Coexistence of glass and ferroelectric order in deuterated betaine phosphate0.05 betaine phosphite0.95 crystals

The dielectric spectra of ferroelectric hydrogen bonded betaine phosphate_0.05 betaine phosphite_0.95 (DBP_0.05DBPI_0.95) was investigated in the very wide temperature (300–20 K) and frequency (20–35 GHz) regions. The dielectric dispersion was analyzed in terms of distribution of relaxation times, using Tichonov regularization method. Strongly asymmetric and broad distribution of relaxation times below ferroelectric phase transition temperature T _c?≈?253 K clearly differs from the one that is usually observed in ferroelectrics. The observed disorder in deuterons system close to ferroelectric phase transition temperature is an embryo of coexistence ferroelectric order and dipolar glass disorder observed at low temperatures.     

Absence of relaxor-like ferroelectric phase transition in (Pb,Sr)TiO3 thin films

We have performed dielectric and micro-Raman spectroscopy measurements in the 298–673 K temperature range in polycrystalline Pb_0.50Sr_0.50TiO₃ thin films prepared by a soft chemical method. The phase transition have been investigated by dielectric measurements at various frequencies during the heating cycle. It was found that the temperature corresponding to the peak value of the dielectric constant is frequency-independent, indicating a non-relaxor ferroelectric behavior. However, the dielectric constant versus temperature curves associated with the ferroelectric to paraelectric phase transition showed a broad maximum peak at around 433 K. The observed behavior is explained in terms of a diffuse phase transition. The obtained Raman spectra indicate the presence of a local symmetry disorder, due to a higher strontium concentration in the host lattice. The monitoring of some modes, conducted in the Pb_0.50Sr_0.50TiO₃ thin films, showed that the ferroelectric tetragonal phase undergoes a transition to the paraelectric cubic phase at around 423 K. However, the Raman activity did not disappear, as would be expected from a transition to the cubic paraelectric phase. The strong Raman spectrum observed for this cubic phase is indicative that a diffuse-type phase transition is taking place. This behavior is attributed to distortions of the perovskite structure, allowing the persistence of low-symmetry phase features in cubic phase high above the transition temperature. This result is in contrast to the forbidden first-order Raman spectrum, which would be expected from a cubic paraelectric phase, such as the one observed at high temperature in pure PbTiO₃ perovskite. PACS 78.30.-j; 77.80.Bh; 64.70.Kb; 68.55.-a; 77.22.-a; 77.55.+f     

Coexistence of magnetic and ferroelectric properties in Y0.1Co1.9MnO4

The magnetic,conductivity,and dielectric properties have been investigated in single-phase polycrystalline Y0.1 Co1.9 MnO4.The temperature-dependent magnetisation reveals the ferromagnetic transition in sample at a low temperature (～186 K).Magnetisation as a function of field H (M-H loop) indicated the weak ferromagnetism of the sample at room temperature.The constant ε and dielectric loss tgδ measurements represent a ferroelectric phase transition at a higher temperature (～650 K),while the conductivity shows an insulator-metallic transition.The ferroelectric hysterisis loops and capacitance-voltage measurements confirm the ferroelectric nature of the sample at room temperature.The observed ferromagnetism and ferroelectric nature in this material suggests a potential multiferroic application.     

Phase transitions in xPbZr0.53Ti0.47O3-(1 − x)Mn0.4Zn0.6Fe2O4 magnetoelectric composites

The elastic, inelastic, and dielectric properties of the magnetoelectric composite xPbZr_0.53Ti_0.47O₃-(1 ? x)Mn_0.4Zn_0.6Fe₂O₄ (PZT-MZF) are studied in the temperature range from room temperature to 673 K. The influence of the ferroelectric PZT phase on the magnetic phase transition and the magnetic MZF phase on the ferroelectric phase transition is revealed. It is established that, as the PZT content increases, the degree of diffuseness of the phase transition decreases and a gradual crossover from a pronounced relaxor behavior to a more ordered ferroelectric behavior occurs.     

