Physical properties of BaMg1/3Nb2/3O3-BaCo1/3Nb2/3O3 solid solutions

Structural, electric and magnetic properties of Ba₃Mg_1−xCo_xNb₂O₉ based dielectric ceramic compounds have been studied. The samples, prepared by a solid state reaction method, were characterised by X-ray powder diffraction (XRPD), electron microscopy (SEM), dielectric (ε(T)) and magnetic measurements (χ⁻¹(T)). The XRPD analyses showed that the crystal structure of these compounds does change by the increase of substitution degree, passing from a superstructure hexagonal-type, (no. 164), space group (SG) to a simple structure cubic-type, (no. 221), SG. However, the evolution of the elementary unit cell lattice parameter can be followed and it exhibit a linear increasing tendency with increase in the substitution, indicating the existence of a solid solution through out the investigated range of substitution (0-1). The microstructure analysis shows a variation in the grain size and also the porosity of the samples with the degree of substitution. The results are in good agreement with that of dielectric measurements, which also showed that the dielectric constant (ε) increases with the increase of cobalt content. The magnetic characterization of cobalt substituted samples showed an antiferromagnetic type super-exchange interaction between these magnetic ions. At the same time, the values of effective magnetic momentum (μ_eff) are close to the value that corresponds to Co²⁺ free ions. The study highlights the possibility of modelling these materials by substitutions, in order to improve properties of negative-positive-zero (NPO) type dielectric applications.     

Pyroelectric properties in three phases coexistence Pb[(Mn0.33Nb0.67)0.5(Mn0.33Sb0.67)0.5]0.08(ZrxTi1−x)0.92O3 lead ceramics

The bulk dense Pb[(Mn_0.33Nb_0.67)_0.5(Mn_0.33Sb_0.67)_0.5]_0.08(Zr_xTi_1−x)_0.92O₃ pyroelectric ceramics have been successfully prepared by the conventional solid method. The effect of three phases coexistence in the ceramics is studied. When x = 0.95 and 0.85 in the ceramics, the maximum pyroelectric coefficient peaks appear at 23 °C and 45 °C, and the maximum values are 26.5 × 10⁻⁴ C/m² °C and 25.5 × 10⁻⁴ C/m² °C, respectively. The maximum pyroelectric coefficient appears large while the peaks widths are small. When the two kinds of ceramic powders mixed with the mol ratio of 2:1, the pyroelectric coefficient of the ceramics is above 10.0 × 10⁻⁴ C/m² °C in a broad temperature range from 20 °C to 55 °C. The possible physical mechanism of the temperature broadened phenomenon is briefly discussed.     

Electrical characteristics and low-temperature sintering of BiFeO3-modified Pb(Zr,Ti)O3–Pb(Fe2/3W1/3)O3–Pb(Mn1/3Nb2/3)O3 ceramics with ZnO addition

Effects of ZnO addition on electrical properties and low-temperature sintering of BiFeO₃-modified Pb(Zr,Ti)O₃–Pb(Fe_2/3W_1/3)O₃–Pb(Mn_1/3Nb_2/3)O₃ were investigated. The investigations revealed that the sintering temperature can be decreased to 950 °C, and the favorable properties were obtained with 0.10 wt% ZnO added ceramics. The electrical properties were as follows: d₃₃ = 313 pC/N, K_p = 0.56, tan δ = 0.0053, ε_r = 1407 and T_c = 295 °C, which showed that this system was a promising material for the multilayer devices application.     

Impedance and dielectric studies of ferroelectric SrBi2Nb2O9 ceramics

This paper reports the dielectric and impedance characteristics of ferroelectric SrBi₂Nb₂O₉ (SBN) ceramics in the 100 Hz-1 MHz frequency range at various temperatures (300-823 K). A strong low frequency dielectric dispersion (LFDD) associated with an impedance relaxation has been found to exist in these ceramics in the temperature range 573-823 K. The Z″ of the AC complex impedance showed two distinct slopes in the frequency range 100 Hz-1 MHz suggesting the existence of two dispersion mechanisms. This non-ideal behavior has been explained on the basis of the expression, Z*=R₀/(1+(iω/ω₁)^m+(iω/ω₂)ⁿ) [J. Phys. Chem. Solids 53 (1992) 1] where ω₁ and ω₂ characterize the lattice response and the charge carrier behavior, respectively. The exponents m and n were obtained from the curve fitting. The exponent n was found to exhibit a minimum at the Curie temperature, T_c (723 K) whereas the m was temperature independent.     

