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Une etude dielectrique fine du systeme BaTiO3–BaLiF3 a montre que les ceramiques de composition Ba(Ti1—xLix)O3—3xF3x presentent deux comportements differents: l'un de type ferroelectrique classique pour (0 ≤ x ≲ 0,04), l'autre de type relaxeur ferroelectrique pour (0,04 ≲ x ≲ 0,15). Ces derniers materiaux constituent les premiers relaxeurs ferroelectriques oxyfluores et comportent les caracteristiques dielectriques typiques correspondantes: transition de phase diffuse, dispersion en frequence, croissance de Tm (temperature du maximum de e'r) avec la frequence, ecart a la loi de Curie-Weiss, etc. C'est l'occupation des memes sites cristallographiques par des cations (Ti4+, Li+) et anions (O2—, F−) heterovalents qui est a l'origine de telles proprietes. A fine dielectric study of the BaTiO3–BaLiF3 system has shown that ceramics with composition Ba(Ti1—xLix)O3—3xF3x present two different behaviours: a classical ferroelectric one for (0 ≤ x ≲ 0.04), a ferroelectric relaxor one for (0.04 ≲ x ≲ 0.15). Such materials constitute the first oxyfluoride ferroelectric relaxors with corresponding typical dielectric characteristics: diffuse phase transition, frequency dispersion, increase of Tm (temperature of the maximum of e'r) with frequency, deviation from the Curie-Weiss law. The occupation of the same crystallographic sites by heterovalent cations (Ti4+, Li+) and anions (O2—, F−) is the cause of such properties.

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.

The successive phase transitions of KTa1−xNbxO3 (x = 0.3 and 0.4) (KTN) ceramics have been investigated by various techniques including DTA, dielectric, piezoelectric and pyroelectric measurements from 100 to 450 K. In this temperature range, all the phase transactions have been detected by pyroelectric measurements. All the phase transitions in KTa0.6Nb0.4O3 were detected by piezoelectric measurements as a function of temprature. The d31 piezoelectric coefficient presents around Tc a maximum value (of 20·10−12C/N). The DTA, dielectric and pyroelectric measurements allow to deduce that the ferroelectric-paraelectric phase transition is probably of first order for KTa0.6Nb0.4O3

Tetragonal-tungsten-bronze-type Ba5Li2Ti2Nb8O30 ceramics were synthesized and sintered under different atmospheres. The dielectric properties of such materials were determined via impedance-spectroscopy measurements. Based on an equivalent circuit, the dielectric relaxation was described. The temperature dependence of various dielectrical parameters was also determined and discussed.

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.

Resume X-ray diffraction, differential thermal analysis, polarized light microscopy, and dielectric measurements have been used to study phase transitions and solid solutions in the K3MO3F3–A3MO3F3(A= Rb, Na;M= Mo, W) systems. K3MO3F3–Na3MO3F3(M= Mo, W) systems show two solid solutions; the first (SSI) nearx= 0 (K3−xNaxMO3F3) involves only one transition atTCwhich is practically independent of composition. The sharp transition atT1(

The dielectric permittivity, ∊∗ = ∊′ — i∊″, of a KTa0.6Nb0.4O3 ceramic was measured in the frequency and temperature ranges 106 to 109 Hz and 298 to 450 K, respectively. A dipolar-type relaxation was reported. A minimum of the relaxation frequency occurs close to the ferroelectric Curie-temperature T c. The complex plane plots ∊″r vs ∊′r characterize a relaxation with a Cole-Cole behavior