Dielectric properties of the nanoporous MCM-41 matrix filled with the (NH4)2SO4 ferroelectric

Variations with temperature of the linear dielectric permittivity and amplitude of the third harmonic were studied for nanoporous MCM-41 matrices with 4.0-nm channel pores filled with the (NH₄)₂SO₄ ferroelectric, in comparison with bulk ammonium sulfate. The measurements were performed upon heating and cooling in the temperature range from 100 K to room temperature. A noticeable shift to low temperatures (by approximately 25 K) for the ferroelectric phase transition in the MCM-41/(NH₄)₂SO₄ nanocomposite as compared to bulk (NH₄)₂SO₄ was revealed. The temperature hysteresis observed at the phase transition in the nanocomposite was approximately 2 K which is close to that in bulk ammonium sulfate. The significant decrease of the transition temperature in nanostructured ammonium sulfate agrees with the theoretical predictions based on the Landau and Ising models of the size effect on the ferroelectric phase transition in isolated small particles.     

Phase transitions sequence in pyrochlore Cd2Nb2O7 studied by IR reflectivity

The infrared reflectivity of Cd₂Nb₂O₇ single crystal was studied in the temperature interval of 10-540 K, together with complementary dielectric measurements. A ferroelectric soft mode was revealed above the ferroelectric phase transition at T _c = 196 K coupled with a central-mode type dispersion in the near-millimetre range. This proves the mixed displacive and order-disorder nature of the transition. Below T _c many new modes were detected due to lowering of the symmetry, especially below the previously suggested incommensurate transition at 85 K. Discussion of the possible phase transitions based on symmetry considerations is presented with the conclusion that the ferroelectric transition is proper with the F_1u symmetry of the order parameter, whereas the intermediate ferroelastic transition is improper and triggered by the coupling with the ferroelectric order parameter. Received 17 July 2000     

A Raman and dielectric study of a diffuse phase transition in (Pb<Subscript>1-x</Subscript>Ca<Subscript>x</Subscript>)TiO<Subscript>3</Subscript> thin films

Dielectric and Raman scattering experiments were performed on polycrystalline Pb_1-xCa_xTiO₃ thin films (x=0.10, 0.20, 0.30, and 0.40) as a function of temperature. The results showed no shift in the dielectric constant (K) maxima, a broadening with frequency, and a linear dependence of the transition temperature on increasing Ca²⁺ content. On the other hand, a diffuse-type phase transition was observed upon transforming from the cubic paraelectric to the tetragonal ferroelectric phase in all thin films. The temperature dependence of Raman scattering spectra was investigated through the ferroelectric phase transition. The temperature dependence of the phonon frequencies was used to characterize the phase transitions. Raman modes persisted above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of a breakdown of the local cubic symmetry due to chemical disorder. The lack of a well-defined transition temperature and the presence of broad bands in some temperature interval above the FE–PE phase transition temperature suggested a diffuse-type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films. PACS 77.80.Bh; 77.55.+f; 78.30.-j; 77.80.-e; 68.55.-a     

铌酸锂钠在低温时的介电铁电和热电性

在300—20K的温度范围内观测了用丘克拉斯基法生长的Li_0.025Na_0.975NbO₃晶体的介电、铁电和热电特性。介电常数,极化强度和热电性的反常表示该晶体在低温时发生相变。此相变有特别大的热滞(约80K),降温时发生于180K附近,升温时发生于260K附近。测量了晶体的室温结构,指出了可能的低温相点群。观测了热电电荷的时间响应,报道了热电电荷随时间改变符号的特异现象,认为其起因是相变过程中两相共存。 关键词：     

Lattice mediated interactions and ferroelectric anomalies in the crystal (CH3NH3)5Bi2Cl11 (PMACB)

Abstract

A two-sublattice compressible pseudospin model has been constructed to describe the behaviour of the spontaneous polarisation of PMACB in the temperature region comprising the ferroelectric phase transition (T=307K) and the isomorphous anomaly (T= 180 K). A strong coupling of pseudospins with a “pancake” strain ?(2ε₃₃ ? ε₁₁ - ε₂₂) /3 resulting from the comparison of the model with the experimental spontaneous polarisation agrees with the existing data on the dielectric susceptibility, and on the specific heat. The ferroelectric phase transition then turns out to be close to a tricritical point. The numerical values of the parameters of the model show how the ferroelastic phase transition (T=260K) can occur in this crystal without any discernible effect on the specific heat and on the dielectric susceptibility.     

