J. Ravez

The crossover from a ferroelectric to a relaxor state in lead-free solid solutions

We investigate the gradual crossover from ferroelectric to relaxor behaviour versus substitution in lead-free ceramics. We show that the Vogel?Fulcher law in the relaxor state is the extrapolation of the ferroelectric line. To explain this extrapolation, we suggest that both the host matrix and the impurity induced clusters are triggered by the BaTiO3 soft mode correlation length. The Ti?O bond oscillations are the key mechanism.

Classical or Relaxor Ferroelectric Behaviour of Ceramics with Composition Ba1—xBi2x/3TiO3

Crystallographic and low frequency dielectric studies were performed on lead-tree non-stoichiometric perovskite ceramics of Ba 1-x Bi 2x/3 □ x/3 TiO 3 (0 ≤ x ≤ 0.15) composition. A classical ferroelectric behaviour was shown for x lower than 0.09. A relaxor behaviour appears for higher values of x (0.09 ≤ x < 0.15). The corresponding relaxor characteristics increased as the composition deviated from BaTiO 3 . Such kind of materials are of interest for environmental-friendly applications, in particular the ceramic with composition corresponding to x = 0.15, whose maximum of permittivity occurs at 280 K. i.e. close to room temperature.

Non-Stoichiometric Perovskites Derived from BaTiO3 with a Ferroelectric Relaxor Behaviour

Crystallographic and low frequency dielectric studies were performed on lead-free non-stoichiometric perovskite ceramics of Ba 1-x A x/2 (Ti 1-x Nb x )O 3 (A = Ca, Sr, Ba) compositions. A classical ferroelectric behaviour was shown for x varying from 0 to 0.05. A relaxor behaviour appears for higher values of x. The corresponding relaxor characteristics increased as the composition deviated from BaTiO 3 . They are only slightly dependent on the substituted cations in the 12-coordination number (12-C.N.) crystallographic site. Such kind of materials are of interest for environmentalfriendly applications.

Dielectric and pyroelectric studies on the Ba1−3aBi2aTiO3 classical and relaxor ferroelectric ceramics

Low frequency dielectric studies in the temperature range 80–500 K were performed on the solid solution derived from BaTiO3, Ba1−3aBi2aTiO3 with 0<a<0.117. The temperature and frequency variations of the real part of the relative permittivity show two different behaviors, depending upon the substitution rate. A classical ferroelectric behavior was observed for a varying approximately from 0 to 0.03. However, a relaxor behavior appears for higher values of a. The replacement of Ba2+ by Bi3+ in the A site shows a remarkable decrease of TC. The dielectric studies were confirmed by pyroelectric study.

New lead-free relaxor ceramics derived from BaTiO3 by cationic heterovalent substitutions in the 12 C.N. crystallographic site

Abstract Crystallographic and low frequency dielectric studies were performed on new lead-free ceramics derived from BaTiO 3 by cationic substitutions in the 12 C.N. crystallographic site and having the composition Ba 1− x K x /2 La x /2 TiO 3 or Ba 1− x A 2 x /3 TiO 3 (A=La, Bi). A classical ferroelectric behaviour was shown for compositions close to BaTiO 3 , while a relaxor one occured when the composition deviated from BaTiO 3 . In any case, the value of T C (or T m ) and the relaxor characteristics are higher as x increases. Concerning potassium-free ceramics, bismuth compositions compared with lanthanum ones favoured both high ferroelectric and relaxor characteristics due to the lone pair of Bi 3+ . Such new materials are of interest for environmental friendly applications.

A new ferroelectric phosphate family

A crystal chemical study carried out on phosphate single crystals with the formula AIBIIPO4 (AI=Na, K; BII=Ca, Zn, Sr, Cd, Ba, Pb) has permitted to follow the evolution of the crystal symmetry inside these series. Second harmonic generation (SHG) tests held on powder or small single crystals and hysteresis loops displayed on ceramic samples confirmed the existence of ferroelectricity in these material series.

Dielectric Properties of KTN ferroelectric ceramics sintered with LiF

Abstract Ceramics of the two compositions of KTa1−xNbxO3 (KTN) with x = 0.3 and 0.4 have been densified thanks to the addition of LiF. Densities been determined as a function of sintering temperature, time and amount of LiF. Dielectric investigations have shown that the composition corresponding to x = 0.4 is more densified than x = 0.3. The two compositions x = 0.3 and x = 0.4 exhibit a paraelectric-ferroelectric phase transition at about 310 and 360 K respectively. The phase transition is more diffure for x = 0.3. Oxygen is partially substituted for fluorine in the crystalline network.

Study of disorder in a tetragonal tungsten bronze ferroelectric relaxor: a structural approach

Electron diffraction analysis, nuclear magnetic resonance measurements, structure determinations and refinements were performed on two ferroelectric lead-containing ceramics (classical and relaxor), with tetragonal tungsten bronze structure, in order to investigate the origin of the relaxor behaviour in such compounds. In both compounds the lead atoms are found to be off-centred in the CN (coordination number) 15 (A2) site. The main difference between the two compositions is the proportion of lead in this site: in the relaxor composition, the A2 site is at least half occupied by lead atoms. Disorder in the repartition of lead and potassium atoms in the A1 (CN 12) and A2 sites was also studied via combinatorial calculations to correlate the evolution of the relaxor behaviour in the Pb5Ta10O30–Li10Ta10O30–K10Ta10O30 ternary system with composition. The combinatorial study demonstrated that the relaxor behaviour is enhanced by the disorder in cationic repartition.

Relaxations in new ferroelectric tantalates with tetragonal tungsten bronze structure

Abstract Ceramics belonging to the Pb5xK6(1−x)Li4(1−x)Ta10O30 solid solution were prepared and characterized by dielectric and a. c. impedance complex measurements. Two types of relaxations were observed: a relaxor behavior and an impedance relaxation. Characteristics of these two relaxations were studied and explained for a particular composition Pb3.75K1.50LiTa10O30.

Copper ordering in lamellar CuMP2S6 (M= Cr, In): Transition to an antiferroelectric or ferroelectric phase

Abstract The transition to an antiferroelectric or ferroelectric phase in the lamellar compound CuMP2S6 (M= Cr or In) is shown to be triggered by the appearance of an ordered copper sublattice. Cu(I) ions in CuCrP2S6, which are distributed over several sites at room temperature, freeze into an intermediate quasi-antipolar state between 190 and 175K, and then form an array of alternating up and down dipoles below 150K. Evidence for these transitions were obtained using calorimetry, diffraction and electric permittivity measurements. CuInP2S6 at 298K contains antiparallel polar sublattices of Cu(I) and In(III) which yield a Ps =2.6 μC-cm−2, as calculated from crystallographic results. The existence of copper disorder and the central octahedral location of In(III) in the paraelectric phase (T>315K) were also confirmed.