Dielectric relaxations in Tb0.91Yb1.38Bi0.71Fe5O12 ceramics

Dielectric relaxations of Tb_0.91Yb_1.38Bi_0.71Fe₅O₁₂ ceramics were investigated. A Debye-type relaxation was observed in the temperature range of 125-620 K with an activation energy of 0.29 eV. This activation energy agreed well with that of carriers hopping between Fe²⁺ and Fe³⁺, indicating that this relaxation might be a dipolar-type relaxation associated with the hopping carries. A high relaxorlike dielectric peak with a very strong frequency dispersion in the high temperature range of 400-620 K might be originate from the oxygen vacancies related dielectric relaxation.     

Electric-field dependence of dielectric properties of sol-gel derived Ba(Zr0.2Ti0.8)O3 ceramics

The effect of a dc bias field on the diffuse phase transition and nonlinear dielectric properties of sol-gel derived Ba(Zr_0.2Ti_0.8)O₃ (BZT) ceramics are investigated. Diffuse phase transitions were observed in BZT ceramics and the Curie–Weiss exponent (CWE) was γ∼2.0. The dielectric constant versus temperature characteristics and the γ in the modified Curie–Weiss law, ε ⁻¹=ε _m ⁻¹[1+(T−T _m ) ^γ /C₁](1≤γ≤2), as a function of the dc bias field was obtained for BZT ceramics. The results indicated that γ is a function of dc bias field, and the γ value decreased from 2.04 to 1.73 with dc bias field increasing from 0 to 20 kV/cm. The dielectric constant decreases with increasing dc bias field, indicating a field-induced phase transition. The dc bias field has a strong effect on the position of the dielectric peak and affects the magnitude of the dielectric properties over a rather wide temperature range. The peak temperature of the dielectric loss does not coincide with the dielectric peak and an obvious minimum value for the dielectric loss at the temperature of the dielectric peaks is observed. At room temperature, 300 K, the high tunability (K=80%), the low loss tangent (≈0.01) and the large FOM (74), clearly imply that these ceramics are promising materials for tunable capacitor-device applications.     

Giant dielectric relaxation in SrTiO3-SrMg1/3Nb2/3O3 and SrTiO3-SrSc1/2Ta1/2O3 solid solutions

Ceramic samples of (1−x)SrTiO₃-xSrMg_1/3Nb_2/3O₃ and (1−x)SrTiO₃-xSrSc_1/2Ta_1/2O₃ were prepared, and their dielectric properties were studied at x=0.005–0.15 and 0.01–0.1, respectively, at frequencies 10 Hz–1 MHz and at temperatures 4.2–350 K. A giant dielectric relaxation was observed in the temperature range 150–300 K, and not so strong but well-developed relaxation was found in the temperature range 20–90 K. The activation energy U and the relaxation time τ₀ were determined to be 0.21–0.3 eV and from 10⁻¹¹ to 10⁻¹² s for the high-temperature relaxation and 0.01–0.02 eV and 10⁻⁸–10⁻¹⁰ s for the low-temperature relaxation, respectively. The additional local charge compensation of the heterovalent impurities Mg²⁺ and Nb⁵⁺ (or Sc³⁺ and Ta⁵⁺) by free charge carriers or the host ion vacancies is suggested to be the underlying physical mechanism of the relaxation phenomena. On the basis of this mechanism, the Maxwell-Wagner model and the model of reorienting dipole centers Mg²⁺ (or Sc³⁺) associated with the oxygen vacancy are proposed to explain the high-temperature relaxation with some arguments in favor of the latter model. The polaron-like model with the Nb⁵⁺-Ti³⁺ center is suggested as the origin of the low-temperature relaxation. The reasons for the absence of ferroelectric phase transitions in the solid solutions under study are also discussed. From Fizika Tverdogo Tela, Vol. 44, No. 11, 2002, pp. 1948–1957. Original English Text Copyright ? 2002 by Lemanov, Sotnikov, Smirnova, Weihnacht. This article was submitted by the authors in English.     

