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Electronic version of an article published as Journal of Advanced Dielectrics, 2011, vol. 1, n° 2, p. 251-255. DOI: 10.1142/S2010135X11000240] © [copyright World Scientific Publishing Company : http://www.worldscinet.com/jad/]; International audience; New lead-free compositions of AMO3 perovskite type were synthesized in the BaTi1-x(Nb1/2Sc1/2)xO3(0 ≤ x ≤ 1) system, with regards to the comparable electronic configurations of the various 6-coordinated cations. Special solid state reaction was used. The effect of the replacement of titanium by scandium and niobium in the B-site on structural and dielectric properties was investigated. For this, we have used X-ray diffraction and dielectric characterizations. Dielectric measurements performed on ceramics with 0 ≤ x ≤ 0.25 composition revealed that the small change in composition from BaTiO3 has a strong influence on the dielectric properties: fast decreasing of the ferroelectric transition temperature for low values of x and evolution from classical ferroelectric for 0 ≤ x ≤ 0.075 to relaxor behavior for 0.075 < x ≤ 0.25.

In order to investigate the effect of A-site homovalent and heterovalent substitution on phase transition in barium titanium zirconium oxide (Ba(Ti0.7Zr0.3)O3), dielectric studies were performed on ceramics of Ba 1 − x A x (Ti0.7Zr0.3)O3 and Ba 1 − x A 2 x / 3 ′ (Ti0.7Zr0.3)O3 compositions ( A = Ca , Sr; A ′ = Y , La, Bi). A preliminary X-ray powder diffraction study has allowed us to determine the limits of solid solution for each composition. A relaxor behaviour appears whatever the value of x in the solid solution domains. The corresponding relaxor characteristics were determined and discussed according to the substituted cation in the twelve coordination number (12-C.N.) crystallographic site. Such new materials are of interest for environmental-friendly applications.

The present work concerns the preparation and dielectric characterization of some new non-stoichiometric lead-free ceramics with perovskite compositions derived from BaTiO 3 . Lead-free compositions could be of great interest for environment-friendly applications (dielectric for capacitors, actuators, etc.). These materials have been shown to be either classical or relaxor ferroelectrics. In the first part of this article, the BaTiO 3 Ba 0.5 NbO 3 system concerns the cationic non-stoichiometry occurring only in the 12-coordination crystallographic site. In the second part, the influence of non-stoichiometry also occurring in the anionic site was determined in compositions belonging to the ternary system 2BaOTiO 2 Nb 2 O 5 . The variation in the Curie temperature T C or the temperature T m of the maximum of e ′ r decreased when the composition deviated from BaTiO 3 and when the number of vacancies increased. On the contrary, the relaxor state was favored by such a deviation.

Ferroelectric tantalates with the tetragonal tungsten bronze (TTB) type structure were studied over a wide frequency range from 10 2 to 10 9 Hz. Complementary microwave measurements at 36 GHz and micro-Raman experiments were performed in order to extend the dielectric study. A high frequency relaxation was observed above 10 8 Hz and fitted to the Cole–Cole formula. Different qualitative models are proposed to explain the understanding of the origin of the high frequency relaxation in ferroelectrics. Among these, polar clusters of ferroelectric active off-centred Ta 5+ ions are considered.

International audience; By partial cationic and anionic substitutions in NaNbO3-SrNb2O6 and KNbO3-SrNb2O6 systems, two solid solutions have been obtained: Sr2−xNa1+xNb5O15−xFx and Sr2−xK1+xNb5O15−xFx. They crystallize in the tetragonal tungsten bronze (TTB) type structure, which is related to that of the perovskite. The replacement of oxygen by fluorine causes a decrease of the Curie temperature. A weak F-O substitution rate increases the compactness, which leads to higher dielectric constant with good piezoelectric and pyroelectric characteristics. A relatively higher F-O substitution rate induces aferroelectricrelaxorbehavior. Such relaxorferroelectricceramics with lead-free compositions can be of great interest for environment-friendly applications.