Pb5Al3F19 at 160 K is ferroelectric, crystallizing in the tetragonal system. With Mr = 1477.9, the space group is I4cm and a = 14.07 (2), c = 7.30 (1) Å, V = 1445 (3) Å3, Z = 4, Dx = 6.793 Mg m−3. For λ(Mo Kα) = 0.71073 Å, μ = 56.36 mm−1. F(000) = 2480 Pb5Al3F19 undergoes a first-order phase transition on cooling at 120 K and on heating at 270 K. The structure was determined from 656 independent F(hkl) with I> 3σ(I) and refined by least squares to R = 0.0527, wR = 0.0480, S = 1.065. No atom is further than 0.83 Å, from the hypothetical I4/mcm atomic positions, hence the crystal is structurally ferroelectric. The mean atomic polar displacement of the two independent Al atoms from the zero spontaneous-polarization location, allowing for the F atoms, is 0.207 (28) Å, which leads to a predicted Tc = 860 (230) K. Several additional phase transitions are known to form before the hypothetical phase is attained. Each independent Al atom occupies a fluorine octahedron with average Al—F distance either 1.818 (5) or 1.824 (1) Å. One Pb atom is nine coordinated, occupying a distorted tricapped trigonal prism with average Pb—F = 2.66 (22) Å. The other is ten coordinated, forming a distorted bicapped cuboid, with average Pb—F = 2.61 (28) Å. Pyroelectric pulses are readily detectable in crystals cooled below 120 K, or subsequently on heating to 270 K. Similarly, numerous piezoelectric resonances are observed in the same thermal ranges; the compression mode at ~1197 kHz has been identified by its characteristic admittance circle. The mechanical quality factor Qm for ferroelectric Pb5Al3F19, determined from the admittance-circle-derived equivalent circuit, is ~423.

Dielectric spectroscopy in the 10 6 to 10 9 Hz range is reported for ceramics of the Pb 2 K(Nb 1-x Ta X ) 5 O 15 solid solution of tetragonal tungsten bronze-derived structure. Two dielectric relaxations are observed. The first one, which exists for all compositions in both ferroelectric and paraelectric states, must be related to the structure of the materials. The second one, which appears only in the ferroelectric state and whose contribution to the permittivity increases with increasing diffuseness of the paraelectric-ferroelectric transition, is related to composition fluctuations.

Dielectric measurements on Pb2K(Nb0.1Ta0.9)5O15 ceramics have been reported at 140

The crystal structure of Cu(OH)2, recently redetermined at room temperature by Oswald, Reller, Schmalle & Dubler [Acta Cryst. (1990), C46, 2279–2284], is shown to satisfy the structural criteria for ferroelectricity. The Cu2+ ion at 295 K is displaced 0.131 Å from the most likely paraelectric atomic arrangement, allowing the prediction to be made that the ferroelectric Curie temperature is Tc = 343 K. This prediction is in excellent agreement with an earlier report of a reversible structural transition at 325–335 K in Cu(OH)2. The presence of a λ-shaped anomaly in the heat capacity at about 335 K has been confirmed, and a dielectric anomaly is also observed at similar temperatures over a wide frequency range; the frequency dependence is dispersive, owing at least partly to ionic conductivity. The prediction is hence verified, and Cu(OH)2 is demonstrated experimentally to be a new ferroelectric.

Low-frequency dielectric dispersion phenomena in a LiTaO{sub 3}-type single crystal have been analyzed by impedance spectroscopy in directions parallel and perpendicular to the polar c-axis (rhombohedral system). An empirical expression has been deduced for the complex permittivity {epsilon}*({omega}), {epsilon}*({omega}) = {epsilon}{infinity} + {sup {epsilon}{sub s}-{epsilon}{infinity}}/{sub 1 + (i{omega}/{omega}{sub 1}){sup m}} + {sup {sigma}{sub 0}}/{sub {epsilon}{sub 0}{omega}} [1 + (i{omega}/{omega}{sub 2}){sup n}], where the ({omega}{sub 1}, m) and ({omega}{sub 2}, n) couples characterize respectively the lattice and the charge carrier responses. This relation may be considered as a generalization of the Cole-Cole dielectric expression. Excellent agreement has been obtained in a wide frequency domain (1-10{sup 6} Hz) between the measured and calculated permittivities in the 500-650{degrees}C temperature range ({Tc} = 600{degrees}C). The temperature dependence of various dielectrical parameters has been determined and discussed. The relaxations are correlated to Li atom motions.