Experimental and numerical investigation of a ceramic dielectric resonator (DRA): CaCu3Ti4O12 (CCTO)

In this study, the CaCu₃Ti₄O₁₂ (CCTO) ceramic phase was synthesized by microwave heating in a much shorter time compared to the conventional heating methods. The results indicate that microwave processing is a promising method for preparing CCTO ceramics. CCTO was prepared using a domestic microwave oven operated at 2.45 GHz with 800 W. After a few minutes of microwave irradiation the formation of CCTO was confirmed by X-ray powder diffraction. The CCTO ceramic was studied in the medium-frequency (MF) range (100 Hz-1 MHz) and in the microwave range of frequencies. The experimental and theoretical characteristics of the dielectric resonator antenna are investigated.     

Phase transition, structural and electrical properties of Pb(Zn1/3Nb2/3)O3-doped Pb(Ni1/3Nb2/3)O3-PbTiO3 ceramics prepared by solid-state reaction method

Phase pure perovskite (1−x−y)Pb(Ni_1/3Nb_2/3)O₃-xPb(Zn_1/3Nb_2/3)O₃-yPbTiO₃ (PNN-PZN-PT) ferroelectric ceramics were prepared by conventional solid-state reaction method via a B-site oxide mixing route. The PNN-PZN-PT ceramics sintered at the optimized condition of 1185 °C for 2 h exhibit high relative density and rather homogenous microstructure. With the increase of PbTiO₃ (PT) content, crystal structure and electrical properties of the synthesized PNN-PZN-PT ceramics exhibit successive phase transformation. A morphotropic phase boundary (MPB) is supposed to form in (0.9−x)PNN-0.1PZN-xPT at a region of x=32-36 mol% confirmed by X-ray diffraction (XRD) measurement and dielectric measurement. The MPB composition can be pictured as providing a “bridge” connecting rhombohedral ferroelectric (FE) phase and tetragonal one since crystal structure of the MPB composition is similar to both the rhombohedral and tetragonal lattices. Dielectric response of the sintered PNN-PZN-PT ceramics also exhibits successive phase-transition character. 0.64PNN-0.1PZN-0.26PT exhibits broad, diffused and frequency dependent dielectric peaks indicating a character of diffused FE-paraelectric (PE) phase transition of relaxor ferroelectrics and 0.40PNN-0.1PZN-0.50PT exhibits narrow, sharp and frequency independent dielectric peaks indicating a character of first-order FE-PE phase transition of normal ferroelectrics. The FE-PE phase transition of 0.56PNN-0.1PZN-0.34PT is nearly first-order with some diffused character, which also exhibits the largest value of piezoelectric constant d₃₃ of 462pC/N.     

Piezoelectric, dielectric and magnetic properties of (1-x)Pb[Zr, Ti, (Mg1/2W1/2), (Ni1/3Nb2/3)]O3+x(Ni, Co, Cu)FeO4 composites

The phase structure, microstructure, piezoelectric properties, dielectric characteristic and the ME effect of magnetoelectric Pb[Zr_0.23Ti_0.36+0.02(Mg_1/2W_1/2)+0.39(Ni_1/3Nb_2/3)]O₃ (PZT)+xNi_0.8Co_0.1Cu_0.1Fe₂O₄ (NCCF) composite ceramics were prepared by the conventional solid state reaction method. The structural analysis of both the constituent phases and their composites was carried out by X-ray diffraction, energy dispersive spectrometry and scanning electron microscopy. The results showed cubic spinel structure for ferrite phase and tetragonal perovskite structure for ferroelectric phase. The piezoelectric constant, dielectric constant, Curie temperature, remanent polarization and coercive electric field decreased with increase of ferrite content. The coercive field strength, saturation magnetization and remanent magnetization increased with increasing ferrite content.     

Impedance spectroscopy study and ground state electronic properties of In(Mg1/2Ti1/2)O3

The complex perovskite oxide In(Mg_1/2Ti_1/2)O₃ (IMT) is synthesized by a solid state reaction technique. The X-ray diffraction of the sample at 30 °C shows a monoclinic phase. The dielectric properties of the sample are investigated in the temperature range from 143 to 373 K and in the frequency range from 580 Hz to 1 MHz using impedance spectroscopy. An analysis of the dielectric constant ε′ and loss tangent (tan δ) with frequency is performed assuming a distribution of relaxation times. The Cole-Cole model is used to explain the relaxation mechanism in IMT. The scaling behavior of imaginary part of electric modulus (M″) shows that the relaxation describes the same mechanism at various temperatures. The electronic structure and hence the ground state properties of IMT is studied by X-ray photoemission spectroscopy (XPS). The valence band XPS spectrum is compared with the electronic structure calculation. The electronic structure calculation indicates that the In-5s orbital introduces a significant density of states at the Fermi level, which is responsible for a high value of conductivity in IMT.     