Some solid state chemistry aspects are investigated in relaxor ferroelectrics of both perovskite and tetragonal tungsten bronze (TTB) structure. Various issues regarding composition, cationic charge, electronic configuration, and structure are discussed. The relaxor behavior appears when two different cations occupy the same crystallographic site. One of the cations in the octahedral site has to be ferroelectrically active. The relaxor effect is correlated to cationic distribution disorder in the same site, depending on composition and structure (perovskite or TTB).

The ac conductivity of three fluorides of A2MII+ MIII + F9 type, Sr2NiAlF9, Sr2ZnGaF9 and Ba2MnCrF9, between 293 and 773K, has been analyzed within a combined complex impedance and modulus formalism. The antiferroelectric-paraelectric phase transition of Sr2ZnGaF9 has been characterized. A hopping mechanism for charge carriers is suggested for the three materials; their electrical performance are relatively weak. Sr2NiAlF9 and Sr2ZnGaF9 can be considered as ionic conductors of the F- ion. On the contrary, the conductivity in Ba2MnCrF9 is rather of an electronic nature.

Three solid solutions were prepared from Ba 2 NaNb 5 O 15 by coupled cationic and/or anionic substitutions: Ba 2-x Na 1+x (Nb 5-x W x )O 15 . Ba 2-x Na 1+x Nb 5 O 15-x F x and Ba 2 NaNb 5-x Ti x O 15-x F x . They crystal-lize with a TKWB-type structure. In the case of oxyfluoride compositions, large frequency range dielectric studies performed on ceramics demonstrated a relaxor behavior. The greater the substitution rate, the higher the relaxor corresponding characteristics. Such a behavior is in agreement with the cationic disorder of Ba 2+ and Na + in the same crystallographic site of BaNa 2 Nb 5 O 14 F. In addition, the following transition sequence was obtained: relaxor → ferroelectric → paraelectric. Such ceramics constitute a new family of lead-free relaxor compositions with TKWB structure and can be used in environmental-friendly applications.

New compositions derived from Pb{sub 5}Ta{sub 10}O{sub 30} have been synthetized to study the influence of cationic substitutions [heterovalent (Pb{sup 2+}La{sup 3+} and Pb{sup 2+}K{sup +}) and homovalent (Pb{sup 2+}Ba{sup 2+} and Pb{sup 2+}Ca{sup 2+})] on its ferroelectric properties. The ferroelectric behavior of each composition was evidenced by way of dielectric measurements as a function of both temperature and frequency. Structure determinations and refinements were also performed to determine which crystallographic sites are occupied by the substituted cations, Whatever the nature of the substitution, the relaxor behavior is induced by a weak rate of substitution. Furthermore, except in the case of the calcium-containing composition, no significant difference was found concerning crystallographic data. All the results obtained are compared with those described in the literature and discussed in terms of charge, ionic radius, and steric effect.

The phase transitions of two ternary fluorides have been reinvestigated by thermal and X-ray diffraction analyses. Four phase transitions have been detected at T 1 =483 ± 5 K, T 2 = 548 ± 5 K, T 3 = 765 ± 5 K and T 4 = 867 ± 5 K for Li 3 AlF 6 . Only two transformations occurred at T 1 = 550 ± 5 K and T 2 = 660 ± 10 K for Li 3 CrF 6 . Dielectric and optical measurements performed respectively on ceramics and single crystals revealed a ferroelectric behaviour coupling partially with ferroelastic coupling for the two investigated fluorides.

Two ceramics of Ba 2 NaNb 5 O 15 and BaNa 2 Nb 5 O 14 F compositions, which have a tetragonal tungsten bronze (TKWB)-type structure, are synthesized and characterised by dielectric measurements. Their ferroelectric properties are correlated with the ionic order or disorder in the crystalline network. The oxide is a classical ferroelectric, whereas the oxyfluoride shows a relaxor ferroelectric behaviour in agreement with cationic disorder of Ba 2+ and Na + in the same 15 C.N. crystallographic site. The relaxor characteristics are compared to those of well-known perovskites. BaNa 2 Nb 5 O 14 F is the first lead-free oxyfluoride relaxor of TKWB type.