On the nature of the phase transitions in cadmium pyroniobate

The existing, doubtful interpretations of the numerous phase transitions in cadmium pyroniobate Cd₂Nb₂O₇, which is a ferroelectric with a pyrochlore-type structure and a Curie temperature lying near 200 K, are critically analyzed. The manifestations of all phase transitions in cadmium pyroniobate, the mechanisms proposed for these transitions, and doubts about their validity are discussed. Various dielectric anomalies observed near 200 K are related to three phase transitions in this temperature range. The causes of the large number of phase transitions observed in this material, the dislocation structure of the crystal, the diffuseness of the ferroelectric phase transition, and the relaxor properties of the crystal are also explained.     

Dielectric properties of ferroelectric diisopropylammonium iodide

Temperature dependences of dielectric constant, amplitude of the third harmonic and heat capacity for the organic ferroelectric of diisopropylammonium iodide (C₆H₁₆NI) have been investigated. The measurements were carried out through heating and cooling cycles in the range of 300–400?K. It was found that upon the first heating, only one phase transition occurred without the presence of the ferroelectric phase. For samples preheated over 420?K, two phase transitions at 363 and 378?K appeared in the heating process, and the ferroelectric state was also observed between them. Upon cooling, the ferroelectric phase was detected in the range of lower 361?K and persisted up to room temperature.     

Time relaxation of dielectric constant in the commensurate phase of TlGaSe2

The results of measurements of the time dependences of the dielectric constant of TlGaSe₂ in the commensurate ferroelectric phase are presented. From the result of the observation of the decay of ε at different stabilized temperatures below the commensurate phase transition temperature after cooling from the incommensurate phase, the presence of two different characteristic relaxation time constants with the same temperature behaviour has been revealed. This peculiarity is considered as a result of a coexistence of two polar sublattices in the temperature range below 110 K. According to these results, the previously reported dielectric anomaly at about 103 K is considered as a final lock-in phase transition accompanied by the forming of the antiferroelectric state in TlGaSe₂.     

Investigation of ferroelectric phase transition for modified barium titanate in multilayer ceramic capacitors by in situ Raman scattering and dielectric measurement

The ferroelectric phase behaviors of modified BaTiO₃ in X7R multilayer ceramic capacitors (MLCCs) were investigated by dielectric measurements and in situ Raman scattering. The in situ thermo-Raman scattering shows that because of a residual stress existing in the MLCCs, the tetragonal to cubic phase transition for modified BaTiO₃ in MLCCs takes place over a wide temperature range of 373 K to 473 K, suggesting a diffuse-like characteristic which can be well explained by a modified phenomenological thermodynamic model, while the dielectric measurement indicates that the tetragonal to cubic transition occurs at 393 K. A disagreement exists between the two experimental results. Furthermore, a dc field-induced paraelectric to ferroelectric transition was identified by the two measurement techniques, but their phase-transition mechanisms are different. The dielectric measurement reveals the polar-micro-region to macro-domain transformation in the shell part of the core–shell structure while the in situ Raman scattering shows the cubic to tetragonal transition under a dc bias field. PACS 74.20.De; 77.22.Ch; 77.84.Dy; 77.80.Bh; 78.30.-j     

Phase transition and linear thermal expansion of (Pb1− x Ba x )ZrO3 ceramics

(Pb_{1? x} Ba _x )ZrO₃ ceramics for the composition range 0?≤?x?≤?0.30 were prepared by the mixed oxide solid state reaction method. Phase transition was studied by dielectric and dilatometric measurements. The ferroelectric to paraelectric phase transition temperature was progressively shifted to a lower temperature by replacing lead with barium. The x?=?0.20 sample showed the maximum dielectric constant of 16,300 at the transition temperature. For compositions 0?≤?x?≤?0.075, the antiferroelectric to ferroelectric phase transition exhibited a large linear thermal expansion. However, the antiferroelectric to ferroelectric phase transition did not exist for 0.10?≤?x?≤?0.30 samples. A phase diagram for PBZ ceramics prepared by the conventional mixed oxide method was also present.     

Effect of an organic dye on the ferroelectric phase transition in KH2PO4 (KDP)

This paper reports on the first measurement of the dielectric permittivity and heat capacity of a KDP crystal doped by Chicago Sky Blue organic dye within a temperature interval including the ferroelectric phase transition at T _c =122 K. Similar measurements were made on a pure KDP crystal under the same conditions for the sake of comparison. The heat capacities of the pure and doped crystals were shown to differ substantially within an interval 1 K wide in the vicinity of T _c , where an anomaly in the heat capacity of the doped crystal was observed to wash out without producing any change in the temperature position of its maximum. The doping reduces the permittivity in the polar phase markedly. The observed effects are associated with the influence of nonisomorphic defects on the ferroelectric phase transition in a piezoelectric crystal.     