Effect of Ca doping on the magnetic properties of Nd0.5Sr0.5MnO3 studied by electron spin resonance

The magnetic properties of Ca-doped Nd_0.5Sr_0.5MnO₃ have been studied by electron spin resonance (ESR) and dc magnetization measurements. The antiferromagnetic order and charge order are found to occur separately at T_N=200 K and T_co=150 K, respectively. Compared to the undoped Nd_0.5Sr_0.5MnO₃, the ferromagnetic correlations are suppressed by doping of the small Ca²⁺ ion. In addition, the antiferromagnetic transition temperature is enhanced to 200 K, which can be explained by an increase of superexchange interaction between Mn³⁺ and Mn⁴⁺ ions as their distance decreases.     

Maxwell-Wagner characterization of dielectric relaxation in Ni0.8Zn0.2Fe2O4/Sr0.5Ba0.5Nb2O6 composite

Dense composites were prepared through incorporating the dispersed Ni_0.8Zn_0.2Fe₂O₄ ferromagnetic particles into Sr_0.5Ba_0.5Nb₂O₆ ferroelectric matrix. Extrinsic dielectric relaxation and associated high permittivities of the materials are reported in the composites. We used an ideal equivalent circuit to explain electrical responses in impedance formalism. A Debye-like relaxation in the permittivity formalism was also found. Interestingly, real permittivity (ε′) of the sample containing 30% Ni_0.8Zn_0.2Fe₂O₄ shows obvious independence of the temperature at 100 kHz. Dielectric relaxation and high-ε′ properties of the composites are explained in terms of the Maxwell-Wagner (MW) polarization model.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Surface spin-glass-like behavior of monodispersed superparamagnetic Mn0.5Zn0.5Fe2O4 magnetic fluid

Zero field cooled dc-magnetization measurements of monodispersed Mn_0.5Zn_0.5Fe₂O₄ nanoparticles dispersed in kerosene exhibit two transitions at low temperatures. These transitions correspond to (i) the superparamagnetic to blocked superparamagnetic and (ii) the blocked superparamagnetic to surface spin-glass like/quantum superparamagnetic state upon lowering the temperature. The existence of a disorder surface is confirmed by recording small-angle neutron scattering data below and above the Curie temperature. Magnetic relaxation analysis shows a plateau at low temperature (below 5 K) with a slight minimum at 3 K, which is a characteristic of the surface spin-glass-like state. This is analyzed considering the energy distribution n(E)∼1/E. The existence of surface disorder dominates at low temperature and mimics the transition from superparamagnetic to quantum superparamagnetic states.     

Relaxor behavior of (Sr,Ba,Bi)TiO3 ferroelectric ceramic

The dielectric behavior of (Sr_0.4Ba_0.6)_0.925Bi_0.05TiO₃ (SBBT) ceramic was investigated in the temperature range from 100 K to 450 K. Broad dielectric maxima, which shift to higher temperature with increasing frequency, and the value of the relaxation parameter γ=1.6-2 estimated from the linear fit of the modified Curie-Weiss law, indicated the relaxor nature of the SBBT ceramic. The dielectric relaxation which follows the Vogel-Fulcher relationship with  K, E_a=0.0392 eV, and ν_o=2.98×10¹¹ Hz, further supports such a relaxor nature. The P-E hysteresis loop at different temperatures and ‘butterfly’ shape dc bias field dependence of permittivity at T>T_m (the temperature of permittivity maximum) signifies the occurrence of nanopolar clusters, which is the typical characteristic of relaxor ferroelectrics. At 300 K and 10 kHz, the dielectric constant and loss tgδ are ∼2210 and 0.00118, respectively. The tunability (28%) and figure of merit (237) at room temperature show that the SBBT ceramic could be a promising candidate for tunable capacitor applications.     

Ferroelectric-relaxor behavior of (Na0.5K0.5)NbO3-based ceramics

We report that ferroelectric-relaxor behavior is induced by doping of SrO and TiO₂, or BaO and TiO₂ into classic ferroelectric (Na_0.5K_0.5)NbO₃. It is found that [(Na_0.5K_0.5)_0.9Sr_0.1](Nb_0.9Ti_0.1)O₃ ceramics exhibit a pronounced ferroelectric-relaxor behavior, comparable to that of [(Na_0.5K_0.5)_0.9Ba_0.1](Nb_0.9Ti_0.1)O₃ ceramics. Our results indicate that the relaxor behavior is closely related to the appearance of micropolar regions in these systems. The relaxor behavior should arise from the dynamic response of micropolar clusters. Raman spectra of [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ ceramics measured in the wavenumber range from 100 to 1200 cm⁻¹ confirm that the first order scattering is dominant in phonon bands should result from both short-range ordered region (micropolar regions) and disordered matrix. The frequency dependence of dielectric permittivity measurements show that the relaxor behavior of SrO and TiO₂, or BaO and TiO₂ doped (Na_0.5K_0.5)NbO₃ ceramics is not a Debye type in the radio frequency range.     

Dielectric properties and relaxation behavior of [TMA]2Zn0.5Cu0.5Cl4 compound

The [TMA]₂Zn_0.5Cu_0.5Cl₄ hybrid material was prepared and its dielectric spectra were measured in the 10⁻¹ Hz-10⁶ Hz frequency range and 200-305 K temperature interval. The dielectric permittivity showed a ferroelectric-paraelectric phase transition at 293 K. Double relaxation peaks are observed in the imaginary part of the electrical modulus, suggesting the presence of grain and grain boundary in the sample. The frequency dependent conductivity was interpreted in term of Jonscher's law: σ(ω)=σ_dc+Aωⁿ. The temperature dependent of the dc conductivity (σ_dc) was well described by the Arrhenius equation: σ_dcT=σ_o×exp(−E_a/kT).     

Dielectric parameters in Se70Te30 and Se70Te28Zn2 chalcogenide glasses

Temperature and frequency dependence of dielectric constant (ε′) and dielectric loss (ε″) are studied in glassy Se₇₀Te₃₀ and Se₇₀Te₂₈Zn₂. The measurements have been made in the frequency range (8-500 kHz) and in the temperature range 300 to 350 K. An analysis of the dielectric loss data shows that the Guintini's theory of dielectric dispersion based on two-electron hopping over a potential barrier is applicable in the present case.No dielectric loss peak is observed in glassy Se₇₀Te₃₀. However, such loss peaks exist in the glassy Se₇₀Te₂₈Zn₂ in the above frequency and temperature range. The Cole-Cole diagrams have been used to determine some parameters such as the distribution parameter (α), the macroscopic relaxation time (τ₀), the molecular relaxation time (τ) and the Gibb's free energy for relaxation (ΔF).     

Dielectric property of CaCu3Ti4O12 thin film grown on Nb-doped SrTiO3(100) single crystal

Thin film of CaCu₃Ti₄O₁₂ (CCTO) has been deposited on Nb-doped SrTiO₃(100) single crystal using pulsed laser deposition. The dielectric constant and AC conductivity of CCTO film in the metal–insulator–metal capacitor configuration over a wide temperature (80 to 500 K) and frequency (100 Hz to 1 MHz) range have been measured. The small dielectric dispersion with frequency observed in the lower temperature region (<300 K) indicates the presence of small defects in the deposited CCTO thin film. The frequency-dependent AC conductivity at lower temperature indicates the hopping conduction. The dielectric dispersion data has been analyzed in the light of both conductivity relaxation and Debye type relaxation with a distribution of relaxation times. Origin of dielectric dispersion is attributed to the distribution of barrier heights such that some charge carriers are confined between long-range potential wells associated with defects and give rise to dipolar polarization, while those carriers which do not encounter long-range potential well give rise to DC conductivity.     

