbO21 single crystal

Abstract Single crystals of the ferroelectric mixed Aurivillius phase Bi 7 Ti 4 NbO 21 have been prepared by the flux method in Bi 2 O 3 excess. Thermal dependence of dielectric permittivity, X-ray diffraction patterns, birefringence and optical observations as well as differential scanning calorimetry have been studied between room temperature and 900 °C. Two temperature driven phase transitions were observed, the first one close to 630–675 °C within a ferroelectric state (ferro I-ferro II), the second one at ≈ 830 °C, between the ferroelectric state II and the paraelectric state.

New oxyfluorides and highly densified ceramics related to LiNbO3

Abstract New ferroelectric oxyfluorides with structures related to that of LiNbO 3 have been prepared in sealed tubes by the compensated substitution Nb 5+ + 3O 2− = Mg 2+ + 3F − . Highly densified ceramics can be obtained at 900°C by sintering in an atmosphere of free air. Partial hydrolysis of the fluorides by moisture in the air enhances the densification of these materials.