Investigation of the optoelectronic properties of columbite ZnNb2O6 crystal

A high-quality ZnNb₂O₆ single-crystal grown by optical floating zone method has been used as a research prototype to analyze the optoelectronic parameters by measuring the absorption coefficient and transmittance spectra along the b-axis from 200 nm to 1000 nm at room temperature. The optical interband transitions of ZnNb₂O₆ have been determined as a direct transition with a band gap of 3.84 eV. The refractive index, extinction coefficient, and real and imaginary parts of the complex dielectric constants as functions of the wavelength for ZnNb₂O₆ crystal are obtained from the measured absorption coefficients and transmittance spectra. In the Urbach tail of 3.16–3.60 eV, the validity of the Cauchy–Sellmeier equation has also been evaluated. Using the single effective oscillator model, the oscillator energy E_o is found to be 4.77 eV. The dispersion energy E_d is 26.88 eV and ZnNb₂O₆ crystal takes an ionic value.     

On the power-law behavior of the AC conductivity of the mixed crystal (NH4)3H(SO4)1.42(SeO4)0.58

A power law used to describe the AC conductivity from 299 to 393 K of the mixed crystal (NH₄)₃H(SO₄)_1.42(SeO₄)_0.58 led to fractional exponent values ranging from 1.08 to 0.91, depending on structural changes induced on temperature variation [B. Louati, M. Gargouri, K. Guidara and T. Mhiri, J. Phys. Chem. Solids 66 (2005) 762]. In the present note, we suggest that the fractional law exhibits features of lattice relaxation. Despite the structural changes, the parameters of the power law are mutually interconnected to yield a temperature independent phenomenon. Such behavior is probably of general validity and characterizes the universal fractional dispersion of the AC conductivity, as it was also observed in glasses of different composition.     

Complex impedance spectroscopic analysis of Mn-modified Pb(Zr0.65Ti0.35)O3 electroceramics

The polycrystalline samples of Pb(Zr_0.65−xMn_xTi_0.35)O₃ (PZMT) (x=0, 0.05, 0.10, 0.15) were prepared by a high-temperature solid-state reaction technique. Detailed studies on the effect of compositional variation of manganese (Mn) on the electrical behavior (complex impedance Z*, complex modulus M*, electrical conductivity and relaxation mechanisms) of the PZMT systems have been carried out by a nondestructive complex impedance spectroscopy (CIS) technique at 400 °C. The Nyquist plots suggest that the grains only are responsible in the conduction mechanism of the materials. The occurrence of single arc in the complex modulus spectrum of all the compositions of Mn confirms the single-phase characteristics of the PZMT compounds, and also confirms the presence of non-Debye type of multiple relaxation in the material.     

Phase transformation behavior and dielectric characteristics of BaTi1−x(Al1/2Nb1/2)xO3 (0.01≤x≤0.40) ceramics

Microstructure, phase transformation behavior and dielectric properties of BaTi_1−x(Al_1/2Nb_1/2)_xO₃ (0.01≤x≤0.40) ceramics were investigated. A high level of (Al_1/2Nb_1/2)⁴⁺ substitution for Ti⁴⁺ ions was not conducive to the stability of the perovskite structure and resulted in the formation of BaAl₂O₄. As x was increased, lattice constants and unit cell volume decreased, reached a minimum at x=0.10 and then increased. The BaTi_1−x(Al_1/2Nb_1/2)_xO₃ ceramics at room temperature experienced a transformation from ferroelectric to paraelectric phase with increasing (Al_1/2Nb_1/2)⁴⁺ concentration. Meanwhile, permittivity of the BaTi_1−x(Al_1/2Nb_1/2)_xO₃ ceramics was markedly reduced, while Q value was slightly increased. Frequency dispersion of dielectric peak was obviously increased as x was increased from 0.01 to 0.10. It is of great interest that a dielectric abnormity represented by a broad dielectric peak at 200-400 K was observed for the composition with x=0.40.     

Dielectric properties of Bismuth Titanate (Bi4Ti3O12) synthesized using solution combustion route

Ferroelectric Bismuth Titanate (Bi₄Ti₃O₁₂) was prepared by solution combustion route with glycine as fuel. The single phase Bismuth Titanate was obtained after calcination at 800 °C, which was confirmed with the help of X-ray diffraction studies and EDS analysis. SEM micrographs of the calcined powders show agglomerated particles, which is typical of combustion synthesis. Behavior of dielectric constant and dielectric loss as a function of temperature of as prepared sample are reported here. Ferroelectric to paraelectric phase transition occurs at the temperature T_c∼650 °C. Impedance studies were made in the frequency range from 1 KHz to 1 MHz. The semicircles observed in the complex impedance diagrams indicate deviation from the Debye behavior. Activation energy of the sample around T_c is found to be ∼0.35 eV and below T_c is ∼0.13 eV, which was calculated using the Arrhenius plots.     