Low-temperature sintering of Pb(Zr,Ti)O{sub 3} (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 PbF{sub 2} (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.

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.

A new oxyfluoride K 3 (Nb 3 Ti 2 )O 11 F 4 with a tetragonal tungsten bronze type structure has been prepared and characterized in the KNbO 3 TiOF 2 system. The room temperature space group ( P 4/ mbm ) has been determined on a single crystal. The unit cell parameters are a = 12.453 A and c = 3.972 A.

Several radio frequency experiments with perovskites are reviewed. The devices used by several groups were very different, and the chemical nature and shape of the perovskite samples were intentionally changed. Despite this, sub-gigahertz relaxation is always reported. We describe and overcome all the possible extrinsic contributions to this relaxation, and we conclude that it is related to the perovskite structure.

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.

The occurrence of a first-order paraelectric-ferroelectric phase transition at {Tc} = 315(5) K in CuInP{sub 2}S{sub 6} is revealed by calorimetry, X-ray powder diffraction, and dielectric measurements. This is the first observation of such a phenomenon in a thiophosphate of the lamellar MPS{sub 3} family. The data are consistent with the polar character of the room-temperature structure attributable to the acentric positions of the Cu{sup I} and In{sup III} ions and may be related to the probable T dependence of the site-occupation ratios for the former cation and displacement of the latter. Results of the dielectric measurements are suggestive of a two-component phase transition. 21 refs., 8 figs.

The A5Nb3OF18 (A=K, Rb) crystals have been grown by slow cooling from HF solutions. Piezoelectric and pyroelectric measurements were performed from 280 to 550K. A piezoelectric resonance is observed at room temperature. The materials are pyroelectric, The pyroelectric current reverses at the phase transition T=500K for K and 485K for Rb. This transition seems to be related to the thermal behavior of the [NbX5] (X=O, F) infinite chains of the framework.

The Curie temperature of materials isomorphous with Pb5Cr3F19 and with formula Pb5M3F19 is found to decrease with decreasing radius of the M3+ ion, in contrast with expectation. This effect, which remains on substituting the M atom by Al or Fe, is due to the influence of the Pb2+ 6(sp)2 lone pair of electrons on the displacement of the M3+ ion from the center of its MF6 octahedron in the ferroelectric phase.

Pb5Al3F19 undergoes a succession of three transitions on heating, from ferroelectric phase IV with point group 4mm to antiferroelectric phase III with point group 4/m to ferroelastic phase II with point group 2/m to prototype phase I with point group 4/mmm over the temperature range 265 to 345 K. The properties of each phase, and the family of which Pb5Al3F19 is the archetype, are considered.

The Pb5(Cr1 − xAlx)3F19 system forms four phases. Ferroelectric phase IV, with point group 4mm, undergoes a first-order transition on heating to paraelectric phase I, point group 4/m, in the composition range 0 ≤ x ≲ 0.80. The Curie temperature varies linearly with x over this range, from 555 K at x = 0 to 365 K at x = 0.80. Phase IV transforms to monoclinic phase II in the range 0.80 ≲ x ≲ 0.88 in a first-order transition before undergoing a second-order transition at higher temperatures to phase I. In the range 0.88 ≲ x ≲ 1, phase IV initially transforms on heating to phase III, point group 4/m, in a first-order transition; at higher temperatures, this phase transforms to phase II in another first-order transition; at still higher temperatures, it transforms to phase I in a second-order transition. Multiple phase formation at x ≲ 0.80 is a result of structural instabilities that are related to the small size of the Al3+ cation and to the decreased interaction strength between the 6(sp)2 electron lone pair of Pb2+ and the residual 3d electron spin moment at the Cr-atom site as it is increasingly substituted by aluminum.