Influence of mobile space charges on electrical properties of ferroelectric Bi3.5La0.5Ti3O12 thin films

Lanthanum-substituted bismuth titanate, Bi_3.5La_0.5Ti₃O₁₂ (i.e., x=0.5 in Bi_4−xLa_xTi₃O₁₂), thin films have been grown on Pt/Ti/SiO₂/Si substrates using pulsed laser deposition. The frequency dependence of the real part ε′(ω) and the imaginary part ε″(ω) of the dielectric constant has been studied. The ε′(ω) does not show any sudden change within the frequency range of 10²-10⁶ Hz. In contrast, the ε″(ω) shows a large dispersion as frequency decreases. The observed relaxation behavior in ε″(ω) can be explained in terms of a migration of oxygen vacancies in (Bi₂O₂)²⁺ layers, not in Bi₂Ti₃O₁₀ perovskite layers.     

Oxygen-defects-related dielectric response in CaCu3Ti4O12 ceramics

A 10 mm thickness columned CaCu₃Ti₄O₁₂ ceramic was fabricated by the conventional solid-state reaction method and the dielectric properties of different parts in ceramic had been investigated. For the sample close to the surface, only one Debye-type relaxation around 10⁷ Hz was observed at room temperature. However, for the sample close to the core, another relaxation peak was observed at about 10⁴ Hz. The results were explained in terms of the equivalent circuit model by showing in the impedance spectroscopy. Moreover, it was introduced that the low-frequency dielectric relaxation is associated with the electrode-sample contact effect based on varying sample thickness and an annealing treatment in the nitrogen atmospheres.     

Effect of Cr doping on the magnetic and electrical transport properties of charge-ordered Bi0.5Ca0.5MnO3

An experimental study on the magnetic and electrical transport properties of the manganites Bi_0.5Ca_0.5Mn_1−xCr_xO₃ (BCMCO) (0≤x≤0.12) is carried out. The results show that Cr doping can suppress the charge-ordering transition, favoring the ferromagnetic clusters. For x=0.12, the charge-ordering transition disappears but a very broad paramagnetic-ferromagnetic-like transition is detected at the Curie temperature T_C=72.6 K. It is caused by phase separation or coexistence of the charge-ordering and ferromagnetic phase. Moreover, the critical Cr content to destroy charge ordering phase in BCMCO does not match the general monotonous tendencies shown by Cr-doped Re_0.5Ca_0.5MnO₃ (Re-rare-earth). These differences are ascribed to the fact that the ground state in BCMCO differs markedly from the ferromagnetic metallic phase in Cr-doped Re_0.5Ca_0.5MnO₃ compounds.     

Effect of Cu-stoichiometry on the dielectric and electric properties in CaCu3Ti4O12 ceramics

CaCu_3+yTi₄O₁₂ (y=0, ±0.025, ±0.05, ±0.1 and −0.15) ceramics are prepared by the conventional solid-state reaction technique under sintering condition of 1050 ^°C, 10 h. X-ray diffraction shows that they all have the good crystalline structure. Cu-deficient ceramics exhibit the microstructures of uniform grain size distribution, whereas both Cu-stoichiometric and Cu-rich ceramics display microstructures of bimodal grain size distribution. The largeness of low-frequency dielectric permittivity at room temperature is found to be very sensitive to the Cu-stoichiometry. Upon raising the measuring temperature, all of the ceramics present commonly three semicircles in the complex impedance plane. It indicates that there exist three distinct contributions, which are ascribed to arising from domains, grain boundaries and domain boundaries. In addition, the influence of CuO segregation on the dielectric and electrical properties is also discussed.     

Ferroelectric relaxor behaviour in Pb(Fe0.5Ta0.5)O3

The relaxor ferroelectric lead iron tantalate, Pb(Fe_0.5Ta_0.5)O₃ (PFT) is synthesized by Coulombite precursor method. The X-ray diffraction pattern of the sample at room temperature shows a cubic phase. The field dependence of dielectric response is measured in a frequency range 0.1 kHz — 1 MHz and in a temperature range from 173–373 K. The temperature dependence of permittivity (ɛ′) shows broad maxima at various frequencies. The frequency dependence of the permittivity maximum temperature (T _m ) has been modelled using Vogel-Fulcher relation.      