Dielectric and optical behaviors in relaxor ferroelectric Pb(In1/2Nb1/2)1−xTixO3 crystal

Dielectric permittivities (ε′,ε″) have been measured as functions of temperature (140-535 K) and frequency (500 Hz-2.0 MHz) in a (001)-cut Pb(In_1/2Nb_1/2)_0.7Ti_0.3O₃ (PINT30%) single crystal grown by the modified Bridgman method with Pb(Mg_1/3Nb_2/3)_0.71Ti_0.29O₃ (PMNT29%) seed crystal. A diffused phase transition was observed in the temperature region of ∼430-460 K with strong frequency dispersion. Above the Burns temperature T_B≅510 K, the dielectric permittivity was found to follow the Curie-Weiss behavior, ε′=C/(T−T_C), with parameters C=3.9×10⁵ and T_C=472 K. Below T_B≅510 K, polar nanoclusters are considered to appear and are responsible for the diffused dielectric anomaly. Optical transmission, refractive indices, and the Cauchy equations were obtained as a function of wavelength at room temperature. The unpoled crystal shows almost no birefringence, indicating that the average structural symmetry is optically isotropic. The crystal exhibits a broad transparency in the wavelength range of ∼0.4-6.0 μm.     

Dielectric and AC conductivity behavior of BaBi2Nb2O9 ceramics

The complex dielectric and AC conductivity response of BaBi₂Nb₂O₉ relaxor ferroelectric ceramics were studied as a function of frequency (100 Hz-10 MHz) at various temperatures. The observed dielectric behavior was characterized by two types of relaxation processes which were described by the ‘universal relaxation law’. The frequency dependence of conductivity which showed a classical relaxor behavior followed the Jonscher's universal law σ(ω)=σ₀+Aωⁿ. The exponent n exhibited a minimum in the vicinity of temperatures of dielectric anomaly while the pre-factor A showed a maximum. The temperature dependence of n followed the Vogel-Fulcher relation with activation energy of about 0.14 eV.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).     

铁电单晶0.62Pb(Mg1/3Nb2/3)O3-0.38PbTiO3折射率的研究

用最小偏向角法在20℃下精确测量了0.62Pb(Mg_1/3Nb_2/3)O_{3< /sub>-0.38PbTiO3( 0.62PMN-0.38PT)单晶的折射率，给出了该温度下折射率色散的Sellmeier方程.研究了能带 结构与折射率的关系，计算了样品的Sellmeier光学系数：对no，E0=5.50eV,λ0=0.2 26μm，Ｓ０=1.004×10¹⁴m^-2，Ed=28.1 0eV；对ne，Ｅ0=5.57eV,λ 0=0.223μm，S0=1.017×10¹⁴m^-2，Ed=28.10eV.A BO3型钙钛矿材料中，BO6八面体基元决定了晶体的能带结构，对折 射率产生重要影响. 关键词： PMNT单晶 折射率 Sellmeier光学系数}     

Crystal structure,NMR study,dielectric relaxation and AC conductivity of a new compound [Cd3(SCN)2Br6(C2H9N2)2]n

The crystal structure, the ¹³C NMR spectroscopy and the complex impedance have been carried out on [Cd₃(SCN)₂Br₆(C₂H₉N₂)₂]_n. Crystal structure shows a 2D polymeric network built up of two crystallographically independent cadmium atoms with two different octahedral coordinations. This compound exhibits a phase transition at (T=355±2 K) which has been characterized by differential scanning calorimetry (DSC), X-rays powder diffraction, AC conductivity and dielectric measurements. Examination of ¹³C CP/MAS line shapes shows indirect spin–spin coupling (¹⁴N and ¹³C) with a dipolar coupling constant of 1339 Hz. The AC conductivity of this compound has been carried out in the temperature range 325–376 K and the frequency range from 10⁻² Hz to 10 MHz. The impedance data were well fitted to two equivalent electrical circuits. The results of the modulus study reveal the presence of two distinct relaxation processes. One, at low frequency side, is thermally activated due to the ionic conduction of the crystal and the other, at higher frequency side, gradually disappears when temperature reaches 355 K which is attributed to the localized dipoles in the crystal. Moreover, the temperature dependence of DC-conductivity in both phases follows the Arrhenius law and the frequency dependence of σ(ω,T) follows Jonscher's universal law. The near values of activation energies obtained from the conductivity data and impedance confirm that the transport is through the ion hopping mechanism.     

Electron hopping mechanism in hematite (α-Fe2O3)

The frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric constant of polycrystalline hematite (α-Fe₂O₃) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O²⁻ ion into one of the magnetic ions Fe³⁺) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the −ln τ vs 10³/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy −0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature T_Mo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains.