Magnetic-field-induced phase transition in Tb0.95Bi0.05MnO3+δ multiferroic

The effect of a magnetic field on the dielectric properties of Tb_0.95Bi_0.05MnO_3+δ single crystals has been analyzed. It has been shown that the state of the crystal for temperatures of 5–440 K is inhomogeneous and restricted domains of polar and spin correlations are exhibited in it. A phase transition in which the inhomogeneous state of the crystal changes substantially has been observed at a temperature of T ? 180 and 225 K (in the absence and presence of a magnetic field, respectively). The high-temperature phase contains large dielectric domains with a high dielectric constant (ε ～ 10⁵) and thin conducting layers at the boundaries of these domains. The magnetic field significantly affects the state of both low-and high-temperature phases of the crystal, shifting the temperature of the phase transition between them, and induces an additional phase transition at T ? 441 K.     

由BaTiO3晶体结构相变时的介电特性研究其电场作用下的偶极子偏转路径

在不同频率和偏置电场下测量了BaTiO₃晶体从三方相到正交相再到四方相相变过程中的介电温谱.基于BaTiO₃晶体在电场作用下的偶极子偏转假设和介电特性实验结果,提出BaTiO₃各结构相在电场作用下的偶极子偏转路径可以由其相邻相的介电常数随温度的变化特性表现出来.推断出各结构相的偶极子偏转路径,以及偏置电场对偶极子偏转路径的影响. 关键词： 3晶体')" href="#">BaTiO₃晶体 介电特性 结构相变 偶极子偏转路径     

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：     

The aging effect within incommensurate phase in TlGaSe2

The effects of physical aging on the dielectric constant of TlGaSe₂ layered ferroelectric crystal were investigated. Isothermal aging processes were done within incommensurate phase in TlGaSe₂ ferroelectric single crystals when slow cooling was interrupted for a time at a constant temperature. The dielectric constant exhibited an unusual behaviour: it decreased exponentially with time. The results can be interpreted taking into account the interaction of the structure of the incommensurate phase with crystal fields of defects randomly distributed in the crystal structure. The measurements show that there are two relaxation mechanisms associated with two relaxation sites in mentioned temperature interval. This means that two different types of incommensurate phases occur in mentioned temperature interval.     

Dielectric and acoustic properties of the new lead-free BiLi<Subscript>0.6</Subscript>W<Subscript>0.4</Subscript>O<Subscript>3</Subscript> ceramic

The properties of the new lead-free compound BiLi_0.6W_0.4O₃, obtained using ceramic technology, were studied by dielectric and mechanical spectroscopy. The anomalies of internal friction are, along with the elastic and dielectric properties observed experimentally at a temperature of 350°C, related to its structural phase transition. Dielectric and mechanical relaxation due to processes of interaction between crystal lattice defects was revealed in the low-temperature phase.     

Time relaxation of permittivity in [(CH3)2NH2]3CuCl5 crystal under external electric field

Continuation of our previous studies connected with dielectric properties of tris-dimethylammoniumpentachlorocuprate crystal [(CH₃)₂NH₂]₃CuCl₅ crystal under the influence of electric field are presented in this article. We studied the time dependence of the permittivity under the influence of electric field at constant temperatures above T _c. In experiment, slow but significant decrease of permittivity was observed, which can be described with two main relaxation processes. The observed dielectric behavior can be related to the coexistence of phases and slow transition to ferroelectric one through long-time metastable states connected with the presence of incommensurate phase in the studied crystal. The coexistence of phases and slow transition was confirmed from the appearance of induced polarization under external electric field, measured as depolarization current.     

Dielectric properties of a triglycine sulfate crystal grown with a transition through the Curie point

The dielectric properties of the triglycine sulfate (TGS) crystal grown by lowering the temperature with a transition through Curie point T_C during its growth are studied. The greatest values of low-frequency dielectric permittivity ε_max(T = T_C) and effective dielectric permittivity ε_eff(E₀) correspond to the layer of crystal formed at the temperature of the phase transition.     