Dielectric measurements performed on a Pb ( In 1 2 Nb 1 2 ) O 3 ceramic revealed behaviour strongly dependent on the frequency and temperature. A high frequency (HF) relaxation is observed over the entire temperature range (200–450 K) at frequencies close to 108 Hz. There is a minimum of the relaxation frequency fr (HF) close to Tm, the temperature of the maximum of er′. A second relaxation occurs below Tm, at lower frequency, with fr(LF) values decreasing on cooling. In addition, very small ordered nanoregions were observed by transmission electron microscopy. This is in agreement with a high degree of disorder arising probably from the “fast firing” process used for ceramic preparation. The dielectric response was correlated with the nanostructure.

Two new oxyfluoride ferroelectric solid solutions derived from NaNbO3 have been prepared; their compositions are the followings : Na1−xAx(Nbl-xMgx)O3−3xF3x with 0≤ × ≤0.15 for A = Na and 0≤ × ≤ 0.20 for A = K. The variations of both the symmetry and the unit cell parameters have been determined at room temperature. Dielectric measurements have been performed on ceramic samples in the temperature and frequency ranges 100 – 800 K and 102–105 Hz respectively. Each sample exhibits a maximum of ϵ′r at the Curie temperature TC.

Antiferroelectric phase III of lead aluminium fluoride, Pb 5 Al 3 F 19 , M r =1477.9, tetragonal space group P4/n. At T=295 K, a=20.1738 (4) and c=7.2205 (1) A V=2939 (1) A 3 , Z=8, D m =6.66 (5), D x =6.681 Mg m -3 . For λ(Mo Kα)=0.71069 A, μ=58.0 mm -1 , F(000)=4960. The structure was determined from 18502 (1276 independent) F m 2 ≥3σ(F m 2 ) with (sin θ)/λ≤0.703 A -1 . Least-squares refinement on wF m 2 resulted in R(F m )=0.0579 with R int (F m )=0.048. Pb 5 Al 3 F 19 undergoes a first-order phase transition from antiferroelectric to ferroelectric at about 110 K, with a wide thermal hysteresis

Dielectric measurements have been performed on poled and unpoled Pb(Mg0.25Nb0 50Ti0.25)O3 ceramics in the frequency and temperature ranges 103–109 Hz and 150–450 K respectively. The temperature dependence of the permittivity, er, shows for both samples a maximum at Tm ≃ 400 K (103 Hz), a phase transition much less diffuse than PMN and no dielectric dispersion until 106 Hz. On the contrary a strong dielectric relaxation has been detected at high frequency (107–109 Hz). Both for the poled and unpoled ceramic, the thermal evolution of the relaxation frequency, fr, shows the existence of the relaxation in both ferroelectric and paraelectric phases and a minimum of fr close to Tm. On the other hand, higher fr values are obtained in all the temperature range for the poled sample.

Un comportement dielectrique dependent fortement de la frequence et de la temperature est observe pour une ceramique de composition Pb1 – 3x/2Lax(Mg1/3Nb2/3)O3 (PLMN). Une relaxation haute frequence existe dans tout le domaine de temperature considere (200 a 450 K). Une relaxation apparaǐt a plus basse frequence pour T ≦ Tm (Tm = 251 K a 103 Hz). Les proprietes dielectriques du PLMN sont comparees a celles decrites recemment pour Pb(Mg1/3Nb2/3)O3 et pour des compositions de la solution solide Pb(Mg1/3Nb2/3)O3–PbTiO3. Une correlation entre nanostructure et reponse dielectrique est etablie; en particulier, le rǒle determinant de la taille des regions ordonnees (1Mg: 1Nb) est mis en evidence. For ceramics of composition Pb1–3x/2Lax(Mg1/3Nb2/3)O3 (PLMN) a strongly frequency and temperature dependent dielectric behaviour is observed. In the whole temperature range considered (200 to 450 K) a high-frequency relaxation exists. A lower-frequency relaxation occurs at T ≦ Tm (Tm = 251 K at 103 Hz). The dielectric properties of PLMN are compared with those described recently for Pb(Mg1/3Nb2/3)O3 and solid solutions Pb(Mg1/3Nb2/3)O3–PbTiO3. A correlation between nanostructure and dielectric response is established; especially the importance of the size of ordered regions (Mg: Nb = 1:1) is evidenced.

Structural, ferroelectric, pyroelectric, and optical properties of ceramics and crystals of BaAl2O4 have been investigated. A refinement of the atomic positions has been carried out from the X-ray powder data. BaAl2O4 shows anomalous behavior of the dielectric and pyroelectric properties: the dielectric constant e′r is very weak; its maximal value at Curie temperature for a crystal oriented along the hexagonal c-axis is about 15 and that of a ceramic of compactness 0.89 is around 7.5. The Curie-Weiss constant is relatively weak (C = 125). The behavior of the spontaneous polarization Ps is unexpected with respect to that of classical ferroelectric materials: Ps increases with decreasing temperature from Tc and decreases again after having reached a maximum at 200 K. A model based on ferrielectric behavior and supported by the Landau equation has been proposed. La structure et les proprietes ferroelectriques, pyroelectriques et optiques de ceramiques et de cristaux du compose BaAl2O4 ont ete etudiees. Un affinement des positions atomiques a ete entrepris a partir du spectre de diffraction X sur poudre. BaAl2O4 presente un comportement anormal des proprietes dielectriques et pyroelectriques: la constante dielectrique e′r est faible, sa valeur maximale a Tc est voisine de 15 pour les cristaux et de 7,5 pour des ceramiques de compacite 0,89. La constante de Curie est aussi relativement faible (C = 125). Le comportement de la polarisation spontanee Ps differe de celui des materiaux ferroelectriques "classiques": Ps augmente lorsque T decroit en-dessous de Tc pour atteindre un maximum a T = 200 K suivi d'une decroissance. Un modele de comportement ferrielectrique s'appuyant sur la relation de Landau pourrait justifier les proprietes observees.

The successive phase transitions of a BaZnGeO4 crystal have been investigated by various techniques such as X-ray diffraction, DTA, dielectric, pyroelectric, piezoelectric and optical measurements from 100 K to 1300 K. The pyroelectric and optical studies have been reported for the first time in this paper. The material gives rise to a pyroelectric current from 100 K to 520 K. Five phase transitions have been detected in this temperature range by pyroelectric current measurements. The optical measurements allow the observation of four phase transitions. The birefringence decreases with rising temperature between 100 and 1300 K. A mechanism is proposed for the observed pyroelectricty.

Rb5Nb3OF18 crystals have been grown from HF solution. The tetragonal crystallographic network is built u from infinite (NbOF4)−chains running along the c-axis, isolatecf (NbF7)2- groups and Rb+cations; the room temperature space-group is 14cm. A Raman study has been carried out on sinle crystals between 300 and 650 K. The most striking feature concerns Be (788–847 cm-1) TO-LO splitting connected with the Nb-O stretching mode (with A1 symmetry) polarized along the c-axis. The “anomalous” temperature dependence of this TO mode around 485 K can be correlated with the atomic displacements. In fact, a phase transition occurs at this temperature and the Raman spectra show that the high temperature phase remains polar.

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.

Dielectric measurements have been performed on ceramics with composition (1−x) PMN −x( PbTiO 3 ) (x=0,0.05, 0.10, 0.25) , in the frequency and temperature ranges 102–109 Hz and 200–450 K respectively. Compared with PMN, the addition of PbTiO3 leads to higher values of both e r ′ and T m , the temperature of the maximum of e r ′ at low frequency. In addition the transition becomes less diffuse when x increases. Moreover, two different relaxations at high and low frequencies are observed for x=0,0.05 and 0.10 . The two relaxation frequencies depend strongly on both temperature and composition. The low frequency relaxation disappears for x=0.25 . Correlations between nanostructure evolution and dielectric properties are discussed.