Five different phases of Pb5Al3F19 ([I] to [V]) occur on cooling. The ferroelastic one [III] was stabilized at room temperature by a low rate of Cr–Al substitution. The structure was determined by X-ray diffraction analysis on a single-domain crystal. The symmetry is triclinic, P1, with cell parameters a=10.72(3) A, b=10.67(3) A, c=7.24(2) A, α=109.27(5)°, β=110.14(5)°, γ=83.33(5)°. The crystal network, which is closely related to those of the other varieties, is composed of both isolated and trans- corner-linked AlF6 octahedra developing parallel to the c-axis. The structure is closely derived from that of the paraelastic phase [II], mainly by the inclination of the previous tetragonal c-axis on the a–b plane. The loss of the mirror symmetry of the high-temperature phase leads to a break in the Pb2+ ion alignment along the 4-fold axis. Polydomain crystals were observed under a polarizing microscope; the ferroelastic [III]–paraelastic [II] phase transition occurs at 326 K on heating.

Dielectric susceptibility measurements on niobium-modified barium titanate ceramics for Nb compositions extending up to the solubility limit (18%) show that these materials display relaxor-like properties for compositions larger than ∼ 6% Nb. Raman spectra exhibit extra modes attributed to local responses related with the substitution of niobium for titanium ions. The behaviour of this barium titanate modification, in which Nb5+ aliovalent cations are substituted for Ti4+, is discussed by comparison with the zirconium-modified series, in which, oppositely, only a size difference characterizes the substitution on the B site. It is emphasized that electrical random fields created in the former case are more efficient in inducing a relaxor behaviour than elastic random fields created in the latter. The nature of the complex defects expected from the inherent high concentration of barium vacancies is discussed in connection with photoluminescence spectra.

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.

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, theA2 site is at least half occupied by lead atoms. Disorder in the repartition of lead and potassium atoms in theA1 (CN 12) andA2 sites was also studiedviacombinatorial calculations to correlate the evolution of the relaxor behaviour in the Pb5Ta10O30–Li10Ta10O30–K10Ta10O30ternary system with composition. The combinatorial study demonstrated that the relaxor behaviour is enhanced by the disorder in cationic repartition.

New dielectric oxyfluoride ceramics have been obtained in the ternary system BaTiO 3 –BaZrO 3 –CaLiF 3 . X-ray powder diffraction studies determined the limits of perovskite-type solid solutions. Dielectric measurements were performed over the frequency and temperature ranges 10 2 –2×10 5 Hz and 77–450 K respectively. This ternary system shows both classical ferroelectric and relaxor ferroelectric behaviours depending on the composition. These results are related to ionic substitution and chemical bonding. These compositions are of interest for applications (dielectrics for capacitors, actuators) since they are environment-friendly.

This article discusses some state-of-the-art aspects of solid state chemistry within the context of ferroelectric materials such as oxides, fluorides and oxyfluorides, whose crystalline networks include octahedra. Structural considerations make it possible to determine the origin of the ferroelectricity and to predict the existence of polar properties in new families. Crystallographic distortions of different nature lead to highly variable transition sequences. Ferroelectricity is often associated with ferroelasticity. The Curie temperature is linked to the composition and to the chemical bond. In this respect, several factors are to be considered, e.g. size, coordination and configuration of the cations, covalence of bonds, order-disorder, etc. Physical studies are very useful: first, the dielectric characteristics and their variations with temperature, frequency and electric field are typical of ferroelectrics; second, the pyroelectric, piezoelectric, electro-optical and non-linear optical properties are among the most effective. Various types of materials (single crystals, ceramics, thin films) are used nowadays in a wide range of applications. © 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS ferroelectricity / solid state chemistry

Ceramics with composition Ba1-xNaxTi1-xNbxO3 are of either classical ferroelectric (for 0 ≤ x < 0.075) and ferro- or antiferroelectric (for 0.55 < x ≤ 1) or relaxor ferroelectric type (for 0.075 ≤ x ≤ 0.55), the transition at Tc being only diffuse without any frequency dispersion for this last region. All the corresponding dielectric characteristics, i.e. diffusivity of the ferroelectric-paraelectric transition, frequency dispersion of e'r, shift of Tm with frequency deviation from the Curie-Weiss law, are determined. The relaxor behaviour is more relaxor the more the composition deviates from BaTiO3 and NaNbO3. This study is in the field of preparation of relaxor ceramics free from lead in the interest of the environment, which present a transition temperature close to room temperature.