Research on charge-ordering state in La0.5Sr0.5MnO2.88 and La0.5Sr0.5Mn0.5Ti0.5O3 systems

The structural, magnetic and transport properties of La_0.5Sr_0.5MnO_2.88 and La_0.5Sr_0.5Mn_0.5Ti_0.5O₃ samples have been investigated systematically. Indeed, this series has been considered to understand the influence of physical parameters such as oxygen deficiency and titanium doping effect in undoped La_0.5Sr_0.5MnO₃ sample. Ceramic material based on La_0.5Sr_0.5MnO_2.88 exhibits interesting behaviours of charge-ordering (CO), ferromagnetic (FM) states and a good conductivity down to the lowest temperatures. The substitution of Ti for Mn destroyed drastically the CO, damaged the motion of itinerant e_g electrons and changed the local parameters of perovskite cell. A change of the structure from tetragonal to rhombohedral symmetry is observed causing a weakening of double-exchange interaction. The experiment results show that the suppression of the CO is sensitive to the variety of Mn³⁺/Mn⁴⁺ ratio. In a field of 8 T at 10 K, FM and CO phase can be evaluated to be ∼20:80 according to the μ_exp/μ_cal ratio for La_0.5Sr_0.5MnO_2.88, whereas the CO state is suppressed for La_0.5Sr_0.5Mn_0.5Ti_0.5O₃ sample, FM and anti-ferromagnetic (AFM) phase are coexisted and evaluated to be ∼54:46, respectively.     

Local composition and carrier concentration in Pb0.7Ge0.3Te and Pb0.5Ge0.5Te alloys from Te NMR and microscopy

Pb_0.7Ge_0.3Te and Pb_0.5Ge_0.5Te alloys, (i) quenched from 923 K or (ii) quenched and annealed at 573 K for 2 h, have been studied by ¹²⁵Te NMR, X-ray diffraction, electron and optical microscopy, as well as energy dispersive spectroscopy. Depending on the composition and thermal treatment history, ¹²⁵Te NMR spectra exhibit different resonance frequencies and spin-lattice relaxation times, which can be assigned to different phases in the alloy. Quenched and annealed Pb_0.7Ge_0.3Te alloys can be considered as solid solutions but are shown by NMR to have components with various carrier concentrations. Quenched and annealed Pb_0.5Ge_0.5Te alloys contain GeTe- and PbTe-based phases with different compositions and charge carrier concentrations. Based on the analysis of non-exponential ¹²⁵Te NMR spin-lattice relaxation, the fractions and carrier concentrations of the various phases have been estimated. Our data show that alloying of PbTe with Ge results in the formation of chemically and electronically inhomogeneous systems. ¹²⁵Te NMR can be used as an efficient probe to detect the local composition in equilibrium as well as non-equilibrium states, and to determine the local carrier concentrations in complex multiphase tellurides.     

Characterization of lead free (K0.5Na0.5)NbO3-LiSbO3 piezoceramic

(K_0.5Na_0.5)NbO₃ (KNN) based lead free ceramics have been fabricated by a solid state reaction. In this work, LiSbO₃ (LS) modified KNN based ceramics were sintered at atmospheric pressure and high density (>96% theoretical) was obtained. The detailed elastic, dielectric, piezoelectric and electromechanical properties were characterized by using the resonance technique combined with the ultrasonic method. The full set of material constants for the obtained polycrystalline ceramics were determined and compared to the pure hot pressed KNN counterpart. KNN-LS polycrystalline ceramic was found to have higher elastic compliance, dielectric permittivity and piezoelectric strain coefficients, but lower mechanical quality factor, when compared to pure KNN, exhibiting a “softening” behavior. However, a high coercive field (∼17 kV/cm) was found for the LS modified KNN material. The properties as a function of temperature were determined in the range of −50-250 ^°C, showing a polymorphic phase transition near room temperature, giving rise to improved piezoelectric behavior.