Les spectres de diffusion Raman de monocristaux de Rb5Nb3OF18 sont etudies entre 300 et 650 K. Un fort eclatement TO-LO est mis en evidence pour le mode totalement symetrique vNbO des chaǐnes NbOF, montrant ainsi que ces cristaux appartiennent a un groupe d'espace polaire dans tout le domaine de temperature etudie. D'autre part, des “anomalies” se produisent vers 485 K sur l'evolution thermique des frequences et intensites de certaines raies; ces phenomenes pourraient correspondre a l'existence d'une transition de phase reversible a cette temperature. The Raman scattering spectra of Rb5Nb3OF18 single crystals are studied between 300 and 650 K. A strong TO-LO splitting is evidenced for the totally symmetric vNbO mode of the NbOF chains, thus showing that these crystals pertain to a polar space group in the whole temperature range investigated. On the other hand, “anomalies” are observed around 485 K in the frequency and intensity evolutions of a number of lines, as a function of temperature; these phenomena could be due to the existence of a reversible phase transition occurring at this temperature.

The dielectric behaviour of lead magnesium niobate (PMN) was studied, and in particular its dependences on temperature and frequency. Two different relaxations were observed in PMN, one at high frequencies and one at low frequencies. The latter was detected only below T m , the temperature at which e′ r is maximum at low frequency. Possible mechanisms for this behaviour are discussed. At T m , the two relaxations are partially overlapped due to both a minimum in f r high frequency and a mazimum in f r low frequency.

Ferroelectric ceramics of Pb2(K1-xLax/3)Nb5O15 (x = 0, 0.1, and 0.3) and of tetragonal tungsten bronze (TTB) type have been investigated. The Curie temperatures of those TTB-type materials are weakly dependent on x. The ferroelectric-paraelectric transition becomes more and more diffuse when La3+-content increases. Dielectric measurements in the microwave frequency range have revealed the existence of two relaxations. The first one is probably related to the TTB-type structure and the second results from the cationic distribution disorder.

In order to correlate dielectric relaxation properties with chemical homogeneity and microstructure in ferroelectric ceramics, powders are prepared both by sol-gel route and solid-state reaction. This comparative study concerns Ba(Ti0.8Zr0.2)O3 ceramics, with maximum permittivity close to room temperature. Dielectric relaxation is reported in the frequency and temperature ranges 106 to 109 Hz and 250–350 K, respectively. A minimum of the relaxation frequency occurs close to the phase transition temperature. The variations off r are correlated with chemical homogeneity and microstructure.

The occurrence of a first-order paraelectric-ferroelectric phase transition at {Tc} = 315(5) K in CuInP{sub 2}S{sub 6} is revealed by calorimetry, X-ray powder diffraction, and dielectric measurements. This is the first observation of such a phenomenon in a thiophosphate of the lamellar MPS{sub 3} family. The data are consistent with the polar character of the room-temperature structure attributable to the acentric positions of the Cu{sup I} and In{sup III} ions and may be related to the probable T dependence of the site-occupation ratios for the former cation and displacement of the latter. Results of the dielectric measurements are suggestive of a two-component phase transition. 21 refs., 8 figs.

International audience; Based on the Debye theory, we have determined the evolution of the dielectric permittivity of the ceramic with composition KTa0.6Nb0.4O3 as a function of temperature and frequency. This evolution shows that this composition exhibits a ferroelectric–paraelectric phase transition at 360 K with dispersion at high frequencies (106–6 × 108 Hz). A comparison between experimental and numeric calculation of dielectric permittivity for this solid solution has been suggested using a Gaussian distribution of the relaxation times, which was introduced in Debye relations. The numerical results were in agreement with experimental data.

A strong dielectric dispersion is reported for oxyfluoride ceramics derived from BaTiO3, with the composition Ba ( Ti 1−x Li x ) O 3−3x F 3x (x=0; x=0.025; x=0.05) , within the frequency range 106−109Hz. The relaxation is of dipolar type. Cole-Cole analysis leads to a good agreement between theoretical and experimental curves of permitivities ϵr′ and ϵr″ vs. frequency. The transition sequence rhomb.-orth.-tetr.-cub. which occurs for compositions corresponding to x = 0 and x = 0.025 becomes a simple rhomb.-cub. transition for x = 0.05 . Measurements at various temperatures have shown the relaxation frequency to present a minimum at each transition temperature, the deepest minimum is at the Curie temperature. These results are discussed using the lattice-soft-mode phenomena and the model of correlated chains of off-centred Mn+ cations in octahedra (M = Ti, Li).

An impedance spectroscopy experiment has been carried out on a Pb 2 KNb 4 TaO 15 ceramic sample of tetragonal tungsten bronze-type structure ( T c =643 K) from 300 to 900 K in the 20-3×10 5 Hz frequency range. A low frequency dielectric dispersion (LFDD) has been found. An impedance relaxation occurs simultaneously. A model is proposed to account for the observed phenomena. An empirical impedance expression has been deduced: Z ∗ =R 0 /(1+iω/ω 1 +(iω/ω 2 ) n ) where ω 1 and ω 2 characterize respectively the lattice response and the “universal behavior” of the charge carriers. This formula may be considered as generalizing the ideal Debye and non-ideal Cole-Cole expressions. Various dielectric dispersion relations are also obtained. An excellent fitting has been obtained for the dielectric constant spectrum in the 570–780 K temperature range. The ajustable parameters A and n of the “universal law” A (iω) 1− n are accurately determined and discussed. A shows a prominent peak in the paraelectric-ferroelectric transition region, while n exhibits a linear temperature dependence in both ferroelectric and paraelectric phases. At T c a minimum is observed for n .

A systematic study of the dielectric relaxation phenomenon has been carried out for ceramics with compositions derived from barium titanate in the frequency and temperature ranges 1 MHz–1 GHz and 250–500 K, respectively. Whatever the composition may be, a minimum of the relaxation frequency and a maximum of the dielectric dispersion appears at each phase transition temperature. The dipole-type relaxation is correlated to coherent displacements of the ions in the octahedron sites ordered either along chains or in 3D-lattice.

Ferroelectricity has been reported in six families of complex fluorides that contain fluorine octahedra and in four other families that contain both oxygen and fluorine atoms in the octahedra. The experimental Curie temperatures found are in good agreement with those predicted on the basis of structure in the case of the BaMnF4, SrAlF5, Sr3Fe2F12, Pb5Cr3F19 and K3Fe5F15 fluorine-octahedra containing families and are also in agreement in the case of the Pb5W3O9F10 and Na5W3O9F5 oxygen and fluorineoctahedra containing families. The structures of Na2MgAlF7, Bi2TiO4F2, and K3MoO3F3 in their ferroelectric phases are not known with sufficient accuracy to allow prediction of Tc but each family exhibits the experimental attributes of ferroelectricity. Dielectric, calorimetric, and structural properties are presented for all ten ferroelectric families. The range of Curie temperatures and dielectric permittivities in fluorine-octahedra containing ferroelectrics is comparable to that in oxygen-octahedra fer...

Dielectric measurements have been performed on Pb(Mg13Nbbuilt|2/3)O3 ceramics in the frequency and temperature ranges 20–109 Hz and 180–400 K respectively. In addition to the well-known low frequency relaxation characteristic of a relaxor ferroelectric, a second characteristic has been found at high frequency. The latter exists not only in the ferroelectric phase but also in the paraelectric one. A minimum of the corresponding relaxation frequency occurs at the temperature of the maximum of e′r measured at low frequency. This relaxation could be related to the inertia of Nb5+ correlation chains at high frequency.