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.

Four allotropic forms are shown for the K3InF6 ternary fluoride by various experimental investigations, microcalorimetry, high temperature X-ray diffraction, optical and electrical measurements. The ac conductivity data of K3InF6 are analysed in the Z*, M* and e* formalisms and the conductivity relaxation parameters are determined. Conductivity is of Arrhenius-type on either side of a transition temperature Ttr (Ttr=488±10 K) which is very close to the Curie temperature TC (TC=480±5 K). A conductivity jump of about one decade is observed at TC and that transition is of ionic-superionic type.

The evolution of conductivity in tetragonal tungsten bronze (TTB) oxides belonging to the Pb 5(1− x ) Li 10 x Ta 10 O 30 solid solution was studied using two techniques: a.c. complex impedance spectroscopy and the four-probe method. The results obtained from both techniques allowed the correlation of the evolution of conductivity with composition. Indeed, the conductivity mainly appears to be governed by the total number of Li + ions in the structure. Furthermore, it seems that when the number of Li + ions in the structure is small, two separated regions appear: • a low temperature region (LT), characterized by activation energy Δ E LT , a very small number of mobile ions and a low value of the prefactor σ 0 ; • a high temperature region (HT), characterized by activation energy Δ E HT (Δ E HT >Δ E LT ), a larger number of charge carriers and a higher value of the prefactor σ 0 . The existence of these two regions is explained in terms of extrinsic–intrinsic behavior. Dielectric measurements were also performed, with the substitution 2Li + –Pb 2+ leading to a decrease of the Curie temperature which is explained in terms of chemical bonding and antagonist effects.

New dielectric oxyfluoride ceramics have been obtained in the ternary system BaTiO 3 BaZrO 3 BaLiF 3 . X-ray powder diffraction studies allowed to determine the limits of solid solutions. Dielectric measurements were performed in the frequency and temperature range 10 2 –2×10 5 Hz and 77–450 K, respectively. This ternary system shows both classical ferroelectric and relaxor ferroelectric behaviours depending on the composition. These results are related to ionic substitution and chemical bonding. In addition, a spontaneous classical ferroelectric–relaxor ferroelectric transition is evidenced for the first time in a lead-free perovskite-type phase. These compositions are of interest for applications (dielectrics for capacitors, actuators) since they are environment-friendly.

Relaxor lead-free BaTiO3 derived ceramics were prepared and characterised by various physical studies. The relaxor characteristics depend on the substitution rate and on the type of ionic substitution: they are favoured by a stronger composition deviation from BaTiO3 and by heterovalent substitutions in the octahedral crystallographic site of the perovskite.

The paraelectric and the ferroelectric/ferroelastic structures of Pb2KNb5O15 have been refined by the Rietveld method from neutron and X-ray powder diffraction data. The paraelectric phase is tetragonal (space group P4/mbm) with lattice parameters a t = 12.646 (1) and c t = 3.9551 (2) Å at 790 K. The Pb- and K-atom distribution has been determined. It was established that the Pb atoms situated on the 15-coordinate sites are not on the special position (2mm) but are distributed over two disordered positions as in the tetragonal phase of Pb2KTa5O15. The ferroelectric/ferroelastic phase is orthorhombic. The space group of the average structure is Cm2m and the lattice parameters are a o = 17.779 (2), b o = 18.015 (2), c o = 3.9209 (4) Å at 294 K and a o = 17.756 (1), b o = 18.019 (1), c o = 3.9141 (2) Å at 81 K. It is isostructural with the average ferroelectric/ferroelastic structure of PbNb2O6. The orthorhombic distortion is of the same order but the Curie temperature is lowered by the substitution 2K+–Pb2+. The structures of PbNb2O6, PbTa2O6, Pb2KNb5O15 and Pb2KTa5O15 are compared. The ferroelectric polarization of the niobates is due to both the Pb off-site position and the Nb displacements, which induce a distortion of the oxygen lattice. In PbTa2O6, the oxygen-lattice distortion is very weak. The polarization seems to be due only to the Pb atom. In the tantalates the substitution 2K+–Pb2+ should weaken the correlations between the Pb atoms and consequently suppress the ferroelectric transition. Only a local polarization should take place; this might be the origin of the dielectric relaxation observed in this compound.