High-temperature phase transitions in the improper ferroelectric KIO3

The ac conductivity (σ_ac) and dielectric permittivity (?) are determined in the temperature range 300?K?T3 compound. The results indicated that the compound behaves as an improper ferroelectric and undergoes a ferroelectric phase transition from a high temperature rhombohedral phase I to a low temperature monoclinic phase II at T _c?=?(486?±?1)?K. A second structural phase transition was observed around 345?K. The conductivity varies with temperature range and for T?>?428?K intrinsic conduction prevails. Different activation energies in the different temperature regions were calculated. The frequency dependence of σ(ω) was found to follow the universal dynamic response [σ(ω)∝(ω) ^s(T)]. The thermal behaviour of the frequency exponent s(T) suggests the hopping over the barrier model rather than the quantum mechanical tunneling model for the conduction mechanism.     

Diffuse phase transition of Fe doped lead ytterbium tantalate ceramics

The effect of different concentration of Fe on the phase transition behavior of Lead ytterbium tantalate is investigated by dielectric and differential scanning calrimetry measurements. The samples are prepared through solid state reaction method and it has been found that the sintering temperature significantly lowered when the proportion of Pb(Fe_1/2Ta_1/2)O₃ increased. It has been observed that the doping in small amounts (0≤x≤0.2) of Fe could meliorate the dielectric and ferroelectric properties. The diffuseness in the mode of phase transition increases and the phase transition temperature decreases as a function of Fe content. It is revealed that the dielectric data and heat capacity data follow a similar trend in the variation of the mode of phase transition and phase transition temperatures. The phase transition temperature values obtained from the heat capacity measurement well agreed with the values obtained from dielectric measurement.     

Ti-doping induced antiferroelectric to ferroelectric phase transition and electrical properties in Sm-PbZrO3 thin films

The antiferroelectric (Pb_0.985Sm_0.01) (Zr_1-xTi_x)O₃ (Ti-PSZO) thin films were synthesized on Pt(111)/Ti/SiO₂/Si substrates using a chemical solution deposition method. The films were crystallized in the perovskite phase with a preferential orientation along (111) direction. With Ti doping in PSZO, a gradual transformation from antiferroelectric to ferroelectric phase transition was noticed at room temperature owing to the Ti doping induced lattice distortion. The phase transition has been confirmed through the P - E hysteresis loops, X-ray diffraction (peak shifting), capacitance-voltage measurements, and Raman scattering analysis. The thin film with Ti = 0.15 doping displayed a ferroelectric behavior with high dielectric constant and large dielectric tunability of about 62%. Also, Ti doping altered the Curie temperature (T_c) and enhanced the order of dielectric diffuseness. It is believed that Ti-doping in PSZO is an effective way to induce an antiferroelectric - ferroelectric phase transition and to tailor the electrical characteristics of PSZO thin films.     

钨青铜型结构铁电体的低温扩散相转变

对六种钨青铜结构的铌酸盐铁电晶体进行了从15K到室温范围的介电特性及热电特性的研究。分析其介电特性和热电特性与极化电场的关系,用X射线粉末衍射进行佐证。证实在50—70K的范围内,SBN,PBN,KNSBN三类铁电钨青铜型铌酸盐晶体均存在着一个新的相变。相变是由点群4mm铁电相到点群mm²铁电相的转变。铁电自发极化方向由四方晶胞的e轴方向转变到正交晶胞的b轴方向。介电特性的高频及低频测量表明该相变具有扩散(或称瀰散)型特征。比热的实验结果证明相变是属于高于一阶相变的高阶相变。对相变前 关键词：     

Proton conductivity and ferroelectric phase transition in hydrogen-bonded ferroelectric NH4IO3

We report on the ac dielectric permittivity (ε) and the electric conductivity (σ_ω), as function of the temperature 300?K?T4IO₃. The main feature of our measured parameters is that, the compound undergoes a ferroelectric phase transition of an improper character, at (368?±?1)K from a high temperature paraelectric phase I (Pm2₁ b) to a low temperature ferroelectric phase II (Pc21n). The electric conduction seems to be protonic. The frequency dependent conductivity has a linear response following the universal power law (σ_{( ω )}?=?A(T)ω ^{s (T)}). The temperature dependence of the frequency exponent s suggests the existence of two types of conduction mechanisms.