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

A new family of ferroelastic-ferroelectric materials has been prepared with the composition (Na_5?xNd_x) (W_3?2xNb_2x)O₉F₅ (0 ? x ? 1,50), and a related chiolite structure. The Curie temperature decreases as x increases.     

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

The Sr_1-xPb_xTeO₃ solid solution (0 ? x ? 0.30) has been prepared by substitution of strontium by lead in SrTeO₃. The ferroelectric phase, which is limited for SrTeO₃ 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.     

Etude cristallochimique et dielectrique des phases du systeme quaternaire LiNbO3−LiTaO3−NaNbO3−NaTaO3

A crystal chemical study of the quaternary system LiNbO₃ ? LiTaO₃ ? NaNbO₃ ? NaTaO₃ has been carried out. The obtained phase diagram depends on the temperature. At 1370 K for instance three solid solutions appear, they are related to perovskite, LaCoO₃ and LiNbO₃ - type structure. Most of these materials have ferroelectric - paraelectric transitions. The Curie temperature increases by substitution of sodium by lithium and tantalum by niobium. They have potential applications in piezoelectricity, pyroelectricity and electrooptics.     

Elaboration and dielectric study of ferroelectric or relaxor ceramics in the ternary system BaTiO3-NaNbO3-BaSnO3

The present work reports the elaboration and physical investigation of new compounds of the following composition Ba_1−xNa_x(Ti_1−ySn_y)_1−xNb_xO₃ (BTSnNxy). The studied ternary system presents some continuous solid solutions between the next 3 phases: the NaNbO₃ antiferroelectric phase that becomes easily ferroelectric at low rate substitutions, the BaTiO₃ ferroelectric phase and the paraelectric stannate phase BaSnO₃. Two different dielectric behaviors can be observed once some substitutions are made either in A or B sites of an ABO₃ perovskite. These substitutions modify the dielectric properties of the material. The introduction of Sn⁴⁺ and Ti⁴⁺ cations in the B site favors, respectively, a decrease of the transition temperature and an increase in the value of the real dielectric permittivity. The transition temperature should be modulated by varying the rate of cationic substitution. Some relaxor materials can be obtained at a temperature around room temperature.     

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

A new non-stoichiometric solid solution has been identified with the formula Li_1+yTa_1?5yTi_6yO₃ (0 ≤ y ≤ 0.028) and a LiTaO₃-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.     

Etude cristallochimique,dielectrique et pyroelectrique de la solution solide Na1−xLix(Nb0,6Ta0,4)O3 (0,02 ⩽ x ⩽ 0,20)

A solid solution with formulation Na_1?xLi_x(Nb_0.6Ta_0.4)O₃ (0.02 ? x ? 0.20) and a perovskite related structure has been prepared by substituting sodium by lithium in Na(Nb_0.6Ta_0.4)O₃. The ferroelectric Curie temperature increases with x from 308 to 428 K. The pyroelectric properties have been investigated. The high performances may lead to appropriate applications.     

Low-Temperature Sintering of Pb(Zr,Ti)O3 Ceramics with the Aid of Oxyfluoride Additive: X-ray Diffraction and Dielectric Studies

Low-temperature sintering of Pb(Zr,Ti)O₃ (PZT) ceramics has been performed with the aid of oxyfluoride additives in order to compensate the volatilization of PbO, which results in the degradation of electric properties. An additive of a mixture of PbO and PbF₂ (eutectic composition) realizes a liquid-phase sintering and allows high densification at low temperature. The densification of more than 98% of theoretical density was achieved by sintering at 900°C. The characterization of these PZT ceramics was carried out with XRD, TG, SEM, and dielectric measurements. The thermal variation of dielectric constant showed ferroelectric behavior. The highest permittivity value was obtained in the specimen fired at 900°C for 3 or 5 h with 3 wt% additive.     

Influence de l'insertion d'ions lithium sur les proprietes dielectriques de la phase Ba0,725Sr1,775Nb5O15 de structure “bronzes quadratiques de tungstene”

A new solid solution with formula Ba_0,725Sr_1,775Li_xNb_5?xTi_xO₁₅ (0,02 ? x ? 1) has been prepared by insertion of lithium in the lattice of Ba_0,725Sr_1,775Nb₅O₁₅. It is isostructural with the tetragonal tungsten bronzes and has a ferroelectric behaviour. Crystallographic and dielectric studies show that lithium is intercalated preferably in the 15 or 12 coordination sites when 0.02 ? x ? 0.50 and then with coordination 9 when 0,50 < x ? 1. Pyroelectric and electrooptic applications are expected for such materials.     

Etude optique et couplage ferroélectrique-ferroélastique de cristaux de composition Li0, 02Na0, 98NbO3

Crystal of composition Li_0.02Na_0.98NbO₃ is ferroelectric at room temperature (space group Pc2m, T_c = 640 K). Two kinds of domain walls parallel to (101) or (141) planes have been observed at 300 K. The birefringence n_a-n_c and the domain pattern have been investigated from 300 to 950 K. Four transitions have been detected at T₂ = 540 K, T₃ = T_c = 640 K, T₄ = 825 K and T₅ = T′_c = 940 K. The transition at T′_c is of ferroelastic-prototype type. The switching of the polarization associated with the disappearing of the domain walls is in good agreement with a ferroelectric-ferroelastic coupling.     

Dielectric, piezoelectric and pyroelectric properties of Pb(Zr, Ti)O3 ceramics elaborated by low temperature sintering with the aid of oxyfluoride additives

Pb(Zr, Ti)O₃ ceramics have been elaborated by low temperature sintering at 900°C with the aid of oxyfluoride additives. The mixture of PbO and PbF₂ (eutectic composition) allowed to achieve the densification of more than 98% of the theoretical density. The dielectric, piezoelectric and pyroelectric properties of thus obtained ceramics were investigated.