Dielectric and Raman scattering experiments were performed on various ceramics with composition Ba(Ti1-xZrx)O3. Such lead-free, environmental-friendly materials were shown, from dielectric measurements, to exhibit behaviours extending from conventional to relaxor ferroelectrics on increasing the zirconium concentration. The evolution of the Raman spectra was studied as a function of temperature for various compositions, and the spectroscopic signature of the corresponding phases was determined. In the relaxor state, the variation of the integrated intensity of the Raman lines with temperature showed a plateau at low temperature. This anomaly was also detected as a peak in depolarization current measurements, and attributed to ergodicity breaking which characterizes usual relaxor systems. Raman results hint at locally rhombohedral polar nanoregions resulting from the random fields associated with Zr ions.

Analysis of the a.c. conductivity data of Pb5Al3F19 within the complex permittivity formalism allowed, in a previous work, to show a new transition at 670 K corresponding most probably to the expected, but hitherto unobserved, transition from the paraelastic phase II to the paraelectric prototype phase I of Pb5Al3F19. A new investigation of Pb5(Cr0.1Al0.9)3F19 confirms this and allows us to complete the phase diagram of the Pb5(Cr1−x ;Alx)3F19 solid solution system for (0.80 ⩽ x ⩽ 1) in the high-temperature range. Ceramics with compositions corresponding to x = 0 and 0.5 were also characterized by this type of spectroscopy impedance method.

Ferroelectric fluorides have been studied in the two last decades in relation to the performance of ceramics sintered at low temperature for various potential applications. Compositions derived from octahedraltype ferroelectrics by progressive replacement of oxygen by fluorine are particularly interesting. Those considered in the present work belong to perovskite, tetragonal potassium tungsten bronze TKWB and LiTaO3 families. The variation of the Curie temperature TC with the F-O substitution rate is strongly dependent on the crystalline network and on the ferroelectricity-type. The decrease of TC is maximal for TKWB-type materials due to the predominant role of covalency of the M-X (X = O, F) bonds in relation to the same direction for such bonds and for both the displacement vector of the M-atom from the octahedron-centre and the spontaneous polarization vector. Also the ferroelectricity is purely displacive for this type.

The modulation of ferroelectric material properties using ionic substitutions is well known. The Curie temperature, in particular, was previously related to chemical bonding considerations. The present work deals with niobates and tantalates, ferroelectric families particularly interesting for applications. The metal-oxygen covalency is investigated using quantum chemistry methods. The role of covalency on the ferroelectric distortion is discussed and relations between covalency on one side, Curie temperature and microwave relaxation frequency on the other side, are evidenced. Covalency not only softens the short-range repulsions, but also stiffens and stabilizes the metal-oxygen network. This last effect appears to dominate in niobates and tantalates. As an unusual result, covalency tends to inhibit the ferroelectric distortion in these compounds. Covalency effects can be used in a predictive way to modulate chemically ferroelectric and dielectric properties. This approach is validated through v...

KTa 0.3 Nb 0.7 O 3 ceramic was prepared by sintering at high temperature with aid of LiF. The ferroelectric properties of this ceramic have been studied and its phase transition features have been revealed by the measurements of dielectric constant, loss, ac impedance and spontaneous polarization in the temperature range 300–700 K. This study affirms that this ceramic is a ferroelectric with a diffuse phase transition (FDPT) and that it presents a low frequency dispersion phenomenon at high temperature.