Magnetodielectric coupling by exchange striction in Y<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>5</Subscript>

We have studied the magnetodielectric response of Y₂Cu₂O₅, the so-called blue phase in the Y₂O₃-CuO-BaO phase diagram. Based on symmetry principles, we predict and demonstrate magneto-dielectric coupling on a single crystal sample. We report an anomaly in the dielectric constant at the ordering temperature of the Cu spins. We probe the magnetic field-induced phase transitions between four different magnetic phases using magneto-capacitance measurements, demonstrating relatively strong magnetodielectric coupling. We observe an increase in dielectric constant in the spin-flip phase where there exists spontaneous magnetization. We construct a detailed magnetic phase diagram. The magnetodielectric coupling is analyzed in terms of striction induced by symmetric superexchange and optical phonon frequency shifts.     

Sequence of structural phase transitions of CsInF4 crystal

In this work we employ calorimetric and dielectric techniques to study the sequence of structural phase transitions (SPTs) of CsInF₄ crystal in the temperature range from 450 to 250 K. Our results show three first-order SPTs. Based on these results and on direct interferometric observation of the domain patterns, we discuss the elastic state of CsInF₄ phases.     

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.     

Effect of gamma irradiation on the dielectric response of a Sr0.75Ba0.25Nb2O6 relaxor single crystal

The effect of gamma irradiation from a ⁶⁰Co source at a dose D = 4.75 × 10⁴ R on the dielectric properties of a Sr_0.75Ba_0.25Nb₂O₆ (SBN-75) single crystal with a smeared phase transition is investigated in the low-and infralow-frequency ranges. The behavior of the dielectric response is compared with the behavior of the depolarization currents induced in unirradiated and irradiated samples of the Sr_0.75Ba_0.25Nb₂O₆ compound. The observed changes in the electrical properties of the Sr_0.75Ba_0.25Nb₂O₆ single crystal in the temperature range of the smeared phase transition are associated with the stabilization of the ferroelectric phase of the material in an internal bias field generated by radiation-induced defects.     

Dielectric properties of deuterated ammonium Rochelle salt

The dielectric constant of deuterated ammonium Rochelle salt along the three crystallographic axes as well as the pyroelectric charge density developed on the crystal surface perpendicularly to the b-axis have been measured with a high temperature resolution. The para- to ferroelectric transition at T_c = ?159°C shows a thermal hysteresis of ΔT = 0.15°C. This and the discontinuities in the dielectric constant along all three axes show that the phase transition is of the first-order. No phase intermediate between the para- and the ferroelectric phases could be detected.     

Dielectric and calorimetric studies on 1BC1EPOPB feroelectric liquid crystal having a large spontaneous polarization

Experimental investigations on the ferroelectric liquid crystal, R-4′(1-butoxycarbonyl-1-ethoxy) phenyl 4-(4-octyloxy phenyl) benzoate (1BC1EPOPB) of large spontaneous polarization (P _S(+) = 240?nC?cm^?2), using dielectric and calorimetric techniques, are reported. The temperature range of 25.0–125.0°C has been chosen for dielectric measurements. Dielectric dispersion studies are carried out in the temperature range 45.0–75.0°C and in the frequency range 2?Hz to 2?MHz for the smectic A, smectic C* and smectic X phases. A new phase called ‘smectic X’ has been found around 56.3°C. The transition temperatures identified by the dielectric dispersion studies for different phases and those given by DSC techniques are in close agreement.     

High-pressure dielectric studies of a novel hydrogen-bonded ferroelectric (NH4)2H2P2O6

The hydrostatic pressure effect on the dielectric properties of (NH₄)₂H₂P₂O₆ ferroelectric crystal was studied for pressures from 0.1 MPa to 360 MPa and for temperatures from 100 to 190 K. The pressure–temperature phase diagram obtained is linear with increasing pressure. The paraelectric–ferroelectric phase transition temperature decreases with increasing pressure with the pressure coefficient dT_c/dp=?5.16×10^?2 K MPa^?1. Additionally, the pressure dependences of Curie–Weiss constants for the crystal in paraelectric (C₊) and ferroelectric (C_?) phases are evaluated and discussed. The possible mechanism of paraelectric–ferroelectric phase transition is also discussed.     

Characteristics of the clustered structure of Pb(Li, La)(Zr1−y Tiy)O3 in the antiferroelectric-ferroelectric transition region

Mesoscopic inhomogeneities and their influence on the behavior of the physical properties (optical reflectivity, dielectric constant, and Curie temperature) of a system of lead zirconate-titanate Pb_0.85(Li_1/2La_1/2)_0.15(Zr_1−y Ti_y)O₃ solid solutions are investigated. It is established that the disruptions of long-range order have a mesoscopic scale and generate ion groups in the form of interlayers of elements of binary-oxide planes coherently embedded in the crystal structure of the host phase. It is shown that the clustering of the structure is manifested in nonlinearity of the physical properties of the lead zirconate-titanate solid solutions, where the most pronounced variations are characteristic of the coexistence region of the antiferroelectric and ferroelectric phases. Zh. Tekh. Fiz. 68, 43–47 (February 1998)     

Properties of strontium titanate in the SrTiO3/CeO2/Al2O3 multilayered structure

Studies of the crystal structure, elemental composition, and dielectric properties of strontium titanate films in SrTiO₃/CeO₂/Al₂O₃ multilayered structures are reported. Data on the crystal lattice and impurity contents have been obtained, and temperature and electric field dependences of the dielectric properties of SrTiO₃ films in the microwave range have been measured. An analysis of the results is made to establish the reason for the nonmonotonic dependence of the small-signal dielectric permittivity of SrTiO₃ films on temperature. Fiz. Tverd. Tela (St. Petersburg) 39, 1024–1029 (June 1997)     

Investigating the dielectric properties of potassium iodate polycrystals

Linear and nonlinear dielectric properties of KIO₃ polycrystalline samples are investigated. It is shown that linear dielectric permittivity ε' displays four anomalies corresponding to phase transitions in KIO₃. Anomalies of third-harmonic coefficient γ_3ω are observed at temperatures of 113, 263, and 345 K, corresponding to phase transitions between KIO₃ ferroelectric phases. It is established that the third-harmonic coefficient responsible for the nonlinearity of dielectric properties displays no anomalies during the transition to the paraelectric phase of KIO₃ at approximately 485 K. Possible reasons for there being no such anomaly are discussed.     

Optical activity of incommensurate crystals with plane-wave modulation

A phenomenological approach is used to discuss the propagation of polarized light in the incommensurately modulated phases of crystals of the family A₂BX₄ with spatially averaged inversion symmetry. The Jones-matrix technique for an anisotropic inhomogeneous medium is used to calculate the optical parameters of a crystal with sinusoidally modulated dielectric parameters. It is demonstrated that these crystals can exhibit a weak optical activity, i.e., weaker than that of crystals without inversion symmetry. Also discussed are boundary conditions on the phase of the modulation wave at the surface of a crystal plate. Results are obtained suggesting that the optical properties of an incommensurately modulated crystal need not depend on the shape of the modulation wave. Fiz. Tverd. Tela (St. Petersburg) 39, 1360–1365 (August 1996)     

The effect of substrate heating on the tunability of rf-sputtered Bi2O3-ZnO-Nb2O5 thin films

Reactively rf-sputtered Bi₂O₃-ZnO-Nb₂O₅ (BZN) thin films were prepared on Pt(111)/TiO₂/SiO₂/Si with substrate heating. The effects of substrate heating on the structures, morphologies, dielectric properties, and voltage-tunable dielectric properties of the films were investigated. With heating, the BZN thin films could be deposited in crystalline form as the cubic pyrochlore phase. The amounts of secondary phases, such as zinc niobate and bismuth niobate, depended on the substrate temperature. The more compounding of the BZN crystalline phase proceeded at deposition, the less formation of secondary phases and stoichiometric change occurred after post-annealing. Therefore, improvement of the dielectric constant and tunability of thin films by grain-size enlargement might be possible with proper substrate heating and post-annealing. The BZN thin films sputtered with a substrate temperature of 550 °C and annealed at 800 °C showed a maximum tunability of 26.5% at a dc bias field of 1000 kV/cm and measurement frequency of 1 MHz. PACS 81.15.Cd; 77.55.+f; 77.84.Dy; 81.40.Tv     

Effect of enantiomeric excess on the smectic-C* phases under electric field

A series of enantiomeric mixtures of 4-(1-methylheptyloxycarbonyl) phenol 4'-octyloxybiphenyl-4-carbonate (MHPOBC) is reported to understand the structures and stability conditions of emerging liquid crystal phases. The mesomorphic properties are studied by microscopic observation, electro-optic and dielectric measurements. The electric field–concentration (E–x) phase diagram for each phase at constant temperature is constructed. The complete electric field–temperature (E–T) phase diagrams for several enantiomeric mixtures of the antiferroelectric MHPOBC are established. These phase diagrams show the influence of optical purity on the phase stability and the unwinding threshold.