A phase transition at T r = 365±10K is shown for Pb9Al2F24 by high-temperature X-ray diffraction, microcalorimetric and electrical measurements. The AC conductivity data of Pb9Al2F24 are analyzed in the Z*, M* and e* formalisms and the conductivity relaxation parameters are determined. The high-temperature form of Pb9Al2F24 is characterized by an activation energy (Δσ)h.t. close to that of the (x = 0.12) limiting composition of the Pb1-.xAlxF2 2+x solid solution with fluorite-type structure.

Relative permittivity studies of ceramics with a variety of compositions in the BaTiO 3 -BaZrO 3 -CaTiO 3 system showed some of them to be ferroelectric relaxors at temperatures close to 295 K. Such a favourable effect is due to the Ca 2+ for Ba 2+ cationic substitution. In addition a spontaneous classical ferroelectric-relaxor ferroelectric transition was evidenced. Some of these new compositions are of interest for applications (dielectrics for capacitors, actuators) since they are environmentally friendly.

Dielectric, pyroelectric current and X-ray diffraction studies have been performed on a ceramic of composition Ba0.92Ca0.08(Ti0.75Zr0.25)O3 belonging to the BaTiO3–BaZrO3–CaTiO3 ternary diagram. A spontaneous classical ferroelectric–relaxor transition was evidenced at T = 210 K. Such behaviour has been reported previously in other perovskite relaxors (PSN, PST, …). This lead-free ceramic is suitable for dielectric applications in capacitors and actuators because it is environmentally friendly.

In perovskites, the relaxor behavior occurs dominantly in lead based compositions. The study of the BaTiO 3 -KNbO 3 binary diagram provided previously evidence for new free from lead ferroelectric relaxor properties but unfortunately at low temperature. The present dielectric studies of ceramics with composition inside the BaTiO 3 -KNbO 3 -CaTiO 3 showed some of them to be ferroelectric relaxor at temperature close to 300K. Such a favourable effect is due to the Ca 2+ -Ba 2+ cationic substitution. These new compositions are of interest for applications (dielectric for capacitors, actuators) in the way of environmental protection.

Dielectric and impedance measurements were performed on ceramics with composition Pb 5x K 6(1-x) Li 4(1-x) Ta 10 O 30 , in wide frequency and temperature ranges, 10 1 to 10 9 Hz and 100 to 650 K, respectively. At low frequency and low temperature, a relaxor behavior is found for compositions corresponding to x > 0.70. Moreover, two other relaxations are observed: an ionic conduction relaxation at low frequency and high temperature and a network relaxation at high frequency and low temperature. The former is explained on the basis of the motion of Li + cations in sites of coordination number 9 and the latter on the basis of correlation chains.

Solid solutions of formula Pb 5− x Mg 2 x Cr 3− x F 19− x ; Pb 5−5 x Sr 4 x Mg 2 x Cr 3− x F 19− x and Pb 5−5 x Sr 4 x Zn 2 x Ga 2 x Cr 3−3 x F 19− x were found in systems Pb 5 Cr 3 F 19 –A 2 M II M III F 9 (A=Sr,Pb; M II =Mg, Zn; M III =Cr, Ga). An X-ray diffraction study allowed characterization of a restricted single-phase domain attached to each end member: 0≤ x ≤0.15 and 0.8≤ x ≤1. Phase transition studies were carried out by differential thermal and scanning calorimetric measurements. The results are discussed in relationship to the nature of the cation of the host anions and compared with those obtained from A 5 M 3 F 19 family (A=Sr, Pb, Ba; M=Al, Cr. Fe) and related compounds.

Ceramic materials constituted of stacking of dense and porous layers of alumina were prepared and characterised at low (10 3 –10 5 Hz) and high (10 9 –10 10 ) frequencies. At low frequency, the permittivity was measured perpendicularly to the layers. It depends on the thickness of porous layers but not on frequency. Low values were obtained which gives to these materials a real interest as substrates. In the microwave range, a microstrip circular patch was deposited on one of the sample faces and the resonances of this structure are characterised and compared to the resonances of an identical patch on a dense alumina substrate. Resonance frequency are only slightly modified but the return losses are increased: the laminar substrate improves the coupling of the circular patch to the excitation circuit.

The ac (alternating current) conductivity of Pb5Al3F19 between 293 and 743 K has been analyzed within a combined complex impedance, modulus, and permittivity formalism. The antiferroelectric phase IV to ferroelastic phase III, and the phase III to paraelastic phase II, transitions have been characterized; the latter is shown to be accompanied by higher F− ion mobility above T=368±5 K. A new dielectric anomaly observed isochronously in e′(f,T) at 670 K corresponds to an expected, but hitherto unobserved, transition from phase II to the paraelectric prototype phase I of Pb5Al3F19. A 19F NMR investigation between 125 and 430 K of F− ion diffusion properties in Pb5Al3F19 shows that half the anions have become mobile at the ferroelectric phase V to phase IV transition temperature and that almost all the anions are mobile at the phase IV to phase III transition temperature. These motions are shown to be short range, and no correlation is found between phase formation and F− ion diffusion properties in Pb5Al3F19...

Correlations between Curie temperature and chemical bonding were found in ferroelectric tetragonal potassium tungsten bronze type ceramics [TKWB] using the extended Hckel tight-binding (EHTB) method. The influence of Pb for K,Li substitution is shown. AC complex impedance measurements were performed on ceramics with compositions corresponding to Pb 5x K 6(1–x) Li 4(1–x) Ta 10 O 30 (PKLT) in a wide temperature range. The conductivity relaxation parameters of Li + conducting PKLT ceramics were determined. Transport properties in these materials appear to be due to a Li + ion hopping mechanism. The influence of the substitution 6K + +4Li + →5Pb 2+ on the mobility is discussed with the assistance of the results obtained from EHTB calculations.

L’organisation de l’enseignement en trois reseaux determine les missions assignees aux inspecteurs. Dans les deux reseaux subventionnes, elles se limitent au controle. Elles sont plus larges dans le reseau officiel gere par le ministre et concernent le controle, le conseil et l’elaboration des programmes. Les mutations du systeme educatif rendraient necessaires le recours a des approches pluridisciplinaires et la pratique d’audit d’etablissement.

The lamellar compound ${\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$ is presented as an unusual example of a collinear, two-sublattice ferrielectric system. Single-crystal x-ray diffraction is used to study the thermal evolution of the polar character of its structure. The results are consistent with the occurrence of a first-order, order-disorder transition at ${T}_{c}=315\mathrm{K}$ and provide indirect evidence for copper hopping motions. Estimates for the spontaneous polarization are calculated at various temperatures from the atomic coordinates and found to agree well with dielectric measurements. The example of ${\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$ suggests that cooperative dipole behavior may generally arise when ${d}^{10}$ cations (e.g., ${\mathrm{Cu}}^{\mathrm{I}}$ and ${\mathrm{In}}^{\mathrm{III}}$) susceptible to second-order Jahn-Teller distortions are embedded in a matrix of restricted dimensionality.

Resume Le present travail fait le point sur les ceramiques ferroelectriques classiques ou relaxeurs derivees de BaTiO 3 . Les substitutions homovalentes en site de coordinence 12 (exemple : Ba 1 − x Sr x TiO 3 ) donnent naissance a une transition abrupte a T C et a une phase ferroelectrique classique. En revanche, les substitutions en site de coordinence 6 conduisent a une transition diffuse et a une phase ferroelectrique relaxeur ; ces substitutions peuvent etre soit homovalentes (exemple : Ba(Ti 1 − x Zr x )O 3 ) soit heterovalentes (exemple : Ba 1 − x K x (Ti 1 − x Nb x )O 3 ).

A new device for high frequency dielectric measurements of ferroelectric ceramics at temperatures from 300 up to 800 K is presented. The apparatus involves a circular coaxial guide short-circuited at its end and extended by an air line for the measurement in temperature. The calculations of the real and imaginary parts of the complex permittivity were computed using a mode matching method. In fact, the main innovation of the present work is the technical adaptation in temperature. The device was preliminary tested with BaTiO 3 ceramics and validated up to 450 K. The first new study concerns KNbO 3 derived ceramics from 300 K to temperatures greater than T c ∼ 670 K. A high frequency relaxation was shown and characterized. Such a new device will be very useful for the study of ferroelectrics with Curie temperature higher than 450 K; this temperature is the limit generally reached for the measurement of high permittivities at frequency up to 10 9 Hz.

Correlations between Curie temperature, relaxation frequency and chemical bonding have been evidenced in ABO3 type ferroelectric perovskites. This approach is extended to Pb(B, B')O3 type relaxor ferroelectric materials for which a high frequency relaxation has been recently observed. A study of relaxation frequency—chemical bonding relations using Extended Huckel theory is proposed. The influence of the covalency of oxygen — cation (B, B') bond on the relaxation frequency in Pb(In1/2Nb1/2)O3, Pb(Sc1/2Nb1/2)O3, Pb(Sc1/2Ta1/2)O3 is shown.

The perovskite-type structure has a composition and symmetry flexibility able to modify drastically the physical properties. As a significant example it will be shown in particular that the resulting modifications of the covalency influence strongly lattice distortion, spontaneous polarization and Curie temperature of the ferroelectrics concerned. The predictive character of the property changes will be illustrated in a relevant way.

A dielectric response relation is proposed to differentiate the lattice response from the charge carrier response in ferroelectric materials. This relation may be considered as a generalization of the Cole-Cole dielectric expression. It is applied to a LiTaO3-type single crystal.

Tetragonal-tungsten-bronze-type Ba5Li2Ti2Nb8O30 ceramics have been densified. Such materials are both ferroelectric and ionic conductor. A maximum of e'r appears at Tc ≈580K. Another small peak has been found at about 305K, it probably results from a crystallographic transition. In addition, a very strong frequency dispersion, related to the ionic conduction, occurs at T≥Tc.

ac electrical measurements are reported for the paraelectric phases of the ferroelectric CuInP2S6 and antiferroelectric CuCrP2S6. A complex permittivity analysis confirms previous dielectric results for CuInP2S6. Arrhenius behaviors with nearly identical activation energies were observed for the dc conductivities and modulus relaxation frequencies. A hopping mechanism for Cu(I) transport in these materials is thus suggested.

Ceramics of Ba3(TINb4)O15 composition and with tetragonal potassium tungsten bronze structure have been prepared. The dielectric properties have been determined as a function of sintering atmosphere and annealing under oxygen pressure. A maximum of e, occurs at the Curie temperature Tc ≈ 450K. No dispersion appears in the ferroelectric phase. In contrast, a strong relaxation takes place in the paraelectric phase, just above Tc, for ceramics sintered in air. Such a dispersion can be strong restricted by annealing under oxygen pressure. Both ac and dc conductivity measurements have been made on ceramics sintered in air. Oxygen-deficient Ba3(TiNb4)O15—type ceramics are mixed conductors. The conductivity nature in these materials is correlated to a partial reduction of the cations located in the octahedral sites and the simultaneous presence of oxygen vacancies. Such materials are an example of ferroelectrics with both electronic and ionic conductivities

Fluorine additives were used with a view to sinter related BaTiO 3 ceramics at low temperature. One of them sintered at 1000°C as a result of the additive CaF 2 , 4LiF. A large frequency range (10 2 –10 9 Hz) dielectric study shows that this material relaxes at high frequency. This type of material is of potential interest as dielectric for a capacitor, as a modulator for a high frequency system or as an infrared detector.

Piezoelectric and pyroelectric properties of KT a0.3 Nb 0.7 O 3 ceramics which belong to the perovskite-ferroelectric ABO 3 family, were studied. The piezoelectric constants (d 31 = 18 pC/N, d 33 = 28 pC/N and d 15 = 19 pC/N) of these ceramics are higher than that for KNbO 3 . We found that the amplitudes of the fundamental frequency and its overtones for the thickness vibrations depend on the temperature in the ferroelectric phase. The value at room temperature of the pyroelectric coefficient (p = 10 nC cm -2 K -1 ) and the spontaneous polarization (P s = 35 μC cm -2 ) are in agreement with that of KNbO 3 .